Welcome back to Launch Pad, I'm Christian
Ready, your friendly neighborhood
astronomer, and in today's video, we're
going to investigate neutron stars, which
are among the strangest and most exotic
stars in the universe.
In fact, neutron stars really aren't
proper stars at all, but instead are the
stellar mutant power zombies that began
their lives in the death of massive
stars. As we saw in our previous video on
how high mass stars evolve and die, the
cores of these stars eventually become
iron. Since iron can't fuse, it cannot
produce the energy needed to hold itself
up. For a while it hangs in there, but the
pressure is so strong it's squeezed into
electron degeneracy. But the core keeps
gaining mass, and when it reaches 1.4
solar masses, the electron
degeneracy fails and the core implodes.
The core collapses from the size of Mars
to the size of Manhattan in just a few
milliseconds. But during those few
milliseconds, protons and electrons are
squeezed into each other to become
neutrons and neutrinos. The neutrinos
escape, but the neutrons are squeezed
together so tightly they exert an even
more powerful neutron degeneracy
pressure. The collapse comes to a ringing
halt at a radius of just a few
kilometers. The layers surrounding the
core crash down a few milliseconds
later and rebound in a titanic supernova
explosion. The core is now an exposed
neutron star. Because they're both
massive and tiny, neutron stars are
unimaginably dense; we're talking about
something that's between 1.2 and 2.8
times the mass of the Sun squeezed down
to the size of a city! That may seem
impossible, but really isn't. If an atom
were enlarged to the size of a football
stadium, its entire nucleus would only be
the size of a marble and its electrons
would be tiny dust grains whizzing
around way up in the upper decks. Now if
that seems like a lot of empty space to
you, well that's because atoms are in
fact mostly empty space! But in a neutron
star, every bit of that empty space would
be filled with neutrons. This is what
makes neutron stars so mind-bogglingly
dense. A
teaspoon of neutronium - the name for
neutron star material - would weigh five
and a half trillion kilograms. That's
like squeezing Mount Everest into a
small lump and dropping it into your tea
cup. Although, it would crash right
through the bottom of the tea cup, then
through the table, then through the floor, then
through the Earth, all the way through
the core, out the other end, and then it
would come back and forth through the
planet for about a billion years turning
the whole Earth into Swiss cheese. The
insane density of neutron stars creates
an equally insane gravitational pull; a
typical neutron star has a surface
gravity that's a hundred billion times
stronger than Earth! But forget even
thinking about standing on one of these
things to weigh yourself; you'd be
instantly flattened into a thin film of
quantum goo. In fact, if you dropped a
golf ball onto a neutron star's surface
from a height of just one meter, it would
hit the ground half a millionth of a
second later. In that short time, tidal
forces on the star would have shredded
the golf ball into a string of atoms.
Those atoms would then hit the surface
at a substantial fraction of the speed
of light, releasing more energy than all
of the thermonuclear bombs and the world
combined. That extreme gravity gives the
star some truly weird and exotic
properties. Understanding the conditions
inside the neutron stars are difficult
because it pushes the limits of our
understanding of quantum mechanics and
gravity. But even what we do know about
neutron stars is pretty mind-flattening.
For example, neutron stars are the most
perfectly smooth objects in the universe.
The tallest mountain on the star's surface
is only a few millimeters high. The crust
is likely a crystalline lattice
structure of iron and maybe some helium
nuclei that weren't destroyed when the core
imploded. It's effectively a solid crust
with a temperature of around a million
Kelvin. Underneath is a mantle of almost
pure degenerate neutrons. The conditions
in the core are the least well
understood.
Inside, pressures are 10 to the 16th
times greater than in the core of the
Sun. Some models suggest that under these
conditions, the neutrons are squeezed so
tightly together they cease being
neutrons and dissolve into an exotic
quark superfluid.
But one thing we do
for sure about neutron stars is that
they rotate, and they rotate fast. The
star was already rotating before it
exploded, but as the core collapsed, it
spun up much faster, like a figure
skater pulling in her arms to increase
her rotation. But when more than a Sun's
worth a mass shrinks down to just 20
kilometers across, the rotation speeds up
by a ridiculous amount. A newly-formed
neutron star can spin up to a hundred
times a second! The fast rotation sets up
insanely strong magnetic fields. How
strong? Well,
Earth's magnetic field is somewhere
around 30 micro Tesla that's 30
millionths of a Tesla...not the car, the
actual unit of magnetism, but hey, if you
want to ship me a Tesla, that's
okay too. The Sun's magnetic field is a
lot stronger, around 0.001 Tesla. Sunspots,
which are very strong magnetic
concentrations on the Sun, reach 0.3
Tesla. MRI machines generate fields of
around 3 Tesla, and the most powerful
magnets inside the Large Hadron Collider
reach 8 Tesla. But a typical neutron star
generates up to 100 million Tesla! In
other words, a neutron star at the Moon's
distance would erase your credit cards.
And it would pay them off as well
because all of the hard drives on Earth
would also be erased. It'd be the end of
civilization as we know it, but you'd be
debt-free.
The magnetic field lines are so strong
they become particle accelerators and
launch beams of radiation along their
magnetic poles. Typically, these poles are
tilted with respect to the star's
rotation, turning the star into a cosmic
lighthouse. If the beam sweeps along our
line of sight, we can detect them as
repeating pulses of radiation. We call
these types of neutron stars "pulsars". The
first pulsar was discovered in 1967 by
Jocelyn Bell who was then a graduate
student at Cambridge
She kept detecting a strange repeating
radio signal/
After ruling out any problems with the
radio telescope she was using, Bell
realize that the signal was coming from
space! At first, nobody knew what was
causing the pulses; the signal was so
perfectly timed they half-jokingly
designated the source as LGM-1,
for "Little Green Men One". Pulsars lie at
the hearts of most supernova remnants.
Their magnetic fields churn the
surrounding gas like a cosmic egg beater, and
the radiation ionizes the surrounding
gases. Pulsars are extremely stable
rotators, so their pulses act as ultra-
precise clocks. In fact, some pulsars are
as accurate as atomic clocks. Each pulsar
has its own rotation period, so for a
while they thought they could be used to
determine your exact location in the
Galaxy. As a matter of fact, the covers of
the golden records affixed to the
Voyager 1 and Voyager 2 spacecraft
include a pictogram of some known
pulsars surrounding Earth. The tick marks
indicate the pulsation periods and the
lengths represent the relative distances.
The idea was that if an alien species
could identify the depicted pulsars, they
could reverse-engineer Earth's location
in the Galaxy. At least, that was the
thinking back in the 1970s. We've since
discovered thousands more pulsars
throughout the galaxy, and that makes
identifying the ones used on the Voyager
records a lot more difficult. But we've
also learned that pulsars can wobble or
precess, and that changes their
orientations over time. So some of the
pulsars that were once aimed at Earth
when the Voyagers were launched aren't
aimed at us anymore. At least not as we
see them, so we don't think of them as
pulsars, we just see them as ordinary
neutron stars. Still, it's interesting to
think that perhaps with enough
information about all the known pulsars,
we might one day be able to use them as
a kind of Galactic Positioning System. We
can even call it GPS for short. I should
patent that. A pulsar's rotation is very
stable on the timescales of years to
centuries, but their magnetic fields drag
against the surrounding remains of the
supernova, so they slow down over
time. For example, the Crab Pulsar is
roughly
1000 years old and rotates about 33
times a second.
[audio signal of Crab Pulsar playing]
but the Vela Pulsar is about 10,000
years old and it rotates about
11 times a second. left to its own
devices a pulsar should slow down over
[audio of Vela Pulsar]
Left to its own devices, a pulsar should
slow down over time.
But if the Pulsar is in a binary
star system, it can actually speed up! As
the companion star evolves, it expands
into a red giant. If the two stars are
close enough together, matter eventually
flows toward the pulsar. The pulsar
accretes this material and gains mass.
But in order to conserve angular
momentum, it must speed up in response.
These pulsars can spin so fast, their
rotation periods are measured in
milliseconds! The closest millisecond
pulsar is PSR J0437-4715. 
Its name comes from its coordinates on
the sky. This pulsar completes one
rotation in just 5.75 milliseconds!
That's 174 rotations a second! But that
is nothing compared to the record holder
PSR J1748-2446ad, located
in the globular cluster Terzan 5.
It's approximately 18,000
light-years from Earth in the
constellation Sagittarius, and is home to
about 30 additional pulsars. But this
pulsar clocks in at a ridiculous 1.4
milliseconds! That's 716 rotations a
second. That's 43,000 rotations per
minute! The pulsar is moving so fast that
surface is actually spinning at 15% the
speed of light. Now this pulsar is too
far away so it's signal is too weak to
be directly converted into sound using
our current radio telescopes. But it
should sound similar to another pulsar,
PSR B1937+21, which rotates 10%
slower at 642 times a second. 
that's annoying the magnetic fields of
[high-pitched audio of pulsar]
That's annoying. The magnetic fields of
millisecond pulsars are super-powerful,
but some neutron stars generate magnetic
fields that can be up to a million times
stronger than a typical neutron star, and
a quadrillion times stronger than the
Sun. We call these beasts "magnetars".
Magnetars are so powerful their
magnetic fields actually stretch atoms
into long cylinders. At 10 billion
Tesla's, a hydrogen atom becomes a
spindle 200 times thinner
than its normal diameter. Don't even
think about getting anywhere near one of
these things. Even if you could somehow
survive the onslaught of x-ray radiation
from the magnetar, its magnetic field
with rip you apart at 1000
kilometers. Oh yeah, and that stuff
about the neutron star erasing the hard
drives on Earth from the Moon's distance?
Well a magnetar could do the same job
from Pluto. Magnetars are thought to be
rare - only one in ten supernovae ever
produce them - and they probably don't
last very long either. It's thought that
their magnetic fields drag on the
surrounding interstellar medium so hard,
the star would slow to normal pulsar
speeds after just a few years. But while
they're still active, they are the most
powerful magnets in the Universe. And
when things go wrong on a pulsar, all
hell literally breaks loose.
When our Sun becomes active, magnetic
field lines connect and short out,
unleashing the equivalent of a million
hydrogen bombs worth of energy in just a
few minute. But magnetars are on a
whole other level; their magnetic fields
are bound to their spinning crusts, so a
change in one automatically results in
a change in the other. Magnetars
rotate at a significant fraction of
the speed of light and the crust is
fighting ultra-strong centrifugal forces.
That puts the crust under incredible
stress until it finally adjusts itself
to alter its shape. That adjustment
creates a tiny crack in less than a
millionth of a second. Even though the
crack is less than a micron in size, a
titanic amount of energy is released. The
event is called a "starquake" and it's
one of the most violent episodes this
side of a supernova. The largest star-
quake ever recorded was on December 27
2004. It came from the magnetar SGR 1860-20.
The energy released in the
starquake would have been equivalent to
a magnitude 32 quake here on
Earth. The quake occurred 50,0000
light-years from Earth, yet it compressed
Earth's magnetosphere and partially
ionized its atmosphere. By the way 50,000
light-years? That is halfway
across the Galaxy, man! And yet it had the
same effect on Earth as a typical solar
flare. If that starkquake were at ten
light years, it would have triggered a
mass extinction. Luckily, we don't need to
worry about death by magnetar. We only
know of about 30 of them and there's
probably only three or four dozen in the
entire Galaxy. And starquakes are even
more rare - we've only detected a handful
of them in the last 40 years or so. We've
learned a lot about neutron stars in the
last 50-plus years, but they're still
continuing to surprise us. In December
2019, two independent teams of
astronomers made the first ever map of a
neutron star. Although their results were
similar, neither showed the magnetic
poles emerging from the northern and
southern hemispheres of the neutron star.
Instead, both magnetic poles were mapped
to emerge from the star's southern
hemisphere. Does this mean our
understanding of neutron stars is
fundamentally wrong, or is it merely
incomplete? We'll investigate that in a
future video. But before we get to the
next video, let's talk about the last one -
about what's happening with Betelgeuse.
Many of you pointed out that Betelgeuse
may have already supernova but
because it's so far away, its light
hasn't reached us yet. Others noted that
the unusual dimming we're witnessing now
actually happened about 640 years ago.
Both statements are
absolutely correct.
But since light travels at a finite
speed. we have no way of knowing what the
star was doing until its light gets
here anyway.
If Betelgeuse is 640 light years away, we
are by definition seeing it as it
appeared 640 years ago. Now I suppose I
should have pointed this out in the
video, but
then I'd have to point that out for
everything. Light takes about one second
to get from the Moon to Earth, so we always
see the Moon that's it appeared one
second ago. Likewise, we always see the
Sun as it appeared eight minutes ago.
Even this video is what it looked like a
couple of nanoseconds ago because it
took light that much time to go from the
screen to your eyes.
We're always seeing into the past thanks
to the speed of light, and that leaves us
with a relative concept of "now". I also
got a fair amount of comments about my
pronunciation of the name a "Betelgeuse". I
call it "bay-tal-juice" but some said it
should be pronounced "beetle-juice", and
others were careful to note that it was
in fact a translation, or maybe a bad
translation, from the original Arabic.
However it may be, the pronunciations
range from "beetle-juice" to "bay-tal-keys" to
"battle-ghosts" to "beetle-goose" to
"beetle-jeez". I just split the difference
somewhere and call it "bay-tal-juice",
although I'm probably pronouncing that
wrong as well. Maybe I should just start
pronouncing it as Alpha Orionis instead
and just be done with it. Or is it "alpha-
orion-us" or is it "alpha-ori-own-us" or "alpha-or-e-on"...
oh man the comments... Many
thanks to my Patreon supporters who are
helping to keep this channel going and
I'd like to welcome my newest patron
Glenn, and give another shout-out to my
cosmological sponsor
Steven J Morgan. If you'd like to
help support this channel for the price
of a cup of coffee, head on over to my
Patreon page. And if you'd like to see
what's up with Betelgeuse and how
massive stars evolve and die, I've got
some videos on them both that we can
check out once we're done here.
Until next time, stay curious, my friends.
