[MUSIC]
Hi everyone.
I'm Fira from the Lawrence Hall of Science.
In our last few Astrophysics Friday's videos,
we've been talking about the lives of stars.
And today, I'm wondering
what happens to stars after they die?
Well, stars don't really "die,"
they just become a new type of object.
The type of object that a star becomes
after they're no longer star,
generally depends on their initial mass.
Lower mass stars will eventually become white dwarfs,
while higher mass stars will eventually become
neutron stars or black holes.
We generally call these objects, stellar remnants.
And in today's video, let's explore
each type of stellar remnants.
The first type of stellar remnant is a white dwarf.
This is the object our sun will become someday.
There are many differences between what the sun
is currently like as a star
and what it'll be like when it's a white dwarf.
First of all, it'll be much smaller as a
white dwarf —
only about the size of the Earth.
It'll also be supported by a different
type of pressure.
Remember stars always want to be
in hydrostatic equilibrium,
balancing the inward gravity with outward pressure.
A star like the sun is supported by gas pressure,
which is basically pressure that's created by lots of gas particles smashing into each other.
Another type of pressure called radiation pressure
also contributes to the hydrostatic
equilibrium of stars.
Radiation pressure is basically the
pressure that's created by photons,
which you may already know as particles of light.
Radiation pressure only contributes a little bit
to the hydrostatic equilibrium of
Sun-like stars.
But for very massive stars,
radiation pressure is the dominant support against gravity.
White dwarfs are supported by a type of pressure called
electron degeneracy pressure.
Electrons can exist in different energy levels,
which we can think of sort of like floors in a building.
The ground floor represents the lowest energy state
where electrons move the slowest
The higher up we go in the building,
the more energy the electrons have.
So the faster they move.
Electrons don't like to be squeezed together too much.
They're kind of claustrophobic.
So as a star, like the Sun,
collapses,
electrons are being forced
into the same low energy state
But they fight for their personal space by moving
to higher states as each floor gets crowded.
The electrons with the highest energy levels
move around so fast
that their movements actually create pressure.
Electron degeneracy pressure can only support
a white dwarf, up to a certain mass.
The maximum mass that a white dwarf can have
in order to stay stable
is 1.4 solar masses.
This is the Chandrasekhar limit
named after the astrophysicist
Subrahmanyan Chandrasekhar.
Over the course of his career,
Chandrasekhar made many important
contributions to astrophysics.
He won the 1983 Nobel Prize
in physics for his work on stellar evolution.
Once the Sun is a white dwarf,
it'll continue to be supported by electron
degeneracy pressure
in shining dimly for trillions of years.
But not all white dwarfs have such a quiet retirement.
A lot of low mass stars are in binary systems
or multiple star systems,
often with higher mass stars.
This just means that two or more stars are basically orbiting each other.
Interactions between a white dwarf
and a larger star or even with another white dwarf
can lead to something called a Type 1a Supernova.
Type 1a supernovae
are beautiful explosions
usually resulting from a white dwarf
gathering material from a large star like a Red Giant.
We call this accretion.
And the disk of gathered material formed
around the white dwarf is called
an accretion disk.
There are some links in the description below
for you to learn more about Type 1a supernovae
and white dwarfs in general.
But now, let's learn about
another type of stellar remnant:
neutron stars!
When an initially 8 to 25 solar masses star
that has become a super giant star goes supernova,
its core collapses.
A neutron star is what's left behind if the collapse stops
once the core reaches the density of an atomic nuclei.
Know that density equals mass divided by volume.
So if something has a high density,
that means it has a lot of mass in a
small volume.
Neutron stars are so dense
that a teaspoon of neutron star material
would weigh about 10 million tons!
That's about a hundred thousand blue
whales!
Neutron stars are in hydrostatic
equilibrium.
The outward pressure battling the inward
gravity for neutron stars
is called neutron degeneracy pressure.
This phenomena is very similar to
electron degeneracy pressure
only it is neutrons and not electrons
that are moving around fast enough to
exert pressure.
One of the most exciting things about neutron stars
is that they're actually spinning super fast
Some rotate all the way to around
several hundred times per second.
Imagine how dizzy you would be if you
were spinning like a neutron star!
All the spinning can make a neutron star
emit strong beams of electromagnetic
radiation
out of its magnetic poles.
Astronomers call neutron stars emitting
electromagnetic beams
Pulsars.
The radiation from pulsars can
only be detected by astronomers
if one of the beams is pointed towards Earth.
The first observation of a pulsar
was made by astrophysicist,
Jocelyn Bell Burnell in November 1967.
She noticed an interesting signal
recorded on the chart paper of observations
made by the radio telescope that she was working with.
She had to check a lot of chart paper by hand
to confirm her suspicion that the signal match another signal that was recorded in August of that year.
She determined that the
signal was emitting a regular pulse
about every 1.3 seconds
This turned out to be the very first
observation of a neutron star.
One of the most exciting astronomical observations involving neutron stars,
is a neutron star merger.
This is what we call two neutron stars
spinning into each other and colliding
This collision releases an incredible amount of energy
as well as something
very cool called gravitational waves.
Gravitational waves are sort of like ripples in water.
But instead of water, it's the very fabric of space-time
that gravitational waves ripple through.
Astronomers think neutron star mergers
may also create black holes.
Speaking of black holes, let's talk about
my favorite type of stellar remnant.
First, it is important to note that
there are actually a few types of black holes.
There are stellar black holes,
theoretical medium-sized black holes,
and supermassive black holes.
This famous image is the accretion disk
around the supermassive black hole
at the center of the M87 galaxy.
It is at least three billion solar masses.
Astronomers don't really know
how supermassive black holes are formed.
Maybe you'll be the scientists to
figure out this mystery someday.
Of course, we do know how
stellar black holes are formed.
When a supermassive star goes supernova,
if it doesn't collapse down to a neutron star,
it collapses all the way down to a singularity.
This is the case when gravity completely
wins the hydrostatic equilibrium battle.
There is no longer any type of pressure
supporting this object.
A stellar black hole's mass is somewhere between
five to several tenths of solar masses.
Like white dwarfs or neutron stars,
black holes are sometimes in
binary systems with other objects.
Like a star or another black hole.
Systems that contain a star and either a
black hole or a neutron star
are called X-ray compact binary systems.
My friend Ellen studies these systems.
Let's ask her to tell us more!
Ellen: Hi everyone. I'm Ellen from the Lawrence Planetarium
When I'm not helping to create fun astronomy content,
I'm working with data from the Nuclear
Spectroscopic Telescope Array
or NuSTAR, which is an X-ray telescope in
space.
X-ray telescopes can be used to find
binary systems
containing a stellar black hole and a star.
Because in these kinds of systems,
material is being transferred
from the star to the black hole,
which releases huge amounts of energy,
heating up the material
to several hundred million degrees Kelvin
and radiating X-rays.
But the amount of energy emitted by
a black hole in a binary system
and the amount of energy emitted by
a neutron star in a binary system
is pretty similar.
So these objects can be hard to tell apart
in these kinds of observations.
Still, I think they're super cool!
Fira: Thanks Ellen!
One last thing I'd like to share about black holes
is the fact that nothing can escape the
gravitational pull of black holes.
Not even light!
This is why we see a black disk in
drawings of black holes
and in the imaging of M87.
That is the accretion disk of light
falling into the black hole.
Hmm...
Thinking about this makes me wonder...
what do you think would happen
if we fell into a black hole?
[DUN DUN DUN]
Well, no one has ever fallen into a black hole,
yet.
So we don't know for sure.
But scientists think that
the effect of gravity on your toes
versus the effect of gravity on your head
would be very different
because the gravity is getting stronger and stronger
the closer and closer you get.
So if you fell into a black hole,
you would get all stretched out.
Sort of like a human spaghetti.
In fact, the scientific term for this
is spaghettification.
But don't worry about getting
spaghettified anytime soon.
You actually have to be pretty close
to a black hole to get sucked in.
And there aren't any black holes
that are close enough to Earth,
for us to worry about falling into one anytime soon.
And on that comforting note, we'll finish up for today.
I hope you enjoyed learning about stellar remnants
from white dwarfs to neutron stars and black holes.
Be sure to give this video a like and
subscribe to the Lawrence Hall of
Science Youtube channel
for more science content.
I'll see you later
and keep asking big questions about the universe.
