There are many different types of stars out
there; some bigger, some smaller than our
own Sun which is technically a yellow dwarf.
However, it’s crucial to know that stars
don’t have nice, tidy boundaries.
They don’t have a rigid surface like a rocky
planet or moons.
Instead, these atomic fireballs have pretty
diffuse surfaces as the super-heated mass
of gas that makes them up slowly thins out
into space voids.
What astronomers use in lieu of a surface
is a star’s photosphere that’s the level
at which the star becomes transparent.
Therefore, a star’s surface means its photosphere.
Another important thing to keep in mind is
that the volume of any star can’t be directly
measured, there are various methodologies
to do that.
Curious to learn more about how we measure
volumes of stars and the biggest stars in
the universe?
Keep Watching!!
Nobody went up to a star with a ruler and
started adding up distances in order to measure
its volume.
What scientists have are estimations — reliable
estimations, for the most part, but estimations
nonetheless.
Depending on a range of factors, such as distance
or structures around stars or between them
and Earth, these estimations can be more or
less accurate, and fall within a smaller or
larger area of confidence.
The question is that “is there any way we
can measure the size of a star more directly?”
After all, real stellar spectra aren't exactly
like those of blackbodies, so these rough
estimates might be wrong.
It turns out that there are several different
methods for measuring the size of a star.
For example, there’re direct imaging, lunar
occultation, eclipsing binaries and interferometry.
If you want to measure the size of a star
via the direct imaging method, just point
your telescope at it and take a picture.
Measure the angular size of the star in the
image, then multiply by the distance to find
the true linear diameter.
What's so hard about that?!
The problem is a phenomenon called diffraction.
If you use a real optical system with an aperture
of diameter D, light rays won't come to a
perfect point at the focus; instead, interference
from rays entering the aperture at different
locations will form a fuzzy blob of light,
surrounded by a series of faint rings forming
a pattern.
This pattern is called the "Airy pattern"
after George Airy, Astronomer Royal of England,
who first derived the angular size of the
central blob and the rings which surround
it.
The important thing is the angular size of
the central blob, measured from its center
to the first minimum in the diffraction pattern.
It helps to work as far into the ultraviolet
as possible, since the decrease in wavelength
shrinks the Airy pattern.
Unfortunately, the Earth's atmosphere prevents
ultraviolet light from reaching the ground.
Astronomers have used telescopes in space
to take near-UV images of a very few stars
in hopes of resolving them.
In at least one case, they have succeeded
-- barely.
As for the lunar occultation method, The basic
idea is simple, first find a star which will
be covered by the Moon as it moves through
the sky, then using a high-speed device, measure
the light from the star as a function of time
and finally, calculate the size of the star
from the light curve.
The problem is that it appears that any lunar
occultation will be a very quick event.
One will need a high-speed photometer or camera,
capable of hundreds of measurements per second
and a big enough telescope to gather enough
photons within each frame to make a decent
measurement.
This requirement of collecting lots of photons
in a very short time is a killer.
The lunar occultation method is therefore
restricted to relatively bright stars.
It's also restricted to stars which happen
to lie near the ecliptic, of course.
But ... it's even worse!
It turns out that diffraction makes life difficult
for astronomers again.
As the Moon's limb begins to pass in front
of the star's disk, it diffracts the light
from the star.
As the Moon's limb approaches and covers a
star, we on the Earth see something like this
pattern of alternating dark and light spots
moving past our detectors.
So the light curves (intensity versus time)
that we record will be somewhat more complicated
than the simple decreasing curve.
Fortunately, the Moon isn't the only "moving
knife edge" we can use to determine the sizes
of stars.
There are many instances in which we can use
one star as the "moving knife edge" to measure
the size of a second star.
All we have to do is find an eclipsing binary
system: a pair of stars orbiting around each
other, oriented in space so that one star
passes in front of the other as seen from
the Earth.
If we measure the light coming from such a
system carefully, we can detect the decrease
in total intensity as a portion of one of
the stars is covered.
Bearing in mind that the diffraction of light
waves means that one would have to build an
impossibly high telescope to be able to resolve
a star like the Sun at a distance of 10 parsecs.
It would take a mirror hundreds of meters
in diameter!
But, in theory at least, it is possible to
combine the light from several small telescopes
separated by a similar distance to achieve
the same resolution.
This technique is called interferometry.
Radio astronomers do it all the time.
The shorter the waves, the harder it is to
combine them properly.
In simple terms, you need to know the distance
between the two telescopes to a precision
smaller than the wavelength of the light you
are combining.
To combine optical light waves properly, you
need to know the distance between the mirrors
to a precision of better than 100 nanometers.
That's hard!
But not impossible.
There are several groups who are currently
using optical arrays to do interferometry.
One of them is located near Flagstaff, Arizona.
Before listing the biggest stars in the universe,
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Here’s the list of The biggest stars in
the universe in a descending order:
1- The largest one spotted in the universe
so far is UY Scuti, a star 9,500 light-years
away, close to the center of the Milky Way
in the constellation Scutum (meaning ‘shield’).
It’s a dust-enshrouded red supergiant (the
largest class of stars out there) that’s
around 1,700 times larger than our Sun in
diameter.
It was first spotted in 1860 by astronomers
at the Bonn Observatory (Germany), who christened
it BD -12 5055.
Subsequent observations showed that BD -12
5055 grows brighter and dimmer over a 740-day
period, so it was classified as a variable
star.
Variable stars regularly expand and shrink
as their brightness changes.
Hypergiants are larger than supergiants, which
themselves are larger than giant stars.
Hypergiants are quite rare and shine brightly.
They also lose more mass than smaller stars
through stellar winds.
To give you an idea of just how huge UY Scuti
is, if it replaced the Sun at the center of
our solar system, its photosphere would extend
past the orbit of Jupiter.
The distance from the Sun to Jupiter is approximately
779 million km, or 484 million miles.
Gas emanating from the star would form a nebula
extending 400 astronomical units.
In effect, this would reach far beyond the
orbit of Pluto, the average orbiting distance
between Pluto and the Sun is 39.5 AU.
2- WOH G64 (1,504 to 1,730 solar radii) — a
red hypergiant star in the Large Magellanic
Cloud in the constellation Dorado (in the
southern hemisphere skies) located about 170,000
light-years away from Earth.
This star’s brightness varies over time
due, in part, to a torus-shaped cloud of dust
that obscures its light.
The torus was likely formed by the star during
its death throes.
WOH G64 was once more than 25 times the mass
of the Sun, but it began to lose mass as it
neared exploding as a supernova.
Astronomers estimate that it has lost enough
component material to make up between three
and nine solar systems.
3- Mu Cephei (around 1,650 solar radii) — a
red supergiant in the constellation Cepheus,
9,000 light-years from Earth.
With more than 38,000 times the Sun’s ​luminosity,
it’s also one of the brightest stars in
the Milky Way.
It appears garnet red and is located at the
edge of the IC 1396 nebula.
Since 1943, the spectrum of this star has
served as the M2 Ia standard by which other
stars are classified.
Mu Cephei is nearing death.
It has begun to fuse helium into carbon, whereas
a main sequence star fuses hydrogen into helium.
When a supergiant star has converted elements
in its core to iron, the core collapses to
produce a supernova and the star is destroyed,
leaving behind a vast gaseous cloud and a
small, dense remnant.
For a star as massive as Mu Cephei the remnant
is likely to be a black hole.
4- V354 Cephei (1,520 solar radii) — a red
hypergiant in the constellation Cepheus.
V354 Cephei is an irregularly variable star,
which means that it pulsates on an erratic
schedule.
It was referred to only by its listings on
relatively obscure catalogs.
It is too faint to be included in catalogs
such as the Henry Draper Catalogue or Bonner
Durchmusterung.
It was included on a 1947 Dearborn Observatory
survey as star 41575, but that ID is hardly
ever used.
5- RW Cephei (1,535 solar radii) — an orange
hypergiant in the constellation of Cepheus;
RW Cephei is also a semi-regular variable
star of type SRd, meaning that it is a slowly
varying yellow giant or supergiant.
The visual magnitude range is from 6.0 to
7.3.
6- Westerlund 1-26 (1,530 to 2,550 solar radii).
That’s quite a large estimate interval;
if the upper estimate is correct, it would
dwarf even UY Scuti, and its photosphere would
reach farther than Saturn’s orbit.
Westerlund 1-26 stands out as its temperature
varies over time, but not its brightness.
7- KY Cygni (1,420 to 2,850 solar radii) — a
red supergiant in the constellation Cygnus.
The upper estimate is viewed with skepticism
as a likely observational error, while the
lower one is consistent with other stars from
the same survey and with our understanding
of stellar evolution.
8- VY Canis Majoris (1,300 to 1,540 solar
radii) — a red hypergiant star that was
previously estimated to be 1,800 to 2,200
solar radii, but that size put it outside
the bounds of stellar evolutionary theory
and were updated.
A hypothetical object travelling at the speed
of light would take 6 hours to travel around
the star's circumference, compared to 14.5
seconds for the Sun.
If placed at the center of the Solar System,
VY CMa's surface would extend beyond the orbit
of Jupiter, although there is still considerable
variation in estimates of the radius, with
some making it larger than the orbit of Saturn.
9- Betelgeuse (950 to 1,200 solar radii) — a
red supergiant in the constellation Orion.
Betelgeuse is one of the most well-known stars
of its kind, as it’s the ninth-brightest
star in the sky and can easily be seen with
the naked eye between October through March
on a clear night.
It’s the closest star to Earth on this list
and is expected to go supernova pretty much
at any time.
Thanks For Watching Everyone!!
Do you know any other observed hypergiant
stars?
Have you understood the methods by which we
can measure the size of different stars?
Have you learned anything new from this video?
Let me know in the comments below, be sure
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on the channel!
