Stellar distance using parallax

WEBVTT
Kind: captions
Language: en

00:00:00.660 --> 00:00:03.210
In the last video, we
talked about how parallax

00:00:03.210 --> 00:00:05.200
is the apparent
change in position

00:00:05.200 --> 00:00:07.590
of something based on
your line of sight.

00:00:07.590 --> 00:00:10.627
And if you experience parallax
in your everyday life,

00:00:10.627 --> 00:00:12.960
if you look outside of the
car window while it's moving,

00:00:12.960 --> 00:00:15.760
you see that nearby
objects seem to be

00:00:15.760 --> 00:00:18.970
moving faster than
far away objects.

00:00:18.970 --> 00:00:20.560
So in the last
video, we measured

00:00:20.560 --> 00:00:23.890
the apparent displacement of
this star at different points

00:00:23.890 --> 00:00:26.060
in the year relative
to straight up.

00:00:26.060 --> 00:00:28.690
But you could also measure it
relative to things in the night

00:00:28.690 --> 00:00:32.900
sky at that same time of year,
that same time of day that

00:00:32.900 --> 00:00:34.180
don't appear to be moving.

00:00:34.180 --> 00:00:36.263
And they won't appear to
be moving because they're

00:00:36.263 --> 00:00:38.640
going to be way, way, way
farther away than this star

00:00:38.640 --> 00:00:39.139
over here.

00:00:39.139 --> 00:00:41.820
There might be other galaxies
or maybe even other clusters

00:00:41.820 --> 00:00:43.070
of galaxies or who knows.

00:00:43.070 --> 00:00:44.990
Things that are not
changing position.

00:00:44.990 --> 00:00:46.032
So that's another option.

00:00:46.032 --> 00:00:47.489
And that's another
way to make sure

00:00:47.489 --> 00:00:49.840
that you're looking at the
right part of the universe.

00:00:49.840 --> 00:00:51.839
So you could measure it
relative to straight up,

00:00:51.839 --> 00:00:54.464
if you know based on the time
of year and the time of day

00:00:54.464 --> 00:00:56.880
that you're looking at the
same direction of the universe.

00:00:56.880 --> 00:00:59.310
Or you could just find
things in the universe that

00:00:59.310 --> 00:01:02.280
are way far back that their
apparent position isn't

00:01:02.280 --> 00:01:03.200
changing.

00:01:03.200 --> 00:01:04.769
So just to visualize
this again, I'll

00:01:04.769 --> 00:01:09.210
visualize it in a
slightly different way.

00:01:09.210 --> 00:01:10.760
Let's say, this is
our-- let's say,

00:01:10.760 --> 00:01:14.620
this is a night field of vision.

00:01:14.620 --> 00:01:16.440
Let me scroll to the
right a little bit.

00:01:16.440 --> 00:01:18.890
Let's say, our night field
of vision looks like this.

00:01:18.890 --> 00:01:21.580
And I'll do it in a dark
color because it's at night.

00:01:21.580 --> 00:01:25.430
So our night field of
vision looks like this.

00:01:25.430 --> 00:01:31.209
And let's say, that this right
over here is straight up.

00:01:31.209 --> 00:01:33.250
This right here is if
we're just looking straight

00:01:33.250 --> 00:01:35.480
up in the night sky.

00:01:35.480 --> 00:01:37.940
And just to make the
convention, in the last video,

00:01:37.940 --> 00:01:39.980
I changed our
orientation a little bit.

00:01:39.980 --> 00:01:43.920
But I'll reorient us in a
traditional orientation.

00:01:43.920 --> 00:01:48.290
So if we make this
north, this is south.

00:01:48.290 --> 00:01:49.930
This will be west.

00:01:49.930 --> 00:01:51.900
And then this will be east.

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So when we're looking at
the star in the summer,

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what will it look like?

00:01:56.030 --> 00:01:59.350
Well, first of all, the sun
is just to beginning to rise.

00:01:59.350 --> 00:02:02.864
So if you could think about
it, this north direction--

00:02:02.864 --> 00:02:05.280
we're looking at the sun from--
we're looking at the Earth

00:02:05.280 --> 00:02:05.950
from above.

00:02:05.950 --> 00:02:10.600
So the north will be the top
of this sphere over here.

00:02:10.600 --> 00:02:13.440
And the south will be the bottom
of the sphere, the other side

00:02:13.440 --> 00:02:15.240
of this sphere that
we're not seeing.

00:02:15.240 --> 00:02:19.140
The east will be this
side of the sphere

00:02:19.140 --> 00:02:25.810
where the sun is just beginning
to rise right over there.

00:02:25.810 --> 00:02:28.920
So what will be the apparent
position of this star?

00:02:28.920 --> 00:02:30.940
Well, it's going to
be towards the east.

00:02:30.940 --> 00:02:32.830
It's going to be
towards the direction

00:02:32.830 --> 00:02:34.090
that the sun is rising.

00:02:34.090 --> 00:02:41.140
So this angle right over here
is going to be right over there.

00:02:41.140 --> 00:02:46.640
So this will be the angle theta.

00:02:46.640 --> 00:02:48.620
So this is in the summer.

00:02:48.620 --> 00:02:49.990
And what about the winter?

00:02:49.990 --> 00:02:52.260
Well, in the winter in
order-- straight up--

00:02:52.260 --> 00:02:55.450
in order for straight up to
be that same point in time,

00:02:55.450 --> 00:02:58.190
or that same direction of
the universe I should say,

00:02:58.190 --> 00:03:01.750
then, the sun will just
to be setting word.

00:03:01.750 --> 00:03:03.240
We're rotating around that way.

00:03:03.240 --> 00:03:06.070
So we're just going to
be capturing the last two

00:03:06.070 --> 00:03:08.840
glimpse of sunlight.

00:03:08.840 --> 00:03:11.410
So in that situation, the
sun is going to be setting.

00:03:11.410 --> 00:03:14.430
So this is our
winter sun-- so I'll

00:03:14.430 --> 00:03:18.070
do a slightly different color--
will be setting on the west.

00:03:18.070 --> 00:03:22.050
And now, the apparent
direction of that star

00:03:22.050 --> 00:03:24.980
is going to be in the
direction of the sun again.

00:03:24.980 --> 00:03:27.890
But it's going to be
shifted away from center.

00:03:27.890 --> 00:03:32.700
So it's going to be to
the right of center.

00:03:32.700 --> 00:03:34.110
Sorry, to the left of center.

00:03:34.110 --> 00:03:36.360
So it's going to
be right over here.

00:03:39.574 --> 00:03:40.990
And it's a little
bit unintuitive,

00:03:40.990 --> 00:03:44.050
the way I drew it
in the last video.

00:03:44.050 --> 00:03:46.910
Well, I won't make any judgment
on whether the last one is

00:03:46.910 --> 00:03:49.250
easier to visualize
or this one is.

00:03:49.250 --> 00:03:51.250
Over here, I just wanted
to make the conventions

00:03:51.250 --> 00:03:53.530
so that north is
up, south is down.

00:03:53.530 --> 00:03:56.080
But I just want to be clear,
over here, the sun is-- well,

00:03:56.080 --> 00:03:57.500
sun always sets to the west.

00:03:57.500 --> 00:04:00.720
So in the winter, the sun
will be right over there.

00:04:00.720 --> 00:04:04.100
This will be shifted from center
in the direction of the sun.

00:04:04.100 --> 00:04:10.820
So it'll be at an angle theta
just like that in the winter.

00:04:10.820 --> 00:04:12.900
Now, that's all review
from the last video

00:04:12.900 --> 00:04:15.310
I just reoriented
how we visualize it.

00:04:15.310 --> 00:04:18.120
What I want to do in this
video, given that we can measure

00:04:18.120 --> 00:04:22.842
theta, how can we figure out
how far this star actually is.

00:04:22.842 --> 00:04:24.550
So let's just think
about it a little bit

00:04:24.550 --> 00:04:27.050
before even give
you a theta value.

00:04:27.050 --> 00:04:30.600
If we know theta,
then we know what

00:04:30.600 --> 00:04:32.810
this angle is right over here.

00:04:32.810 --> 00:04:34.670
Because this is a
right angle, we're

00:04:34.670 --> 00:04:38.420
going to know this angle
right here is 90 minus theta.

00:04:38.420 --> 00:04:44.694
We also know the distance
from the sun to the Earth.

00:04:44.694 --> 00:04:46.860
And let's say, we're just
going to approximate here,

00:04:46.860 --> 00:04:48.004
it's one astronomical unit.

00:04:48.004 --> 00:04:50.170
It changes a little bit
over the course of the year.

00:04:50.170 --> 00:04:54.470
But the mean distance is
one astronomical unit.

00:04:54.470 --> 00:04:57.840
So we know that angle.

00:04:57.840 --> 00:05:00.040
We know a side
adjacent to the angle.

00:05:00.040 --> 00:05:02.730
And what we're trying to do
is, figure out a side opposite

00:05:02.730 --> 00:05:04.200
to the angle--
this distance right

00:05:04.200 --> 00:05:07.500
here, the distance from
the sun to the star.

00:05:07.500 --> 00:05:09.290
And this is, of course,
a right triangle.

00:05:09.290 --> 00:05:10.540
And you can see it right here.

00:05:10.540 --> 00:05:12.416
Here is the hypotenuse.

00:05:12.416 --> 00:05:13.790
So now, we just
have to break out

00:05:13.790 --> 00:05:17.300
some relatively
basic trigonometry.

00:05:17.300 --> 00:05:20.600
So if we know this angle,
what trig ratio deals

00:05:20.600 --> 00:05:22.760
with an adjacent side
and opposite side?

00:05:22.760 --> 00:05:25.590
So let me write down
my famous SOHCAHTOA.

00:05:25.590 --> 00:05:26.740
I didn't come up with it.

00:05:26.740 --> 00:05:27.770
So the famous SOHCAHTOA.

00:05:31.470 --> 00:05:33.182
Sine is opposite
over hypotenuse.

00:05:33.182 --> 00:05:34.640
Those aren't the
two we care about.

00:05:34.640 --> 00:05:36.417
Cosine is adjacent
over hypotenuse.

00:05:36.417 --> 00:05:38.000
We don't know what
the hypotenuses is.

00:05:38.000 --> 00:05:39.670
And we don't care
about it just yet.

00:05:39.670 --> 00:05:42.240
But a tangent is the
opposite over the adjacent.

00:05:44.780 --> 00:05:48.050
So if we take the
tangent of the angle.

00:05:48.050 --> 00:05:55.330
If we take the tangent
of 90 minus theta,

00:05:55.330 --> 00:05:58.260
this is going to be equal
to the distance to the star.

00:05:58.260 --> 00:06:00.130
This distance right over here.

00:06:00.130 --> 00:06:02.420
The distance to the star,
or the distance from the sun

00:06:02.420 --> 00:06:03.160
to the star.

00:06:03.160 --> 00:06:04.820
We can later figure out
the distance from the Earth

00:06:04.820 --> 00:06:05.360
to the star.

00:06:05.360 --> 00:06:06.776
It's not going to
be too different

00:06:06.776 --> 00:06:08.470
because the star is so far away.

00:06:08.470 --> 00:06:10.950
But the distance from
the sun to the star

00:06:10.950 --> 00:06:13.940
divided by the
adjacent side, divided

00:06:13.940 --> 00:06:15.810
by one astronomical unit.

00:06:15.810 --> 00:06:18.380
I'm assuming everything
is in astronomical units.

00:06:18.380 --> 00:06:20.400
So you can multiply
both sides by one.

00:06:20.400 --> 00:06:22.920
And you'll get the distance
in astronomical units.

00:06:22.920 --> 00:06:28.960
The distance is equal to the
tangent of 90 minus theta.

00:06:28.960 --> 00:06:29.657
Not too bad.

00:06:29.657 --> 00:06:31.740
So let's actually figure
out what a distance would

00:06:31.740 --> 00:06:36.230
be based on some
actual measurements.

00:06:36.230 --> 00:06:38.300
Let's say, you were
to measure some star.

00:06:38.300 --> 00:06:41.150
Measure this apparent-- this
change in angle right here.

00:06:41.150 --> 00:06:43.270
Let's say, you've got
the total change in angle

00:06:43.270 --> 00:06:45.709
right over here from six months
apart, the biggest spread.

00:06:45.709 --> 00:06:47.250
And you're making
sure you're looking

00:06:47.250 --> 00:06:49.570
at a point in the universe
relative to straight up.

00:06:49.570 --> 00:06:50.760
You can do it other ways.

00:06:50.760 --> 00:06:53.310
But this is really just
simplifies our visualization

00:06:53.310 --> 00:06:54.880
and simplifies our math.

00:06:54.880 --> 00:07:01.814
And you get it to be
1.5374 arc seconds.

00:07:01.814 --> 00:07:02.980
And I want to be very clear.

00:07:02.980 --> 00:07:07.080
This is a very, very, very,
very, very, small angle.

00:07:07.080 --> 00:07:08.670
Just to visualize
it or another way

00:07:08.670 --> 00:07:11.060
to think about it
is, one point, there

00:07:11.060 --> 00:07:13.580
are 60 arc seconds
per arc minute.

00:07:13.580 --> 00:07:16.180
And there are 60 arc
minutes per degree.

00:07:16.180 --> 00:07:19.251
Or another way to think about
is a degree is like an arc hour.

00:07:19.251 --> 00:07:21.000
So if you want to
convert this to degrees,

00:07:21.000 --> 00:07:28.010
you have 1.5374
arc seconds times

00:07:28.010 --> 00:07:33.920
one degree is equal
to 3,600 arc seconds.

00:07:33.920 --> 00:07:35.550
The dimensions cancel out.

00:07:35.550 --> 00:07:39.510
And you get this as being equal
to-- get the calculator out.

00:07:39.510 --> 00:07:47.720
This is being equal to
1.5374 divided by 3,600.

00:07:47.720 --> 00:07:52.487
So it's 4.206.

00:07:52.487 --> 00:07:54.820
I'll round because we only
want five significant digits.

00:07:54.820 --> 00:07:56.370
This is of infinite
precision right here

00:07:56.370 --> 00:07:57.870
because that's an
absolute quantity.

00:07:57.870 --> 00:07:59.037
It's a definition.

00:07:59.037 --> 00:08:00.120
So let me write this down.

00:08:00.120 --> 00:08:10.680
So this is going to be 4.2706
times 10 to the negative 4

00:08:10.680 --> 00:08:11.500
degrees.

00:08:11.500 --> 00:08:13.225
Could write it just like that.

00:08:13.225 --> 00:08:14.100
Now, let me be clear.

00:08:14.100 --> 00:08:16.930
This is the total angle.

00:08:16.930 --> 00:08:19.790
This angle that we care about
is going to be half of this.

00:08:19.790 --> 00:08:22.080
So we could divide this by 2.

00:08:22.080 --> 00:08:27.730
So let me just do our
significant digits 4.2706

00:08:27.730 --> 00:08:32.809
divided by 2, or I
could even say times 10

00:08:32.809 --> 00:08:37.820
to the negative
4-- divided by 2 is

00:08:37.820 --> 00:08:41.900
going to be 2.1353 times
10 to the negative 4.

00:08:41.900 --> 00:08:43.809
So that's this angle
right over here.

00:08:43.809 --> 00:08:48.590
This angle, or the shift from
center we could visualize it,

00:08:48.590 --> 00:08:54.361
is going to be 2.1353
times 10 to the negative 4.

00:08:54.361 --> 00:08:55.860
So now that we know
that, we already

00:08:55.860 --> 00:08:57.651
figured out how to
figure out the distance.

00:08:57.651 --> 00:08:59.730
We can just apply
this right over here.

00:08:59.730 --> 00:09:03.520
So let's just take
the tangent-- make

00:09:03.520 --> 00:09:05.260
sure your calculator
is in degree mode.

00:09:05.260 --> 00:09:07.370
I made sure of that before
I started this video.

00:09:07.370 --> 00:09:12.520
--of 90 minus this
angle right here.

00:09:12.520 --> 00:09:15.340
So instead of retyping it, I'll
just write the last answer.

00:09:15.340 --> 00:09:17.360
So 90 minus this angle.

00:09:17.360 --> 00:09:22.510
And we get this large
number-- 268,326.

00:09:22.510 --> 00:09:24.400
Now remember, what
were our units?

00:09:24.400 --> 00:09:32.704
This distance right
here is 268,326.

00:09:32.704 --> 00:09:34.120
I should just round
because I only

00:09:34.120 --> 00:09:36.210
have five significant
digits here.

00:09:36.210 --> 00:09:40.710
300 and-- although with trig
numbered, with the trig,

00:09:40.710 --> 00:09:42.460
when you're using,
involving trigonometry,

00:09:42.460 --> 00:09:44.540
the significant digits
get a little bit shadier.

00:09:44.540 --> 00:09:46.039
But I'll just write
the whole number

00:09:46.039 --> 00:09:55.730
here 268,326 astronomic units.

00:09:55.730 --> 00:10:00.420
So it's this many distances
between the sun and the Earth.

00:10:00.420 --> 00:10:04.690
Now, if we wanted to calculate
that into light years,

00:10:04.690 --> 00:10:06.390
we just have to
know-- and you could

00:10:06.390 --> 00:10:07.620
calculate this multiple ways.

00:10:07.620 --> 00:10:09.150
You could just
figure out how far

00:10:09.150 --> 00:10:12.380
an AU is versus a light year.

00:10:12.380 --> 00:10:15.100
So this is AUs.

00:10:15.100 --> 00:10:23.650
One light year is equivalent
to 63,115 astronomical units,

00:10:23.650 --> 00:10:25.200
give or take a little bit.

00:10:25.200 --> 00:10:27.800
So this is going to
be equal to-- the AUs

00:10:27.800 --> 00:10:29.920
cancel out-- this
quantity divided

00:10:29.920 --> 00:10:32.550
by that quantity in light years.

00:10:32.550 --> 00:10:33.670
So let's do that.

00:10:33.670 --> 00:10:41.750
So let's take this number the
we just got divided by 63,115.

00:10:41.750 --> 00:10:43.550
And we have in light years.

00:10:43.550 --> 00:10:46.146
And so it's about
4.25 light years.

00:10:46.146 --> 00:10:47.770
I'm messing with the
significant digits

00:10:47.770 --> 00:10:58.870
here but just roundabout
answer, 4.25 light years.

00:10:58.870 --> 00:11:00.700
Now remember,
that's about how far

00:11:00.700 --> 00:11:03.390
the closest star
to the Earth is.

00:11:03.390 --> 00:11:05.090
And so the closest
star of the Earth

00:11:05.090 --> 00:11:07.970
has this very,
very, very apparent,

00:11:07.970 --> 00:11:09.977
a very small change in angle.

00:11:09.977 --> 00:11:12.310
You could imagine as you go
to further and further stars

00:11:12.310 --> 00:11:14.440
from this, that angle,
this angle right

00:11:14.440 --> 00:11:16.530
here is going to get
even smaller and smaller

00:11:16.530 --> 00:11:17.050
and smaller.

00:11:17.050 --> 00:11:19.240
And all the way until
you get really far stars.

00:11:19.240 --> 00:11:21.330
And it would be, even
with our best instruments,

00:11:21.330 --> 00:11:23.429
you won't be able to
measure that angle.

00:11:23.429 --> 00:11:25.720
Anyway, hopefully, you found
that cool because you just

00:11:25.720 --> 00:11:28.060
figured out a way
to use trigonometry

00:11:28.060 --> 00:11:33.020
in a really good way to
measure angles in the night sky

00:11:33.020 --> 00:11:36.830
to actually figure out how far
we are from the nearest stars.

00:11:36.830 --> 00:11:39.730
I think that's pretty neat.


Parallax in observing stars

WEBVTT
Kind: captions
Language: en

00:00:00.404 --> 00:00:01.820
What I want to do
in this video is

00:00:01.820 --> 00:00:07.840
explain what parallax
actually is, and then try

00:00:07.840 --> 00:00:10.370
to visualize what
the parallax would

00:00:10.370 --> 00:00:13.030
be like in the context
of observing relatively

00:00:13.030 --> 00:00:14.110
nearby stars.

00:00:14.110 --> 00:00:15.580
And then in the
next video, we're

00:00:15.580 --> 00:00:17.840
going to think
about how we can use

00:00:17.840 --> 00:00:20.360
the parallax of nearby
stars to figure out

00:00:20.360 --> 00:00:23.820
how far they actually
are away from us.

00:00:23.820 --> 00:00:26.990
So parallax really is
just the apparent change

00:00:26.990 --> 00:00:30.380
in position of something based
on a different line of sight.

00:00:30.380 --> 00:00:34.014
So when you see that, while
you're looking out at a car,

00:00:34.014 --> 00:00:35.430
you're going to
see that depending

00:00:35.430 --> 00:00:37.434
on how far different
things are from you,

00:00:37.434 --> 00:00:39.600
it looks like they're moving
relative to each other.

00:00:39.600 --> 00:00:41.474
Right now I'm looking
at my computer monitor.

00:00:41.474 --> 00:00:43.930
And if I move my head around
or shake my head around,

00:00:43.930 --> 00:00:45.780
it looks like the wall
behind the computer

00:00:45.780 --> 00:00:47.905
monitor is moving relative
to the computer monitor.

00:00:47.905 --> 00:00:49.370
We've all experienced this.

00:00:49.370 --> 00:00:51.160
But let's think
about what parallax

00:00:51.160 --> 00:00:53.435
means when looking at stars.

00:00:53.435 --> 00:00:56.670
So let me draw the sun here.

00:00:56.670 --> 00:00:59.460
And obviously none of
this is drawn to scale.

00:00:59.460 --> 00:01:01.150
So let me draw the sun here.

00:01:01.150 --> 00:01:04.160
And let me draw the
earth at some point

00:01:04.160 --> 00:01:06.039
in its orbit around the sun.

00:01:06.039 --> 00:01:07.580
And we're going to
pretend like we're

00:01:07.580 --> 00:01:09.800
looking from above
the solar system.

00:01:09.800 --> 00:01:16.480
So the earth will be rotating in
this direction right over here.

00:01:16.480 --> 00:01:24.820
And let's say the star that we
care about is right over here.

00:01:24.820 --> 00:01:27.770
Obviously, clearly
not drawn to scale.

00:01:27.770 --> 00:01:29.810
And what we're
going to do is we're

00:01:29.810 --> 00:01:32.380
going to wait to the
point in the year, so

00:01:32.380 --> 00:01:34.910
the point in our orbit
around the earth, so

00:01:34.910 --> 00:01:37.145
that right at dawn--
so we're sitting right

00:01:37.145 --> 00:01:38.520
here on the surface
of the earth.

00:01:38.520 --> 00:01:41.020
And to simplify things,
we're on the equator.

00:01:41.020 --> 00:01:43.200
And let's say that
this star is roughly

00:01:43.200 --> 00:01:45.150
in the plane of
our solar system.

00:01:45.150 --> 00:01:49.810
So we're sitting right
here on the equator.

00:01:49.810 --> 00:01:55.380
And then right at dawn, right
when the first light of the sun

00:01:55.380 --> 00:01:57.560
begins to reach me--
remember, right now the sun

00:01:57.560 --> 00:02:00.440
is lighting up this
side of the earth--

00:02:00.440 --> 00:02:05.210
so right when the first light
of the sun is reaching me,

00:02:05.210 --> 00:02:08.090
I'm looking straight up.

00:02:08.090 --> 00:02:10.860
So if I look straight up right
when the first light of the sun

00:02:10.860 --> 00:02:17.110
is reaching me, and I look
straight up like that,

00:02:17.110 --> 00:02:19.566
I will be looking
in that direction.

00:02:19.566 --> 00:02:21.940
Now, so let's say that that
direction that I'm looking at

00:02:21.940 --> 00:02:25.132
is this direction
right over here.

00:02:25.132 --> 00:02:26.590
And then let me
make it clear, this

00:02:26.590 --> 00:02:29.890
is a separate part
of the diagram here.

00:02:29.890 --> 00:02:32.170
Maybe I'll do it over here.

00:02:32.170 --> 00:02:37.830
So if the night sky
looks like this,

00:02:37.830 --> 00:02:41.500
the sun is just beginning
to rise on the horizon.

00:02:41.500 --> 00:02:46.300
If I look straight up, I'm
looking in this direction.

00:02:46.300 --> 00:02:50.339
So where would this star
be relative to straight up?

00:02:50.339 --> 00:02:52.130
Well, straight up is
going to be like that.

00:02:52.130 --> 00:02:57.260
The sun is, the way I drew it
right here, right to my left.

00:02:57.260 --> 00:02:58.520
Straight up is just like that.

00:02:58.520 --> 00:03:00.390
The sun is just coming
over the horizon.

00:03:00.390 --> 00:03:03.650
This star right here,
the apparent position

00:03:03.650 --> 00:03:05.170
of this star
relative to straight

00:03:05.170 --> 00:03:10.810
up is going to be at some angle
to the left of straight up.

00:03:10.810 --> 00:03:12.640
It's going to be
right over there.

00:03:12.640 --> 00:03:15.052
And obviously the star
won't be that big relative

00:03:15.052 --> 00:03:17.260
to your entire field of
vision, but you get the idea.

00:03:17.260 --> 00:03:20.990
Maybe I'll draw it a little
bit smaller, just like that.

00:03:20.990 --> 00:03:23.100
So there's going to
be some angle here.

00:03:23.100 --> 00:03:26.060
And this angle,
whatever it is, let's

00:03:26.060 --> 00:03:30.210
just call it Theta, that's going
to be the same angle as this.

00:03:30.210 --> 00:03:31.710
And when I talk
about the angle, I'm

00:03:31.710 --> 00:03:35.100
talking about if you measure
from one side of the horizon

00:03:35.100 --> 00:03:36.480
to the other side
of the horizon,

00:03:36.480 --> 00:03:39.830
you're essentially looking
halfway around the earth.

00:03:39.830 --> 00:03:41.470
That would be 180 degrees.

00:03:41.470 --> 00:03:43.250
So you could
literally measure what

00:03:43.250 --> 00:03:45.830
this angle is right over here.

00:03:45.830 --> 00:03:49.530
Now let's say we
waited six months.

00:03:49.530 --> 00:03:50.920
What's going to happen?

00:03:50.920 --> 00:03:57.304
Six months, we're going to
be on this side of the sun.

00:03:57.304 --> 00:03:59.220
We're assuming that our
distance is relatively

00:03:59.220 --> 00:04:01.940
constant at one
astronomical unit.

00:04:01.940 --> 00:04:03.720
Now what happens?

00:04:03.720 --> 00:04:07.740
Remember, the earth
is rotating like this.

00:04:07.740 --> 00:04:10.280
So if we wait, right
at sunset, right

00:04:10.280 --> 00:04:12.480
when the last glimpse
of the sun has just

00:04:12.480 --> 00:04:15.060
gone away-- because you
can remember, right now,

00:04:15.060 --> 00:04:17.602
the sun is illuminating
this side of the earth.

00:04:17.602 --> 00:04:19.060
The sun is going
to be illuminating

00:04:19.060 --> 00:04:20.018
that side of the earth.

00:04:20.018 --> 00:04:23.460
So if we're sitting right at
the equator right over there,

00:04:23.460 --> 00:04:27.270
right when the sun is just
setting, we look straight up.

00:04:31.000 --> 00:04:32.920
Let me do that in
the same color.

00:04:32.920 --> 00:04:36.180
We look straight up.

00:04:36.180 --> 00:04:38.230
So six months later when
we look straight up,

00:04:38.230 --> 00:04:41.080
where is the star
relative to straight up?

00:04:41.080 --> 00:04:44.847
Well now the star
will be to the right.

00:04:44.847 --> 00:04:45.930
It'll be in the direction.

00:04:45.930 --> 00:04:49.910
So if this is our field of
vision six months later,

00:04:49.910 --> 00:04:54.320
now the sun is setting
all the way to the right,

00:04:54.320 --> 00:04:56.010
on the right horizon.

00:04:56.010 --> 00:04:58.220
And if we look
straight up, this star

00:04:58.220 --> 00:05:06.840
now is going to be to
the right of straight up.

00:05:06.840 --> 00:05:08.490
So what just happened here?

00:05:08.490 --> 00:05:10.910
Well, it looks like
relative to straight up--

00:05:10.910 --> 00:05:17.330
and we're looking at the exact
kind of position of the earth.

00:05:17.330 --> 00:05:20.310
We're making sure that we're
picking times of year and times

00:05:20.310 --> 00:05:23.970
of day where straight up
is the same direction.

00:05:23.970 --> 00:05:27.040
We're looking in the same
direction of the universe.

00:05:27.040 --> 00:05:31.810
It looks like the position of
that star has actually shifted.

00:05:31.810 --> 00:05:37.197
And let's say that this
is the middle of summer,

00:05:37.197 --> 00:05:38.780
and that this is the
middle of winter.

00:05:38.780 --> 00:05:39.571
Doesn't have to be.

00:05:39.571 --> 00:05:42.670
It could be any other two
points in time six months apart.

00:05:42.670 --> 00:05:45.050
Then when we look at
this star in the summer,

00:05:45.050 --> 00:05:46.330
it's going to be over here.

00:05:46.330 --> 00:05:48.080
Summer, it's going to
be right over there.

00:05:48.080 --> 00:05:49.871
And when we look at
the star in the winter,

00:05:49.871 --> 00:05:51.062
it is going to be over here.

00:05:51.062 --> 00:05:52.770
And, in general, for
any star, especially

00:05:52.770 --> 00:05:55.520
stars that are in the same
plane as the solar system,

00:05:55.520 --> 00:05:58.100
you can find two
points in the year

00:05:58.100 --> 00:06:01.910
where that star is at a kind of
a maximum distance from center.

00:06:01.910 --> 00:06:04.810
And those are the two distances,
those are the two times of year

00:06:04.810 --> 00:06:07.060
that you'll want to care the
most about, because it'll

00:06:07.060 --> 00:06:09.060
be most interesting
to measure this angle.

00:06:09.060 --> 00:06:10.910
And I want to be
clear, this angle

00:06:10.910 --> 00:06:15.890
here is going to be the same
thing as this angle there.

00:06:15.890 --> 00:06:19.150
You can see it's
symmetric this way.

00:06:19.150 --> 00:06:21.830
Whatever this angle is going to
be, and you could look at this.

00:06:21.830 --> 00:06:26.800
This is an isosceles triangle.

00:06:26.800 --> 00:06:30.430
Whatever this distance
is from here to here,

00:06:30.430 --> 00:06:33.902
is going to be the same as this
distance from here to here.

00:06:33.902 --> 00:06:36.110
And so this angle is going
to be equal to that angle,

00:06:36.110 --> 00:06:38.360
and that angle is going
to be equal to that angle.

00:06:38.360 --> 00:06:40.010
What I want to do
in the next video

00:06:40.010 --> 00:06:42.970
is think about if we're
able to precisely measure

00:06:42.970 --> 00:06:46.090
these angles, either one
of them or both of them.

00:06:46.090 --> 00:06:49.230
And let me be clear, if
this angle in the night sky

00:06:49.230 --> 00:06:52.460
is Theta, and this angle
right here is Theta,

00:06:52.460 --> 00:06:54.380
the difference over
here is two Theta.

00:06:54.380 --> 00:06:57.350
So one option, if you
want to kind of make sure

00:06:57.350 --> 00:06:58.890
that your number's
reasonably good,

00:06:58.890 --> 00:07:00.700
you could measure just
the total difference

00:07:00.700 --> 00:07:03.354
that it is around the center
and then divide by two.

00:07:03.354 --> 00:07:04.770
But in the next
video, what I want

00:07:04.770 --> 00:07:09.850
to do is if you
are able to measure

00:07:09.850 --> 00:07:12.890
the apparent change
in angle here,

00:07:12.890 --> 00:07:14.896
if you were to be
able to measure that,

00:07:14.896 --> 00:07:16.770
how would you be able
to use that information

00:07:16.770 --> 00:07:20.469
to actually figure out
the distance to this star?


Star field and nebula images

WEBVTT
Kind: captions
Language: en

00:00:01.045 --> 00:00:05.000
since we've been talking how stars formed and the evolution of stars

00:00:05.000 --> 00:00:08.638
i thought it was about time that we looked at some cool pictures of stars

00:00:08.638 --> 00:00:12.214
forming or stars themselves, or evolution of stars.

00:00:12.214 --> 00:00:23.600
so this right here is from the Eagle Nebula,

00:00:23.600 --> 00:00:30.883
Just so you know, the word Nebula is kind of a general word for any interstellar cloud of gas or dust

00:00:30.883 --> 00:00:34.133
so when we're talking about the Eagle nebula, we're actually talking about a huge nebula

00:00:34.133 --> 00:00:36.733
and actually, it's a nebula that expands.

00:00:36.733 --> 00:00:44.400
and so that you have a sense, this is just one of the pillars of the "Pillars of creation"

00:00:44.400 --> 00:00:48.133
This is just a small part of the actual Eagle Nebula

00:00:48.133 --> 00:00:52.467
And just this pillar right over here, just this pillar here

00:00:52.467 --> 00:01:01.400
so that you have a sense of how large it is, just this pillar itself is 7 light years

00:01:01.400 --> 00:01:04.551
it is 7 light years talls

00:01:04.551 --> 00:01:06.467
so this is an enormous amount of distance

00:01:06.467 --> 00:01:11.425
remember, the distance from earth to the nearest star was about 4 light years

00:01:11.425 --> 00:01:13.839
it would take voyager, if it were pointed in the right direction

00:01:13.839 --> 00:01:17.333
moving at 60 thousand kilometers per hour

00:01:17.333 --> 00:01:21.333
it would take Voyager 80 thousand years to go 4 light years

00:01:21.333 --> 00:01:24.822
just this pillar is 7 light years

00:01:24.822 --> 00:01:29.333
but i wanted to show you this because these type of nebulae, the plural of nebula

00:01:29.333 --> 00:01:32.160
are where stars can form.

00:01:32.160 --> 00:01:37.400
so this right here, you actually see, is actually a breeding ground for the birth of new stars

00:01:37.400 --> 00:01:41.933
this gas is condensing, just like we talked about a couple of videos ago. Until it gets to that critical

00:01:41.933 --> 00:01:45.333
temperature, the critical density, where you can actually get

00:01:45.333 --> 00:01:49.267
fusion of hydrogen

00:01:49.267 --> 00:01:53.933
so this is just a huge interstellar cloud of hydrogen gas

00:01:53.933 --> 00:01:59.200
and over here you can see its just this breeding ground for stars

00:01:59.200 --> 00:02:03.159
and we don't even, we think that this structure doesn't even exist anymore

00:02:03.159 --> 00:02:06.400
because remember, this thing is very very far away from us

00:02:06.400 --> 00:02:11.267
in fact is, just so you have the number, this thing is 7000 light years away

00:02:11.267 --> 00:02:17.416
7000 light years away

00:02:17.416 --> 00:02:23.867
which means that what we are seeing now, the photons that are reaching our eyes or telescopes right now

00:02:23.867 --> 00:02:28.492
left this region of space 7000 years ago

00:02:28.492 --> 00:02:31.133
so we're seeing it as it was 7000 years ago

00:02:31.133 --> 00:02:36.533
so a lot of this gas, a lot of this hydrogen, may have already condensed into many many more stars

00:02:36.533 --> 00:02:39.800
so the structure might not be the way it looks right now

00:02:39.800 --> 00:02:41.267
and actually there was another

00:02:41.267 --> 00:02:45.867
super nova that happened that might have blown away a lot of this stuff

00:02:45.867 --> 00:02:49.000
and we won't even be able to see the effects of this super nova

00:02:49.000 --> 00:02:51.085
for another thousand years

00:02:51.085 --> 00:02:53.600
but anyway, this is just a pretty amazing photograph in my opinion

00:02:53.600 --> 00:02:56.600
especially, and its beautiful at any scale

00:02:56.600 --> 00:02:59.067
and it's even more mindblowing when you think that this is 7, this is a structure

00:02:59.067 --> 00:03:02.200
that is 7 light years tall

00:03:02.200 --> 00:03:04.667
and this is really just part of the Eagle Nebula

00:03:04.667 --> 00:03:06.921
one of the pillars of creation

00:03:06.921 --> 00:03:13.236
this right here is a star field, and this is as we're looking towards the center of our galaxy

00:03:13.236 --> 00:03:13.933
the milkyway

00:03:13.933 --> 00:03:16.162
this is the Sagittarius star field

00:03:16.162 --> 00:03:20.267
the neat thing here you see is such a diversity in stars

00:03:20.267 --> 00:03:24.916
this is also kind of mind numbing because every one of these stars, are inside of our galaxy

00:03:24.916 --> 00:03:27.933
this is looking towards the center of our galaxy

00:03:27.933 --> 00:03:32.333
this isn't one of those where we're looking beyond our galaxy or looking at clusters of galaxies

00:03:32.333 --> 00:03:34.000
this is just stars here

00:03:34.000 --> 00:03:37.153
but the thing here is that you see a huge variety, you see some stars that are

00:03:37.153 --> 00:03:40.467
shining red, right over here

00:03:40.467 --> 00:03:42.819
and obviously, the apparent size, you cannot completely tell

00:03:42.819 --> 00:03:46.933
because the different stars are at different distances and at difference intensities

00:03:46.933 --> 00:03:52.933
but the redder stars, these are stars in their red giant phase

00:03:52.933 --> 00:03:55.400
or they're probably at their red giant phase

00:03:55.400 --> 00:03:57.333
i haven't done specific research on these stars

00:03:57.333 --> 00:03:58.957
but that's what we suspect

00:03:58.957 --> 00:04:00.467
those are in their red giant phase

00:04:00.467 --> 00:04:04.933
the ones that are kind of in the yellowish white part of the spectrum

00:04:04.933 --> 00:04:07.133
these are stars that are probably in their main sequence

00:04:07.133 --> 00:04:10.892
probably not too different than own sun

00:04:10.892 --> 00:04:16.348
the ones that are in the yellowish white, closer to orange-yellowish-white part of the spectrum

00:04:16.348 --> 00:04:22.641
and the ones that look a little more bluish, or a little bit more greenish

00:04:22.641 --> 00:04:24.986
these are burning super fast

00:04:24.986 --> 00:04:27.867
let me see if i can find, this one looks a little big bluish to me,

00:04:27.867 --> 00:04:34.800
these are burning super super fast, and so

00:04:34.800 --> 00:04:40.218
the super massive stars, they burn kind of fast and furious and then just die out

00:04:41.609 --> 00:04:43.000
but the smaller stars, the ones with less mass

00:04:43.000 --> 00:04:48.020
they burn slower over a much much longer period of time

00:04:48.020 --> 00:04:50.017
so the ones that are burning fast

00:04:50.017 --> 00:04:56.101
are emitting a lot of energy at the smaller wavelength part of the light spectrum

00:04:56.101 --> 00:05:00.267
that's why they look bluer or greener and these are going to be more massive stars

00:05:00.267 --> 00:05:03.438
the ones that look white or bluer or greener

00:05:03.438 --> 00:05:04.933
while the redder ones

00:05:04.933 --> 00:05:09.800
the redder ones are less massive stars that are kind of in their super giant phase

00:05:09.800 --> 00:05:14.000
and so they are at this point cooler than the main sequence stars

00:05:14.000 --> 00:05:18.333
this right here, this right here is the cat's eye nebula

00:05:18.333 --> 00:05:20.933
and the word nebula, this is actually a planetary nebula

00:05:20.933 --> 00:05:27.680
this one here is a planetary nebula

00:05:27.680 --> 00:05:34.019
and it's called a nebula because it is kind of this gas that's kinda floating out in space

00:05:34.019 --> 00:05:38.067
but it's at a completely different kind of scale than this Eagle nebula

00:05:38.067 --> 00:05:41.200
that we drew over here

00:05:41.200 --> 00:05:43.200
so when people talk about nebulas, they normally talking about something like the eagle nebula

00:05:43.200 --> 00:05:46.467
these huge masses of interstellar gas

00:05:46.467 --> 00:05:51.318
when people talk about planetary nebulas; this is actually still a huge radius

00:05:51.318 --> 00:05:53.267
but no where near 7 light years

00:05:53.267 --> 00:05:59.538
but this is the bi-product of a star shedding all of its outer material

00:05:59.538 --> 00:06:03.400
so at the center of this, we see kind of a mature star here

00:06:03.400 --> 00:06:07.133
and it's shed of kind of its outer layers

00:06:07.133 --> 00:06:09.174
and it did that in its red giant phase

00:06:09.174 --> 00:06:12.733
so the core would keep flaring up, having these hot explosions

00:06:12.733 --> 00:06:18.485
and every time you have one of these hot explosions, you had more and more of its outer layers getting

00:06:18.485 --> 00:06:21.156
pushed off, pushed off into space

00:06:21.156 --> 00:06:23.467
forming this planetary nebula

00:06:23.467 --> 00:06:25.267
so this as we see it right now

00:06:25.267 --> 00:06:29.467
its still not yet a white dwarf, it is still an active star

00:06:29.467 --> 00:06:35.867
fusion is still occurring in this star, but it is well on its way onto becoming a a white dwarf

00:06:35.867 --> 00:06:37.502
once all its fuel runs out

00:06:37.502 --> 00:06:42.200
it's past its red giant phase, it's thrown all of this material into space and it's on its way onto becoming

00:06:42.200 --> 00:06:43.333
a white dwarf

00:06:43.333 --> 00:06:44.600
anyway hopefully you enjoyed that

00:06:44.600 --> 00:06:49.267
i actually find all of these images to be pretty captivating

00:06:49.267 --> 00:06:50.800
especially the star field one

00:06:50.800 --> 00:06:52.502
because this just inside of our galaxy

00:06:52.502 --> 00:06:55.933
hopefully it gives more appreciation for how many stars there are

00:06:55.933 --> 99:59:59.999
i mean this is just a small fraction of the 200 to 400 billion stars inside the milky way


White and black dwarfs

WEBVTT
Kind: captions
Language: en

00:00:00.620 --> 00:00:03.040
In the last video, we
started with a star

00:00:03.040 --> 00:00:05.840
in its main sequence,
like the sun.

00:00:05.840 --> 00:00:07.670
And inside the
core of that star,

00:00:07.670 --> 00:00:10.300
you have hydrogen
fusion going on.

00:00:10.300 --> 00:00:13.680
So that is hydrogen fusion,
and then outside of the core,

00:00:13.680 --> 00:00:15.370
you just had hydrogen.

00:00:15.370 --> 00:00:17.170
You just hydrogen plasma.

00:00:17.170 --> 00:00:20.930
And when we say plasma, it's
the electrons and protons

00:00:20.930 --> 00:00:23.600
of the individual atoms
have been disassociated

00:00:23.600 --> 00:00:26.066
because the temperatures
and pressures are so high.

00:00:26.066 --> 00:00:27.940
So they're really just
kind of like this soup

00:00:27.940 --> 00:00:33.390
of electrons and protons,
as opposed to proper atoms

00:00:33.390 --> 00:00:36.140
that we associate with
at lower temperatures.

00:00:36.140 --> 00:00:38.970
So this is a main sequence
star right over here.

00:00:43.350 --> 00:00:46.510
And we saw in the last
video that this hydrogen

00:00:46.510 --> 00:00:48.160
is fusing into helium.

00:00:48.160 --> 00:00:52.570
So we start having more
and more helium here.

00:00:52.570 --> 00:00:54.380
And as we have more
and more helium,

00:00:54.380 --> 00:00:56.320
the core becomes
more and more dense,

00:00:56.320 --> 00:00:59.300
because helium is a
more massive atom.

00:00:59.300 --> 00:01:03.290
It is able to pack more
mass in a smaller volume.

00:01:03.290 --> 00:01:05.710
So this gets more
and more dense.

00:01:05.710 --> 00:01:08.485
So core becomes more dense.

00:01:13.500 --> 00:01:15.900
And so while the core is
becoming more and more dense,

00:01:15.900 --> 00:01:19.340
that actually makes the fusion
happen faster and faster.

00:01:19.340 --> 00:01:21.710
Because it's more dense,
more gravitational pressure,

00:01:21.710 --> 00:01:24.080
more mass wanting
to get to it, more

00:01:24.080 --> 00:01:26.600
pressure on the
hydrogen that's fusing,

00:01:26.600 --> 00:01:28.670
so it starts to fuse hotter.

00:01:28.670 --> 00:01:32.400
So let me write this, so
the fusion, so hydrogen

00:01:32.400 --> 00:01:36.470
fuses faster.

00:01:36.470 --> 00:01:38.220
And actually, we even
see this in our sun.

00:01:38.220 --> 00:01:41.070
Our sun today is
brighter and hotter.

00:01:41.070 --> 00:01:42.960
It's fusing faster
than it was when

00:01:42.960 --> 00:01:47.382
it was born 4.5 or
4.6 billion years ago.

00:01:47.382 --> 00:01:49.340
But eventually you're
going to get to the point

00:01:49.340 --> 00:01:54.020
so that the core,
you only have helium.

00:01:54.020 --> 00:01:56.040
So there's going to
be some point where

00:01:56.040 --> 00:01:58.310
the entire core is all helium.

00:01:58.310 --> 00:02:00.800
And it's going to be way denser
than this core over here.

00:02:00.800 --> 00:02:03.397
All of that mass over there has
now been turned into helium.

00:02:03.397 --> 00:02:03.980
Not all of it.

00:02:03.980 --> 00:02:05.672
A lot of it has been
turned into energy.

00:02:05.672 --> 00:02:07.380
But most of it is now
in helium, and it's

00:02:07.380 --> 00:02:09.207
going to be at a much,
much smaller volume.

00:02:09.207 --> 00:02:11.290
And the whole time, the
temperature is increasing,

00:02:11.290 --> 00:02:13.490
the fusion is getting
faster and faster.

00:02:13.490 --> 00:02:16.280
And now there's this
dense volume of helium

00:02:16.280 --> 00:02:17.850
that's not fusing.

00:02:17.850 --> 00:02:22.930
You do have, and we saw this in
this video, a shell around it

00:02:22.930 --> 00:02:25.520
of hydrogen that is fusing.

00:02:25.520 --> 00:02:30.490
So this right here is
hydrogen fusion going on.

00:02:30.490 --> 00:02:33.019
And then this over here
is just hydrogen plasma.

00:02:33.019 --> 00:02:34.810
Now the unintuitive
thing, or at least this

00:02:34.810 --> 00:02:38.870
was unintuitive
to me at first, is

00:02:38.870 --> 00:02:41.180
what's going on the
core is that the core is

00:02:41.180 --> 00:02:43.740
getting more and more dense.

00:02:43.740 --> 00:02:46.530
It's fusing at a faster rate.

00:02:46.530 --> 00:02:48.150
And so it's getting
hotter and hotter.

00:02:48.150 --> 00:02:50.360
So the core is
hotter, fusing faster,

00:02:50.360 --> 00:02:51.860
getting more and more dense.

00:02:51.860 --> 00:02:53.740
I kind of imagine it's
starting to collapse.

00:02:53.740 --> 00:02:57.100
Every time it collapses, it's
getting hotter and more dense.

00:02:57.100 --> 00:02:58.850
But at the same time
that's happening,

00:02:58.850 --> 00:03:03.280
the star itself
is getting bigger.

00:03:03.280 --> 00:03:05.690
And this is actually
not drawn to scale.

00:03:05.690 --> 00:03:09.865
Red giants are much, much
larger than main sequence stars.

00:03:09.865 --> 00:03:11.490
But the whole time
that this is getting

00:03:11.490 --> 00:03:13.350
more dense, the
rest of the star is,

00:03:13.350 --> 00:03:15.370
you could kind of view
it as getting less dense.

00:03:15.370 --> 00:03:18.580
And that's because this is
generating so much energy

00:03:18.580 --> 00:03:24.270
that it's able to more
than offset, or better

00:03:24.270 --> 00:03:27.192
offset the gravitational
pull into it.

00:03:27.192 --> 00:03:28.650
So even though this
is hotter, it's

00:03:28.650 --> 00:03:31.090
able to disperse the
rest of the material

00:03:31.090 --> 00:03:33.700
in the sun over a larger volume.

00:03:33.700 --> 00:03:36.100
And so that volume is so big
that the surface, and we saw

00:03:36.100 --> 00:03:39.550
this in the last video, the
surface of the red giant

00:03:39.550 --> 00:03:44.710
is actually cooler-- let me
write that a little neater-- is

00:03:44.710 --> 00:03:48.660
actually cooler than the
surface of a main sequence star.

00:03:48.660 --> 00:03:50.910
This right here is hotter.

00:03:50.910 --> 00:03:53.360
And just to put
things in perspective,

00:03:53.360 --> 00:03:55.490
when the sun
becomes a red giant,

00:03:55.490 --> 00:03:58.430
and it will become a
red giant, its diameter

00:03:58.430 --> 00:04:01.780
will be 100 times the
diameter that it is today.

00:04:01.780 --> 00:04:05.120
Or another way to be put it,
it will have the same diameter

00:04:05.120 --> 00:04:09.390
as the Earth's orbit
around the current sun.

00:04:09.390 --> 00:04:12.120
Or another way to view it
is, where we are right now

00:04:12.120 --> 00:04:15.110
will be on the surface or
near the surface or maybe even

00:04:15.110 --> 00:04:17.660
inside of that future sun.

00:04:17.660 --> 00:04:20.690
Or another way to put it, when
the sun becomes a red giant,

00:04:20.690 --> 00:04:23.470
the Earth's going to be
not even a speck out here.

00:04:23.470 --> 00:04:25.990
And it will be
liquefied and vaporized

00:04:25.990 --> 00:04:27.070
at that point in time.

00:04:27.070 --> 00:04:28.660
So this is super, super huge.

00:04:28.660 --> 00:04:30.840
And we've even thought about it.

00:04:30.840 --> 00:04:32.970
Just for light to
reach the current sun

00:04:32.970 --> 00:04:35.840
to our point in orbit,
it takes eight minutes.

00:04:35.840 --> 00:04:38.620
So that's how big one
of these stars are.

00:04:38.620 --> 00:04:43.390
To get from one side of the star
to another side of the star,

00:04:43.390 --> 00:04:47.310
it'll take 16 minutes
for light to travel,

00:04:47.310 --> 00:04:48.785
if it was traveling
that diameter,

00:04:48.785 --> 00:04:50.740
and even slightly
longer if it was

00:04:50.740 --> 00:04:53.440
to travel it in a circumference.

00:04:53.440 --> 00:04:55.682
So these are huge,
huge, huge stars.

00:04:55.682 --> 00:04:57.640
And we'll talk about
other stars in the future.

00:04:57.640 --> 00:05:00.950
They're even bigger than this
when they become supergiants.

00:05:00.950 --> 00:05:05.590
But anyway, we have the
hydrogen in the center-- sorry.

00:05:05.590 --> 00:05:07.660
We have the helium
in the center.

00:05:07.660 --> 00:05:09.290
Let me write this down.

00:05:09.290 --> 00:05:13.460
We have a helium
core in the center.

00:05:13.460 --> 00:05:15.490
We're fusing faster
and faster and faster.

00:05:15.490 --> 00:05:16.830
We're now a red giant.

00:05:16.830 --> 00:05:18.830
The core is getting hotter
and hotter and hotter

00:05:18.830 --> 00:05:23.300
until it gets to the temperature
for ignition of helium.

00:05:23.300 --> 00:05:26.464
So until it gets to
100 million Kelvin--

00:05:26.464 --> 00:05:28.380
remember the ignition
temperature for hydrogen

00:05:28.380 --> 00:05:29.590
was 10 million Kelvin.

00:05:29.590 --> 00:05:32.760
So now we're at 100 million
Kelvin, factor of 10.

00:05:32.760 --> 00:05:35.490
And now, all of a
sudden in the core,

00:05:35.490 --> 00:05:38.205
you actually start to
have helium fusion.

00:05:41.562 --> 00:05:43.270
And we touched on this
in the last video,

00:05:43.270 --> 00:05:45.982
but the helium is fusing
into heavier elements.

00:05:45.982 --> 00:05:47.440
And some of those
heavier elements,

00:05:47.440 --> 00:05:49.540
and predominately, it
will be carbon and oxygen.

00:05:52.770 --> 00:05:55.880
And you may suspect this is how
heavier and heavier elements

00:05:55.880 --> 00:05:57.200
form in the universe.

00:05:57.200 --> 00:06:02.210
They form, literally, due to
fusion in the core of stars.

00:06:02.210 --> 00:06:05.260
Especially when we're talking
about elements up to iron.

00:06:05.260 --> 00:06:09.440
But anyway, the core is now
experiencing helium fusion.

00:06:09.440 --> 00:06:13.556
It has a shell around it of
helium that is not quite there,

00:06:13.556 --> 00:06:14.930
does not quite
have the pressures

00:06:14.930 --> 00:06:17.090
and temperatures to fuse yet.

00:06:17.090 --> 00:06:18.460
So just regular helium.

00:06:18.460 --> 00:06:20.330
But then outside
of that, we do have

00:06:20.330 --> 00:06:24.170
the pressures and
temperatures for hydrogen

00:06:24.170 --> 00:06:26.690
to continue to fuse.

00:06:26.690 --> 00:06:29.035
So out here, you do
have hydrogen fusion.

00:06:32.130 --> 00:06:34.590
And then outside
over here, you just

00:06:34.590 --> 00:06:36.140
have the regular
hydrogen plasma.

00:06:36.140 --> 00:06:37.370
So what just happened here?

00:06:37.370 --> 00:06:39.990
When you have helium fusion
all of a sudden-- now

00:06:39.990 --> 00:06:41.790
this is, once again,
providing some type

00:06:41.790 --> 00:06:44.810
of energetic outward
support for the core.

00:06:44.810 --> 00:06:49.944
So it's going to counteract
the ever-increasing contraction

00:06:49.944 --> 00:06:52.360
of the core as it gets more
and more dense, because now we

00:06:52.360 --> 00:06:57.820
have energy going outward,
energy pushing things outward.

00:06:57.820 --> 00:07:01.280
But at the same time
that that is happening,

00:07:01.280 --> 00:07:05.290
more and more hydrogen in this
layer is turning into helium,

00:07:05.290 --> 00:07:06.790
is fusing into helium.

00:07:06.790 --> 00:07:09.820
So it's making this inert
part of the helium core

00:07:09.820 --> 00:07:12.850
even larger and larger and
denser, even larger and larger,

00:07:12.850 --> 00:07:15.600
and putting even more
pressure on this inside part.

00:07:15.600 --> 00:07:19.590
And so what's actually going
to happen within a few moments,

00:07:19.590 --> 00:07:23.660
I guess, especially from a
cosmological point of view,

00:07:23.660 --> 00:07:27.330
this helium fusion is
going to be burning super--

00:07:27.330 --> 00:07:30.642
I shouldn't use-- igniting or
fusing at a super-hot level.

00:07:30.642 --> 00:07:32.600
But it's contained due
to all of this pressure.

00:07:32.600 --> 00:07:35.490
But at some point, the pressure
won't be able to contain it,

00:07:35.490 --> 00:07:39.019
and the core is
going to explode.

00:07:39.019 --> 00:07:41.560
But it's not going to be one of
these catastrophic explosions

00:07:41.560 --> 00:07:43.430
where the star is
going to be destroyed.

00:07:43.430 --> 00:07:45.860
It's just going to release a
lot of energy all of a sudden

00:07:45.860 --> 00:07:47.060
into the star.

00:07:47.060 --> 00:07:48.610
And that's called
a helium flash.

00:07:51.760 --> 00:07:55.450
But once that happens, all of
a sudden, then now the star

00:07:55.450 --> 00:07:57.130
is going to be more stable.

00:07:57.130 --> 00:08:01.180
And I'll use that in quotes
without writing it down

00:08:01.180 --> 00:08:03.120
because red giants, in
general, are already

00:08:03.120 --> 00:08:06.450
getting to be less stable
than a main sequence star.

00:08:06.450 --> 00:08:08.440
But once that
happens, you now will

00:08:08.440 --> 00:08:11.470
have a slightly larger volume.

00:08:11.470 --> 00:08:14.650
So it's not being contained
in as small of a tight volume.

00:08:14.650 --> 00:08:17.530
That helium flash kind
of took care of that.

00:08:17.530 --> 00:08:21.871
So now you have helium fusing
into carbon and oxygen.

00:08:21.871 --> 00:08:24.120
And there's all sorts of
other combinations of things.

00:08:24.120 --> 00:08:27.390
Obviously, there's many elements
in between helium and carbon

00:08:27.390 --> 00:08:28.040
and oxygen.

00:08:28.040 --> 00:08:29.820
But these are the
ones that dominate.

00:08:29.820 --> 00:08:38.100
And then outside of that,
you have helium forming.

00:08:38.100 --> 00:08:40.250
You have helium
that is not fusing.

00:08:40.250 --> 00:08:45.260
And then outside of that, you
have your fusing hydrogen.

00:08:45.260 --> 00:08:48.740
Over here, you have
hydrogen fusing into helium.

00:08:48.740 --> 00:08:50.700
And then out here in
the rest of the radius

00:08:50.700 --> 00:08:53.390
of our super-huge
red giant, you just

00:08:53.390 --> 00:08:57.530
have your hydrogen
plasma out here.

00:08:57.530 --> 00:09:00.742
Now what's going to
happen as this star ages?

00:09:00.742 --> 00:09:02.950
Well, if we fast forward
this a bunch-- and remember,

00:09:02.950 --> 00:09:05.470
as a star gets denser
and denser in the core,

00:09:05.470 --> 00:09:07.500
and the reactions happen
faster and faster,

00:09:07.500 --> 00:09:09.833
and this core is expelling
more and more energy outward,

00:09:09.833 --> 00:09:10.940
the star keeps growing.

00:09:10.940 --> 00:09:12.790
And the surface gets
cooler and cooler.

00:09:12.790 --> 00:09:15.080
So if we fast forward
a bunch, and this

00:09:15.080 --> 00:09:17.580
is what's going to happen to
something the mass of our sun,

00:09:17.580 --> 00:09:19.790
if it's more massive,
then at some point,

00:09:19.790 --> 00:09:22.290
the core of carbon and
oxygen that's forming

00:09:22.290 --> 00:09:24.302
can start to fuse into
even heavier elements.

00:09:24.302 --> 00:09:26.010
But in the case of
the sun, it will never

00:09:26.010 --> 00:09:29.410
get to that 600 million
Kelvin to actually fuse

00:09:29.410 --> 00:09:31.140
the carbon and the oxygen.

00:09:31.140 --> 00:09:36.570
And so eventually you will have
a core of carbon and oxygen,

00:09:36.570 --> 00:09:39.100
or mainly carbon and
oxygen surrounded

00:09:39.100 --> 00:09:51.420
by fusing helium surrounded by
non-fusing helium surrounded

00:09:51.420 --> 00:09:56.860
by fusing hydrogen,
which is surrounded

00:09:56.860 --> 00:10:00.790
by non-fusing hydrogen, or just
the hydrogen plasma of the sun.

00:10:00.790 --> 00:10:03.340
But eventually all of
this fuel will run out.

00:10:03.340 --> 00:10:06.140
All of the hydrogen will
run out in the stars.

00:10:06.140 --> 00:10:08.770
All of this hydrogen, all
of this fusing hydrogen

00:10:08.770 --> 00:10:09.510
will run out.

00:10:09.510 --> 00:10:11.520
All of this fusion
helium will run out.

00:10:13.974 --> 00:10:15.140
This is the fusing hydrogen.

00:10:15.140 --> 00:10:17.620
This is the inert helium,
which will run out.

00:10:17.620 --> 00:10:19.670
It'll be used in
kind of this core,

00:10:19.670 --> 00:10:22.890
being fused into the carbon
and oxygen, until you

00:10:22.890 --> 00:10:27.420
get to a point where
you literally just have

00:10:27.420 --> 00:10:31.225
a really hot core of
carbon and oxygen.

00:10:31.225 --> 00:10:32.100
And it's super-dense.

00:10:32.100 --> 00:10:33.558
This whole time,
it will be getting

00:10:33.558 --> 00:10:36.497
more and more dense as heavier
and heavier elements show up

00:10:36.497 --> 00:10:37.080
in the course.

00:10:37.080 --> 00:10:39.140
So it gets denser and
denser and denser.

00:10:39.140 --> 00:10:42.540
But the super dense thing will
not, in the case of the sun--

00:10:42.540 --> 00:10:45.550
and if it was a more massive
star, it would get there--

00:10:45.550 --> 00:10:47.580
but in the case of
the sun, it will not

00:10:47.580 --> 00:10:50.380
get hot enough for the carbon
and the oxygen to form.

00:10:50.380 --> 00:10:57.260
So it really will just be this
super-dense ball of carbon

00:10:57.260 --> 00:10:59.782
and oxygen and all of the
other material in the sun.

00:10:59.782 --> 00:11:01.115
Remember, it was superenergetic.

00:11:01.115 --> 00:11:03.170
It was releasing tons
and tons of energy.

00:11:03.170 --> 00:11:05.260
The more that we
progressed down this,

00:11:05.260 --> 00:11:06.960
the more energy was
releasing outward,

00:11:06.960 --> 00:11:09.630
and the larger the radius
of the star became,

00:11:09.630 --> 00:11:11.470
and the cooler the
outside of the star

00:11:11.470 --> 00:11:15.360
became, until the outside just
becomes this kind of cloud,

00:11:15.360 --> 00:11:22.140
this huge cloud of gas around
what once was the star.

00:11:22.140 --> 00:11:25.210
And in the center-- so I could
just draw it as this huge--

00:11:25.210 --> 00:11:27.840
this is now way far
away from the star,

00:11:27.840 --> 00:11:31.890
much even bigger than the radius
or the diameter of a red giant.

00:11:31.890 --> 00:11:37.610
And all we'll have left is
a mass, a superdense mass

00:11:37.610 --> 00:11:42.200
of, I would call it,
inert carbon or oxygen.

00:11:42.200 --> 00:11:44.150
This is in the case of the sun.

00:11:44.150 --> 00:11:48.000
And at first, when it's hot, and
it will be releasing radiation

00:11:48.000 --> 00:11:49.020
because it's so hot.

00:11:49.020 --> 00:11:52.400
We'll call this a white dwarf.

00:11:52.400 --> 00:11:56.800
This right here is
called a white dwarf.

00:11:56.800 --> 00:12:00.080
And it'll cool down over many,
many, many, many, many, many,

00:12:00.080 --> 00:12:05.300
many, years, until it
becomes, when it's completely

00:12:05.300 --> 00:12:07.590
cooled down, lost
all of its energy--

00:12:07.590 --> 00:12:12.530
it'll just be this superdense
ball of carbon and oxygen,

00:12:12.530 --> 00:12:17.720
at which point, we would
call it a black dwarf.

00:12:20.590 --> 00:12:23.080
And these are obviously
very hard to observe

00:12:23.080 --> 00:12:24.680
because they're
not emitting light.

00:12:24.680 --> 00:12:28.360
And they don't have quite
the mass of something

00:12:28.360 --> 00:12:30.650
like a black hole that
isn't even emitting light,

00:12:30.650 --> 00:12:33.417
but you can see how it's
affecting things around it.

00:12:33.417 --> 00:12:35.250
So that's what's going
to happen to the sun.

00:12:35.250 --> 00:12:36.480
In the next few
videos, we're going

00:12:36.480 --> 00:12:38.730
to talk about what would
happen to things less massive

00:12:38.730 --> 00:12:41.160
than the sun and what would
happen to things more massive

00:12:41.160 --> 00:12:41.980
than the sun,
although I think you

00:12:41.980 --> 00:12:43.188
can imagine the more massive.

00:12:45.221 --> 00:12:47.220
There would be so much
pressure on these things,

00:12:47.220 --> 00:12:48.606
because you have so
much mass around it,

00:12:48.606 --> 00:12:50.397
that these would begin
to fuse into heavier

00:12:50.397 --> 00:12:53.670
and heavier elements
until we get to iron.


Becoming a red giant

WEBVTT
Kind: captions
Language: en

00:00:00.500 --> 00:00:03.030
In the last video,
we had a large cloud

00:00:03.030 --> 00:00:06.590
of hydrogen atoms
eventually condensing

00:00:06.590 --> 00:00:11.010
into a high pressure, high
mass, I guess you could say,

00:00:11.010 --> 00:00:13.030
ball of hydrogen atoms.

00:00:13.030 --> 00:00:16.450
And when the pressure
and the temperature

00:00:16.450 --> 00:00:19.880
got high enough-- and so this
is what we saw the last video--

00:00:19.880 --> 00:00:22.190
when the pressure and
temperature got high enough,

00:00:22.190 --> 00:00:25.040
we were able to get
the hydrogen protons,

00:00:25.040 --> 00:00:27.050
the hydrogen
nucleuses close enough

00:00:27.050 --> 00:00:29.170
to each other, or
hydrogen nuclei

00:00:29.170 --> 00:00:32.900
close enough to each other, for
the strong force to take over

00:00:32.900 --> 00:00:35.770
and fusion to happen
and release energy.

00:00:35.770 --> 00:00:40.810
And then that real
energy begins to offset

00:00:40.810 --> 00:00:43.290
the actual gravitational force.

00:00:43.290 --> 00:00:45.460
So the whole star--
what's now a star--

00:00:45.460 --> 00:00:47.520
does not collapse on itself.

00:00:47.520 --> 00:00:50.540
And once we're there, we're now
in the main sequence of a star.

00:00:50.540 --> 00:00:51.956
What I want to do
in this video is

00:00:51.956 --> 00:00:53.580
to take off from
that starting point

00:00:53.580 --> 00:00:56.310
and think about what
happens in the star next.

00:00:56.310 --> 00:00:59.390
So in the main sequence, we
have the core of the star.

00:00:59.390 --> 00:01:02.130
So this is the
core-- star's core.

00:01:02.130 --> 00:01:11.590
And you have hydrogen
fusing into helium.

00:01:11.590 --> 00:01:13.670
And it's releasing
just a ton of energy.

00:01:18.210 --> 00:01:21.410
And that energy is what keeps
the core from imploding.

00:01:21.410 --> 00:01:23.770
It's kind of the
outward force to offset

00:01:23.770 --> 00:01:26.310
the gravitational force that
wants to implode everything,

00:01:26.310 --> 00:01:28.130
that wants to crush everything.

00:01:28.130 --> 00:01:31.670
And so you have the core of
a star, a star like the sun,

00:01:31.670 --> 00:01:34.480
and that energy then heats
up all of the other gas

00:01:34.480 --> 00:01:38.760
on the outside of the core
to create that really bright

00:01:38.760 --> 00:01:43.070
object that we see as a star, or
in our case, in our sun's case,

00:01:43.070 --> 00:01:44.310
the sun.

00:01:44.310 --> 00:01:47.380
Now, as the hydrogen
is fusing into helium,

00:01:47.380 --> 00:01:49.480
you could imagine that
more and more helium

00:01:49.480 --> 00:01:51.310
is forming in the core.

00:01:51.310 --> 00:01:53.160
So I'll do the helium as green.

00:01:53.160 --> 00:01:57.590
So more, more, and more
helium forms in the core.

00:01:57.590 --> 00:02:00.460
It'll especially form-- the
closer you get to the center,

00:02:00.460 --> 00:02:01.980
the higher the
pressures will be,

00:02:01.980 --> 00:02:05.950
and the faster that this fusion,
this ignition, will happen.

00:02:05.950 --> 00:02:07.930
In fact, the bigger
the mass of the star,

00:02:07.930 --> 00:02:11.240
the more the pressure, the
faster the fusion occurs.

00:02:11.240 --> 00:02:16.660
And so you have this helium
building up inside of the core

00:02:16.660 --> 00:02:19.245
as this hydrogen in
the core gets fused.

00:02:19.245 --> 00:02:20.620
Now what's going
to happen there?

00:02:20.620 --> 00:02:24.300
Helium is a more dense atom.

00:02:24.300 --> 00:02:27.590
It's packing more mass
in a smaller space.

00:02:27.590 --> 00:02:29.940
So as more and more
of this hydrogen

00:02:29.940 --> 00:02:31.880
here turns into
helium, what you're

00:02:31.880 --> 00:02:36.555
going to have is the core
itself is going to shrink.

00:02:39.570 --> 00:02:43.000
So let me draw a
smaller core here.

00:02:43.000 --> 00:02:47.700
So the core itself
is going to shrink.

00:02:47.700 --> 00:02:54.350
And now it has a lot
more helium in it.

00:02:54.350 --> 00:02:56.240
And let's just take it
to the extreme point

00:02:56.240 --> 00:02:58.610
where it's all helium,
where it's depleted.

00:02:58.610 --> 00:03:00.330
But it's much denser.

00:03:00.330 --> 00:03:03.410
That same amount of mass
that was in this sphere

00:03:03.410 --> 00:03:06.230
is now in a denser sphere,
in a helium sphere.

00:03:06.230 --> 00:03:08.470
So it's going to have just
as much attraction to it,

00:03:08.470 --> 00:03:10.140
gravitational attraction.

00:03:10.140 --> 00:03:12.220
But things can get
even closer to it.

00:03:12.220 --> 00:03:16.400
And we know that the
closer you are to a mass,

00:03:16.400 --> 00:03:18.730
the stronger the
pull of gravity.

00:03:18.730 --> 00:03:25.120
So then instead of having just
the hydrogen fusion occurring

00:03:25.120 --> 00:03:27.650
at the core, you're
now going to have

00:03:27.650 --> 00:03:33.380
hydrogen fusion in a
shell around the core.

00:03:33.380 --> 00:03:38.770
So now you're going to
have hydrogen fusing

00:03:38.770 --> 00:03:44.210
in a shell around the core.

00:03:44.210 --> 00:03:45.120
Let me just be clear.

00:03:45.120 --> 00:03:46.786
This isn't just happens
all of a sudden.

00:03:46.786 --> 00:03:48.170
It is a gradual process.

00:03:48.170 --> 00:03:51.280
As we have more and
more helium in the core,

00:03:51.280 --> 00:03:54.560
the core gets denser
and denser and denser.

00:03:54.560 --> 00:03:59.960
And so the pressures become even
larger and larger near the core

00:03:59.960 --> 00:04:04.600
because you're able to get
closer to a more massive core

00:04:04.600 --> 00:04:05.880
since it is now more dense.

00:04:05.880 --> 00:04:08.130
And as that pressure
near the core

00:04:08.130 --> 00:04:10.840
increases even more and
more, the fusion reaction

00:04:10.840 --> 00:04:13.300
happens faster and
faster and faster

00:04:13.300 --> 00:04:14.910
until you get to this point.

00:04:14.910 --> 00:04:16.060
So here, let me be clear.

00:04:16.060 --> 00:04:17.899
You have a helium core.

00:04:17.899 --> 00:04:21.089
All of the hydrogen in
the core has been used up.

00:04:21.089 --> 00:04:23.590
And then you have the
hydrogen right outside

00:04:23.590 --> 00:04:25.505
of the core is now
under enormous pressure.

00:04:25.505 --> 00:04:26.880
It's actually
under more pressure

00:04:26.880 --> 00:04:29.650
than it was when it was
just a pure hydrogen core.

00:04:29.650 --> 00:04:36.300
Because it's-- there's so
much mass on the outside here,

00:04:36.300 --> 00:04:39.570
trying to, I guess you could
say, exerting downwards,

00:04:39.570 --> 00:04:46.200
or gravitational force trying to
get to that even denser helium

00:04:46.200 --> 00:04:48.980
core because everything
is able to get closer in.

00:04:48.980 --> 00:04:51.165
And so now you have fusion
occurring even faster.

00:04:55.980 --> 00:05:00.140
And it's occurring
over a larger radius.

00:05:00.140 --> 00:05:05.020
So this faster fusion
over a larger radius,

00:05:05.020 --> 00:05:07.890
the force is now going to
expel-- the energy that's

00:05:07.890 --> 00:05:09.490
released from this
fusion is now going

00:05:09.490 --> 00:05:14.630
to expel these outer layers
of the star even further.

00:05:14.630 --> 00:05:16.740
So the whole time,
this gradual process

00:05:16.740 --> 00:05:19.060
as the hydrogen turns
into helium, or fuses

00:05:19.060 --> 00:05:21.900
into helium in the
core, the hydrogen right

00:05:21.900 --> 00:05:24.280
outside of the core,
right outside that area,

00:05:24.280 --> 00:05:27.020
starts to burn
faster and faster.

00:05:27.020 --> 00:05:28.090
I shouldn't say burn.

00:05:28.090 --> 00:05:30.890
It starts to fuse
faster and faster

00:05:30.890 --> 00:05:34.580
and over a larger
and larger radius.

00:05:34.580 --> 00:05:36.670
The unintuitive
thing is the fusion

00:05:36.670 --> 00:05:38.407
is happening faster
over a larger radius.

00:05:38.407 --> 00:05:40.740
And the reason that is is
because you have even a denser

00:05:40.740 --> 00:05:44.410
core that is causing even
more gravitational pressure.

00:05:44.410 --> 00:05:47.050
And as that's happening,
the star's getting brighter.

00:05:47.050 --> 00:05:50.640
And it's also-- the fusion
reactions, since they're

00:05:50.640 --> 00:05:54.260
happening in a more intense
way and over a larger radius,

00:05:54.260 --> 00:05:59.350
are able to expel the material
of the star even larger.

00:05:59.350 --> 00:06:01.950
So the radius of
the star itself is

00:06:01.950 --> 00:06:04.800
getting bigger and
bigger and bigger.

00:06:04.800 --> 00:06:07.980
So if this star looked
like this-- maybe

00:06:07.980 --> 00:06:13.100
let me draw it in
white-- That's not white.

00:06:16.285 --> 00:06:18.060
Now what's happening
to my color changer?

00:06:18.060 --> 00:06:19.230
There you go.

00:06:19.230 --> 00:06:25.380
OK, this star looked like
this right over here.

00:06:25.380 --> 00:06:29.054
Now, this star over here,
since a faster fusion reaction

00:06:29.054 --> 00:06:30.470
is happening over
a larger radius,

00:06:30.470 --> 00:06:32.410
is going to be far larger.

00:06:32.410 --> 00:06:34.480
And I'm not even
drawing it to scale.

00:06:34.480 --> 00:06:36.820
In the case of our sun,
when it gets to this point,

00:06:36.820 --> 00:06:39.230
it's going to be 100
times the diameter.

00:06:39.230 --> 00:06:44.250
And at this point,
it is a red giant.

00:06:44.250 --> 00:06:47.880
And the reason why it's
redder than this one over here

00:06:47.880 --> 00:06:51.430
is that even though the fusion
is happening more furiously,

00:06:51.430 --> 00:06:55.010
that energy is being dissipated
over a larger surface area.

00:06:55.010 --> 00:07:00.060
So the actual surface
temperature of the red giant,

00:07:00.060 --> 00:07:04.500
at this point, is actually
going to be cooler.

00:07:04.500 --> 00:07:10.839
So it's going to emit a light
at a larger wavelength, a redder

00:07:10.839 --> 00:07:12.380
wavelength than this
thing over here.

00:07:12.380 --> 00:07:15.360
This thing, the core, was
not burning as furiously

00:07:15.360 --> 00:07:17.130
as this thing over here.

00:07:17.130 --> 00:07:19.200
But that energy was
being dissipated

00:07:19.200 --> 00:07:21.290
over a smaller volume.

00:07:21.290 --> 00:07:23.390
So this has a higher
surface temperature.

00:07:28.350 --> 00:07:31.580
This over here, the core
is burning more-- sorry,

00:07:31.580 --> 00:07:32.980
the core is no longer burning.

00:07:32.980 --> 00:07:35.030
The core is now helium
that's not burning.

00:07:35.030 --> 00:07:39.370
It's getting denser and
denser as the helium packs in

00:07:39.370 --> 00:07:40.340
on itself.

00:07:40.340 --> 00:07:41.990
But the hydrogen
fusion over here

00:07:41.990 --> 00:07:43.500
is occurring more intensely.

00:07:43.500 --> 00:07:45.910
It's occurring in a hotter way.

00:07:45.910 --> 00:07:49.360
But the surface here is
less hot because it's just

00:07:49.360 --> 00:07:50.790
a larger surface area.

00:07:50.790 --> 00:07:52.650
So it doesn't make--
the increased heat

00:07:52.650 --> 00:07:56.260
is more than mitigated by how
large the star has become.

00:07:56.260 --> 00:07:58.560
Now, this is going
to keep happening.

00:07:58.560 --> 00:08:00.480
And this core is
keep-- the pressures

00:08:00.480 --> 00:08:03.100
keep intensifying because more
and more helium is getting

00:08:03.100 --> 00:08:03.880
produced.

00:08:03.880 --> 00:08:07.700
And this core keeps collapsing.

00:08:07.700 --> 00:08:13.330
And the temperature
here keeps going up.

00:08:13.330 --> 00:08:15.670
So we said that the first
ignition, the first fusion,

00:08:15.670 --> 00:08:20.040
occurs at around
10 million Kelvin.

00:08:20.040 --> 00:08:22.230
This thing will keep
heating up until it

00:08:22.230 --> 00:08:24.466
gets to 100 million Kelvin.

00:08:24.466 --> 00:08:26.090
And now I'm talking
about a star that's

00:08:26.090 --> 00:08:27.700
about as massive as the sun.

00:08:27.700 --> 00:08:29.830
Some stars will never
even be massive enough

00:08:29.830 --> 00:08:33.210
to condense the core so
that its temperature reaches

00:08:33.210 --> 00:08:33.919
100 million.

00:08:33.919 --> 00:08:36.539
But let's just talk about
the case in which it does.

00:08:36.539 --> 00:08:38.539
So eventually, you'll
get to a point--

00:08:38.539 --> 00:08:40.870
so we're still sitting
in the red giant phase,

00:08:40.870 --> 00:08:45.400
so we're this huge
star over here.

00:08:45.400 --> 00:08:46.850
We have this helium core.

00:08:46.850 --> 00:08:48.360
And that helium
core keeps getting

00:08:48.360 --> 00:08:50.580
condensed and condensed
and condensed.

00:08:50.580 --> 00:08:53.790
And then we have a
shell of hydrogen

00:08:53.790 --> 00:08:56.800
that keeps fusing
into helium around it.

00:08:56.800 --> 00:08:59.050
So this is our hydrogen shell.

00:08:59.050 --> 00:09:02.740
Hydrogen fusion is occurring
in this yellow shell over here

00:09:02.740 --> 00:09:04.390
that's expelling,
that's allowed--

00:09:04.390 --> 00:09:09.990
that's causing the radius of the
star to get bigger and bigger,

00:09:09.990 --> 00:09:11.280
to expand.

00:09:11.280 --> 00:09:13.607
But when the temperature
get sufficiently hot--

00:09:13.607 --> 00:09:15.690
and now I think you're
going to get a sense of how

00:09:15.690 --> 00:09:18.569
heavier and heavier elements
form in the universe, and all

00:09:18.569 --> 00:09:20.860
of the heavy elements that
you see around us, including

00:09:20.860 --> 00:09:22.960
the ones that are
in you, were formed

00:09:22.960 --> 00:09:26.190
it this way from,
initially, hydrogen--

00:09:26.190 --> 00:09:32.990
when it gets hot enough at 100
million Kelvin, in this core,

00:09:32.990 --> 00:09:36.550
because of such enormous
pressures, then the helium

00:09:36.550 --> 00:09:38.830
itself will start to fuse.

00:09:38.830 --> 00:09:41.160
So then we're going
to have a core in here

00:09:41.160 --> 00:09:43.860
where the helium itself
will start to fuse.

00:09:47.480 --> 00:09:49.340
And now we're talking
about a situation.

00:09:49.340 --> 00:09:51.245
You have helium, and
you had hydrogen.

00:09:51.245 --> 00:09:53.430
And all sorts of
combinations will form.

00:09:53.430 --> 00:09:55.600
But in general, the
helium is mainly

00:09:55.600 --> 00:09:58.906
going to fuse into
carbon and oxygen.

00:09:58.906 --> 00:10:00.280
And it'll form
into other things.

00:10:00.280 --> 00:10:01.821
And it becomes much
more complicated.

00:10:01.821 --> 00:10:04.040
But I don't want to go
into all of the details.

00:10:04.040 --> 00:10:06.182
But let me just show
you a periodic table.

00:10:06.182 --> 00:10:07.640
I didn't have this
in the last one.

00:10:07.640 --> 00:10:09.030
I had somehow lost it.

00:10:09.030 --> 00:10:11.510
But we see hydrogen
here has one proton.

00:10:11.510 --> 00:10:13.060
It actually has no neutrons.

00:10:13.060 --> 00:10:15.230
It was getting fused
in the main sequence

00:10:15.230 --> 00:10:18.500
into helium, two
protons, two neutrons.

00:10:18.500 --> 00:10:20.740
You need four of these
to get one of those.

00:10:20.740 --> 00:10:23.110
Because this actually
has an atomic mass of 4

00:10:23.110 --> 00:10:24.650
if we're talking about helium-4.

00:10:24.650 --> 00:10:27.430
And then the helium, once we
get to 100 million Kelvin,

00:10:27.430 --> 00:10:28.900
can start being fused.

00:10:28.900 --> 00:10:30.590
If you get roughly
three of them--

00:10:30.590 --> 00:10:32.339
and there's all of
these other things that

00:10:32.339 --> 00:10:33.930
are coming and leaving
the reactions--

00:10:33.930 --> 00:10:35.080
you can get to a carbon.

00:10:35.080 --> 00:10:37.277
You get four of
them, four of them

00:10:37.277 --> 00:10:38.860
at least as the
starting raw material.

00:10:38.860 --> 00:10:39.930
You get to an oxygen.

00:10:39.930 --> 00:10:43.130
So we're starting to fuse
heavier and heavier elements.

00:10:43.130 --> 00:10:46.190
So what happens
here is this helium

00:10:46.190 --> 00:10:48.690
is fusing into
carbon and oxygen.

00:10:48.690 --> 00:10:51.697
So you start building a
carbon and oxygen core.

00:10:51.697 --> 00:10:53.030
So I'm going to leave you there.

00:10:53.030 --> 00:10:56.520
I realize I'm already past
my self-imposed limit of 10

00:10:56.520 --> 00:10:57.094
minutes.

00:10:57.094 --> 00:10:58.510
But what I want
you to think about

00:10:58.510 --> 00:10:59.840
is what is likely to happen.

00:10:59.840 --> 00:11:02.730
What is likely to happen
here if this star will never

00:11:02.730 --> 00:11:06.660
have the mass to begin to
fuse this carbon and oxygen?

00:11:06.660 --> 00:11:10.180
If it does have the mass, if
it is a super massive star,

00:11:10.180 --> 00:11:12.530
it eventually will
be able to raise even

00:11:12.530 --> 00:11:16.390
this carbon and oxygen
core to 600 million Kelvin

00:11:16.390 --> 00:11:18.824
and begin to fuse that
into even heavier elements.

00:11:18.824 --> 00:11:20.240
But let's think
about what's going

00:11:20.240 --> 00:11:22.230
to happen for something like
the sun, where it'll never

00:11:22.230 --> 00:11:24.330
have the mass, it'll
never have the pressure,

00:11:24.330 --> 00:11:26.390
to start to fuse
carbon and oxygen.

00:11:26.390 --> 00:11:29.650
And that'll be the
topic of the next video.


Birth of stars

WEBVTT
Kind: captions
Language: en

00:00:00.520 --> 00:00:02.650
Let's imagine we
have a huge cloud

00:00:02.650 --> 00:00:05.470
of hydrogen atoms
floating in space.

00:00:05.470 --> 00:00:08.240
Huge, and when I say huge
cloud, huge both in distance

00:00:08.240 --> 00:00:09.360
and in mass.

00:00:09.360 --> 00:00:11.720
If you were to combine
all of the hydrogen atoms,

00:00:11.720 --> 00:00:15.000
it would just be this
really, really massive thing.

00:00:15.000 --> 00:00:16.830
So you have this huge cloud.

00:00:16.830 --> 00:00:20.510
Well, we know that gravity
would make the atoms actually

00:00:20.510 --> 00:00:21.650
attracted to each other.

00:00:21.650 --> 00:00:25.100
It's-- we normally don't think
about the gravity of atoms.

00:00:25.100 --> 00:00:27.620
But it would slowly
affect these atoms.

00:00:27.620 --> 00:00:30.380
And they would slowly
draw close to each other.

00:00:30.380 --> 00:00:31.630
It would slowly condense.

00:00:31.630 --> 00:00:34.640
They'd slowly move
towards the center

00:00:34.640 --> 00:00:37.650
of mass of all of the atoms.

00:00:37.650 --> 00:00:39.180
They'd slowly move in.

00:00:39.180 --> 00:00:42.860
So if we fast
forward, this cloud's

00:00:42.860 --> 00:00:44.245
going to get denser and denser.

00:00:46.712 --> 00:00:48.420
And the hydrogen atoms
are going to start

00:00:48.420 --> 00:00:51.300
bumping into each other and
rubbing up against each other

00:00:51.300 --> 00:00:53.280
and interacting with each other.

00:00:53.280 --> 00:00:56.100
And so it's going to get
denser and denser and denser.

00:00:56.100 --> 00:00:58.430
Now remember, it's a huge
mass of hydrogen atoms.

00:00:58.430 --> 00:00:59.760
So the temperature is going up.

00:01:03.400 --> 00:01:05.430
And it'll-- they'll
keep condensing.

00:01:05.430 --> 00:01:07.870
They'll just keep
condensing and condensing

00:01:07.870 --> 00:01:09.980
until something really
interesting happens.

00:01:09.980 --> 00:01:12.677
So let's imagine that they've
gotten really dense here

00:01:12.677 --> 00:01:13.260
in the center.

00:01:16.160 --> 00:01:19.550
And there's a bunch of
hydrogen atoms all over.

00:01:19.550 --> 00:01:20.310
It's really dense.

00:01:20.310 --> 00:01:22.600
I could never draw the
actual number of atoms here.

00:01:22.600 --> 00:01:24.058
This is really to
give you an idea.

00:01:24.058 --> 00:01:28.026
There's a huge amount of
inward pressure from gravity,

00:01:28.026 --> 00:01:29.400
from everything
that wants to get

00:01:29.400 --> 00:01:32.540
to that center of mass
of our entire cloud.

00:01:32.540 --> 00:01:38.900
The temperature here is
approaching 10 million Kelvin.

00:01:38.900 --> 00:01:40.780
And at that point,
something neat happens.

00:01:40.780 --> 00:01:43.680
And to kind of realize the
neat thing that's happening,

00:01:43.680 --> 00:01:46.390
let's remember what a
hydrogen atom looks like.

00:01:46.390 --> 00:01:48.010
A hydrogen-- and
even more, I'm just

00:01:48.010 --> 00:01:49.390
going to focus on
the hydrogen nucleus.

00:01:49.390 --> 00:01:50.900
So the hydrogen
nucleus is a proton.

00:01:50.900 --> 00:01:52.691
If you want to think
about a hydrogen atom,

00:01:52.691 --> 00:01:57.200
it also has an electron orbiting
around or floating around.

00:01:57.200 --> 00:01:59.930
And let's draw another
hydrogen atom over here.

00:01:59.930 --> 00:02:02.050
And obviously this
distance isn't to scale.

00:02:02.050 --> 00:02:04.127
This distance is
also not to scale.

00:02:04.127 --> 00:02:05.835
Atoms are actually--
the nucleus of atoms

00:02:05.835 --> 00:02:07.980
are actually much,
much, much, much smaller

00:02:07.980 --> 00:02:10.199
than the actual
radius of an atom.

00:02:10.199 --> 00:02:11.290
And so is the electron.

00:02:11.290 --> 00:02:13.222
But anyway, this just
gives you an idea.

00:02:13.222 --> 00:02:14.680
So we know from
the Coulomb forces,

00:02:14.680 --> 00:02:17.480
from electromagnetic forces,
that these two positively

00:02:17.480 --> 00:02:20.970
charged nucleuses will
not want to get anywhere

00:02:20.970 --> 00:02:22.300
near each other.

00:02:22.300 --> 00:02:25.520
But we do know from our-- from
what we learned about the four

00:02:25.520 --> 00:02:28.260
forces-- that if they did get
close enough to each other,

00:02:28.260 --> 00:02:31.540
that if they did get-- if
somehow under huge temperatures

00:02:31.540 --> 00:02:35.300
and huge pressures you were able
to get these two protons close

00:02:35.300 --> 00:02:37.210
enough to each other,
then all of a sudden,

00:02:37.210 --> 00:02:39.130
the strong force will overtake.

00:02:39.130 --> 00:02:41.790
It's much stronger
than the Coulomb force.

00:02:41.790 --> 00:02:43.800
And then these two
hydrogens will actually--

00:02:43.800 --> 00:02:48.040
these nucleuses would actually
fuse-- or is it nuclei?

00:02:48.040 --> 00:02:50.750
Well, anyway, they would
actually fuse together.

00:02:50.750 --> 00:02:53.570
And so that is what
actually happens

00:02:53.570 --> 00:02:57.360
once this gets hot
and dense enough.

00:02:57.360 --> 00:02:59.830
You now have enough pressure
and enough temperature

00:02:59.830 --> 00:03:02.620
to overcome the
Coulomb force and bring

00:03:02.620 --> 00:03:05.480
these protons close enough
to each other for fusion

00:03:05.480 --> 00:03:12.186
to occur, for fusion ignition.

00:03:12.186 --> 00:03:14.310
And the reason why-- and
I want to be very careful.

00:03:14.310 --> 00:03:15.020
It's not ignition.

00:03:15.020 --> 00:03:16.894
It's not combustion in
the traditional sense.

00:03:16.894 --> 00:03:20.910
It's not like you're
burning a carbon molecule

00:03:20.910 --> 00:03:22.390
in the presence of oxygen.

00:03:22.390 --> 00:03:23.460
It's not combustion.

00:03:23.460 --> 00:03:24.085
It's ignition.

00:03:24.085 --> 00:03:25.710
And the reason why
it's called ignition

00:03:25.710 --> 00:03:27.880
is because when two
of these protons,

00:03:27.880 --> 00:03:32.760
or two of the nucleuses
fuse, the resulting nucleus

00:03:32.760 --> 00:03:35.064
has a slightly smaller mass.

00:03:35.064 --> 00:03:36.480
And so in the first
stage of this,

00:03:36.480 --> 00:03:42.120
you actually have two protons
under enough pressure--

00:03:42.120 --> 00:03:44.990
obviously, this would not happen
with just the Coulomb forces--

00:03:44.990 --> 00:03:46.781
with enough pressure
they get close enough.

00:03:46.781 --> 00:03:51.150
And then the strong interaction
actually keeps them together.

00:03:51.150 --> 00:03:56.830
One of these guys
degrades into a neutron.

00:03:56.830 --> 00:03:59.460
And the resulting mass
of the combined protons

00:03:59.460 --> 00:04:01.860
is lower than the mass
of each of the original.

00:04:01.860 --> 00:04:04.520
By a little bit, but
that little bit of mass

00:04:04.520 --> 00:04:11.270
results in a lot of
energy-- plus energy.

00:04:11.270 --> 00:04:13.960
And this energy is why
we call it ignition.

00:04:13.960 --> 00:04:16.019
And so what this energy
does is it provides

00:04:16.019 --> 00:04:17.839
a little bit of
outward pressure,

00:04:17.839 --> 00:04:21.420
so that this thing
doesn't keep collapsing.

00:04:21.420 --> 00:04:24.130
So once you get pressure
enough, the fusion occurs.

00:04:24.130 --> 00:04:26.260
And then that energy
provides outward pressure

00:04:26.260 --> 00:04:30.880
to balance what is now a star.

00:04:30.880 --> 00:04:32.660
So now we are at
where we actually

00:04:32.660 --> 00:04:35.800
have the ignition at the center.

00:04:35.800 --> 00:04:39.050
We have-- and we still have all
of the other molecules trying

00:04:39.050 --> 00:04:43.520
to get in providing the pressure
for this fusion ignition.

00:04:43.520 --> 00:04:46.300
Now, what is the hydrogen
being fused into?

00:04:46.300 --> 00:04:50.580
Well, in the first step of
the reaction-- and I'm just

00:04:50.580 --> 00:04:54.640
kind of doing the most basic
type of fusion that happens

00:04:54.640 --> 00:05:00.020
in stars-- the hydrogen
gets fused into deuterium.

00:05:00.020 --> 00:05:01.442
I have trouble spelling.

00:05:01.442 --> 00:05:03.400
Which is another way of
calling heavy hydrogen.

00:05:03.400 --> 00:05:04.775
This is still
hydrogen because it

00:05:04.775 --> 00:05:07.200
has one proton and
one neutron now.

00:05:07.200 --> 00:05:08.100
It is not helium yet.

00:05:08.100 --> 00:05:12.290
This does not have two-- it
does not have two protons.

00:05:12.290 --> 00:05:14.520
But then the deuterium
keeps fusing.

00:05:14.520 --> 00:05:19.010
And then we eventually
end up with helium.

00:05:19.010 --> 00:05:21.100
And we can even see that
on the periodic table.

00:05:21.100 --> 00:05:22.480
Oh, I lost my periodic table.

00:05:22.480 --> 00:05:23.938
Well, I'll show
you the next video.

00:05:23.938 --> 00:05:30.040
But we know hydrogen
in its atomic state

00:05:30.040 --> 00:05:32.320
has an atomic number of 1.

00:05:32.320 --> 00:05:34.560
And it also has a mass of 1.

00:05:34.560 --> 00:05:38.000
It only has one
nucleon in its nucleus.

00:05:38.000 --> 00:05:39.080
But it's being fused.

00:05:39.080 --> 00:05:42.210
It goes to hydrogen-2,
which is deuterium,

00:05:42.210 --> 00:05:46.590
which is one neutron, one proton
in its nucleus, two nucleons.

00:05:46.590 --> 00:05:48.350
And then that
eventually gets fused--

00:05:48.350 --> 00:05:50.475
and I'm not going into the
detail of the reaction--

00:05:50.475 --> 00:05:51.320
into helium.

00:05:51.320 --> 00:05:56.040
And by definition, helium has
two protons and two neutrons.

00:05:56.040 --> 00:05:58.280
So it has-- or we're
talking about helium-4,

00:05:58.280 --> 00:06:00.590
in particular, that
isotope of helium--

00:06:00.590 --> 00:06:03.350
it has an atomic mass of 4.

00:06:03.350 --> 00:06:06.420
And this process
releases a ton of energy.

00:06:06.420 --> 00:06:09.740
Because the atomic mass of
the helium that gets produced

00:06:09.740 --> 00:06:15.220
is slightly lower than
four times the atomic mass

00:06:15.220 --> 00:06:17.440
of each of the
constituent hydrogens.

00:06:17.440 --> 00:06:21.740
So all of this energy, all this
energy from the fusion-- but it

00:06:21.740 --> 00:06:24.900
needs super high pressure, super
high temperatures to happen--

00:06:24.900 --> 00:06:28.820
keeps the star from collapsing.

00:06:28.820 --> 00:06:31.510
And once a star
is in this stage,

00:06:31.510 --> 00:06:35.346
once it is using hydrogen--
it is fusing hydrogen

00:06:35.346 --> 00:06:37.470
in its core, where the
pressure and the temperature

00:06:37.470 --> 00:06:41.350
is the most, to form helium--
it is now in its main sequence.

00:06:41.350 --> 00:06:42.870
This is now a main
sequence star.

00:06:48.130 --> 00:06:50.770
And that's actually where
the sun is right now.

00:06:50.770 --> 00:06:53.580
Now there's questions of,
well, what if there just

00:06:53.580 --> 00:06:56.304
wasn't enough mass to get
to this level over here?

00:06:56.304 --> 00:06:57.720
And there actually
are things that

00:06:57.720 --> 00:06:59.630
never get to quite
that threshold

00:06:59.630 --> 00:07:00.985
to fuse all the way into helium.

00:07:00.985 --> 00:07:02.610
There are a few things
that don't quite

00:07:02.610 --> 00:07:05.090
make the threshold of stars
that only fuse to this level.

00:07:05.090 --> 00:07:06.839
So they are generating
some of their heat.

00:07:06.839 --> 00:07:08.580
Or there are even
smaller objects

00:07:08.580 --> 00:07:10.280
that just get to
the point there's

00:07:10.280 --> 00:07:13.600
a huge temperature and pressure,
but fusion is not actually

00:07:13.600 --> 00:07:15.042
occurring inside of the core.

00:07:15.042 --> 00:07:17.000
And something like Jupiter
would be an example.

00:07:17.000 --> 00:07:20.074
And you could go several
masses above Jupiter

00:07:20.074 --> 00:07:21.490
where you get
something like that.

00:07:21.490 --> 00:07:23.156
So you have to reach
a certain threshold

00:07:23.156 --> 00:07:25.470
where the mass, where the
pressure and the temperature

00:07:25.470 --> 00:07:27.470
due to the heavy mass,
get so large that you

00:07:27.470 --> 00:07:28.770
start this fusion.

00:07:28.770 --> 00:07:31.960
And-- but the smaller you
are above that threshold,

00:07:31.960 --> 00:07:33.830
the slower the
fusion will occur.

00:07:33.830 --> 00:07:35.570
But if you're super
massive, the fusion

00:07:35.570 --> 00:07:37.680
will occur really, really fast.

00:07:37.680 --> 00:07:40.700
So that's the general idea
of just how stars get formed

00:07:40.700 --> 00:07:43.370
and why they don't
collapse on themselves

00:07:43.370 --> 00:07:45.640
and why they are
these kind of balls

00:07:45.640 --> 00:07:48.954
of fusion reactions
existing in the universe.

00:07:48.954 --> 00:07:50.370
In the next few
videos, we'll talk

00:07:50.370 --> 00:07:54.120
about what happens once that
hydrogen fuel in the core

00:07:54.120 --> 00:07:56.878
starts to run out.