This video is brought to you by Magellan
TV. Stars with masses greater than eight
times our Sun are rare; they make up less
than 1-tenth of 1% of all the stars in the
Universe. But these stellar Giants have
an enormous effect on the formation of
stars around them and create the
elements needed to build rocky planets
and even life. But in order to do so
these stars must die, and when they do,
they don't go gently into that good
night. Instead they go out with a bang.
Welcome back to Launch Pad, I'm Christian
Ready, your friendly neighborhood
astronomer and in this video we're going
to talk about how the most massive stars
in the universe evolve and die. But first
I want to take a moment and thank
Magellan TV for sponsoring today's video.
Magellan is a new kind of streaming
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absolutely free of charge so definitely
be sure to check them out. Massive stars
live their lives doing what all stars do:
they burn hydrogen into helium in their
cores and they produce energy along the
way. It's this energy that holds the star
up against its own collapse. Massive
stars start off with a lot more hydrogen
fuel than in our Sun but that extra mass
means extra gravity so their cores are
squeezed far harder. That causes the star
to burn its fuel hotter and faster. That
means the more massive the star the more
luminous it is. For example, Sirius is
only twice the mass of our Sun yet it is
23 times more luminous. PI Andromeda is 6.5 times the mass of our Sun but it
is 800 times brighter. And mu Columbae is
16 times the mass of the Sun yet it is a
whopping 47,500 times brighter! But
before long the core runs out of
hydrogen fuel and all that's left is
inert helium. With no internal fusion to
prop the core up, it contracts and gets
hotter and the hydrogen surrounding the core
is squeezed against it until it begins
fusing in a shell. The outer layers of
the star expand and cool in response and
the star evolves off the main sequence.
This is similar to the way the Sun will
expand to become a red giant,
but because the core of the star is
producing so much more energy, the outer
layers rapidly expand to nearly the size
of Jupiter's orbit. The star has become a
red supergiant. Betelgeuse in the
constellation Orion is a red supergiant
star. It's more than 600 light-years
away but it's so large we can
resolve its surface in our best
telescopes.
This isn't Betelgeuse's light spreading
out over the image, it's the actual
surface of the star. And it's blobby. Its
interior is unstable and its outer
atmosphere is so distended that changes
inside take a long time to ripple around
the star's surface.These undulations
produce powerful stellar winds that
cause Betelgeuse to lose a Sun's worth
of mass every 10,000 years. Interestingly,
the star's overall luminosity hasn't
really changed all that much. And that's
because a star's luminosity is governed
by the square of its radius as well as
its surface temperature to the fourth
power. And this is important because by
now the star's surface temperature is
much much lower than it was while it was
on the main sequence, but it's
overwhelming size basically makes up for
the loss of surface temperature so the
luminosity doesn't really change all
that much. Meanwhile, back in the
contracting core, temperatures quickly
reach a 170 million
Kelvin, and that's hot enough to get
helium to start fusing into carbon and
even some oxygen. In our video on how the
Sun will eventually die we, found that
such low-mass stars won't be able to
ignite their helium cores until it is
squeezed itself into such a dense state
of matter
that it's electrons become degenerate.
But high mass stars have cores that are
already much hotter to begin with so
they're able to ignite helium fusion
while it is still a normal, albeit
extremely hot gas. The core expands and
cools. With less energy pumping into the
rest of the star it contracts and heats
up in response. The star is now a blue
supergiant, In facts, Rigel - also in Orion -
is a blue supergiant star. Even though
it's 860 light years away it's easily
one of the brightest stars in the entire
night sky, putting out 120,000
times the energy of our Sun.
Before long the helium fuel runs out and
all that's left is a core of inert
carbon. Our Sun will develop a carbon
core as well but this is the point where
stars like our Sun start to tap out. And
that's because they can't get their
cores hot enough to fuse carbon into
heavier elements. But the situation is
very different in a massive star. The
sheer weight of the surrounding layers
quickly drive up the core temperatures
to 600 million Kelvin, allowing it to
begin fusing into oxygen, magnesium, and
neon. This cycle of contraction, heating,
and nuclear fusion repeats several more
times, creating heavier nuclei at each
stage. But each heavier nucleus requires
higher and higher temperatures in order
to fuse. Carbon fuses into neon at 600
million Kelvin. Neon fuses into oxygen at
one-and-a-half billion Kelvin. At two
billion Kelvin, oxygen fuses into silicon
and at three-and-a-half billion Kelvin,
silicon begins fusing into Iron. The star
oscillates between the blue and red
supergiant stages as it runs out of one
fuel and ignites another. Eventually, the
core looks like a giant onion with inert
iron at it's very center surrounded by a
silicon burning shell which in turn is
surrounded by an oxygen burning shell
then neon carbon helium and hydrogen
fusing shells, Not only does each stage
have to burn its nuclear fuel hotter, it
must also burn it faster than the one
before.
That's because as the produced
nuclei get heavier, the amount of energy
released in each reaction gets lower.
That means that these successive stages
have to burn their fuel much faster than
the one before it in order to produce
enough energy to hold the star up. For
example a 25 solar mass star will
exhaust its entire supply of hydrogen in
about seven million years. Helium is
burned through in about 700 thousand
years. Carbon burning only lasts
about a hundred and sixty years. Neon is
burned through in about one year. Oxygen
is burnt in about 6 months, and its
entire supply of silicon will be fused
into iron in - believe it or not - one day!
When the core becomes iron the star is
doomed. Remember, every element created so
far produced less energy than the one
before it. By the time it reaches iron it
stops producing energy altogether. On
this last day of the star's life, the
star cannot get hot enough to fuse iron
because iron cannot be fused. At first
the core contracts by a small amount and
the electrons quickly become degenerate.
The core essentially becomes an iron
white dwarf, but the surrounding shells
keep dumping heavy elements onto the
core until it reaches one point four
solar masses. And that's bad. The density
inside the white dwarf is a mind-
crushing 400 billion times
greater than water. It is so dense that
gamma photons disintegrate the iron
nuclei and transforms them into a soup
of free protons and electrons. Wth no
more energy to hold itself up, the core
implodes. In less than 1/10 of one second,
the core collapses from the size of Mars
to the size of Manhattan. During the
collapse the protons and electrons are
squeezed together to form neutrons and
neutrinos. The neutrinos escape the core,
carrying energy away with them and the
collapse accelerates. The neutrons are
squeezed together so tightly they exert
an ultra-powerful neutron degeneracy
pressure and the collapsing core comes
to a ringing halt at a radius of just
a few kilometers. Unfortunately, the rest
of the star doesn't know that. The
surrounding layers just had their legs
kicked out from underneath them and
within a few milliseconds 250,000
Earth's worth of star stuff comes
crashing down on the core at 15% the
speed of light. A powerful shockwave
rebounds off the core. The star has
released more gravitational energy in
just one second than all of the nuclear
energy it released in its entire life.
But that energy has to go somewhere. Most
of it is carried away in the form of
neutrinos: 10 to the 58 neutrinos!
Neutrinos are so tiny that 99.7%
of them pass right through the star at
the speed of light, but the remaining
0.3% of neutrinos collide with the dense
matter in the shockwave. That may not
seem like a lot but 0.3% of 10 to the 58
neutrinos is still 3 times 10 to the 55
neutrinos! That is a colossal amount of
matter all colliding in the shockwave.
The shockwave gets supercharged and
rips through the star at 10% the speed
of light, exploding the star in a
supernova.
The star brightens to 100 billion times
the Sun. That's nearly as bright as all
of the other stars in the galaxy
combined! About one-tenth of a solar mass
worth of neutrons is ripped off the core
surface. These neutrons collide with
heavier nuclei blasting out of the star.
These nuclei then decay and form heavier
elements including elements greater than
iron. In fact, all the elements heavier
than iron, including zinc, copper, silver,
gold, even uranium are created in the
Neutron fusion in the maelstrom of the
supernova. Radioactive elements formed in
the explosion decay over time, releasing
more energy in the process. This keeps
the supernova bright for several months
at a time.
The exploding gases expand away, leaving
behind the exposed core. The core is now
a rapidly spinning ball of degenerate
neutrons called a neutron star. These
objects are at least 1.3
 times the mass of the Sun,
but are only between 10 and 12
kilometers across. That makes these
things mind-bogglingly dense; up to a
hundred-billion-trillion grams per cubic
centimeter! Not only that, but they rotate
at least 10 times per second when they
form. This generates ultra powerful
magnetic fields that are at least a
trillion times stronger than Earth's.
These ultra powerful magnetic fields act
as particle accelerators, generating
powerful beams of radiation along the
magnetic poles. As these beams sweep
across our line of sight, we detect them
as repeating radio pulses so we call
these magnetized
rotating neutron stars "pulsars". Rotating
or not, the neutron star surface is
extremely hot at around 1 million Kelvin.
That's hot enough to ionize the
surrounding gases for about 25 thousand
years. The most well-known supernova
remnant is the Crab Nebula in the
constellation Taurus. It formed in 1054 AD
in a supernova explosion that was so
bright it could be seen during the day.
At the center is the Crab Pulsar
rotating at 30 times a second. It's
magnetic field churns up the surrounding
gas like a stellar eggbeater. Stars
between 8 and 40 solar masses will
ultimately produce neutron stars in
supernova explosions. But if the star
begins life with more than 40 solar
masses, it'll produce an explosion that
with at least 10 times the kinetic
energy of a typical supernova. These
explosions are so energetic we sometimes
call them a "hypernova". The cores of
these ultra-massive stars reach at least
three solar masses. That's too massive
for even Neutron degeneracy pressure to
prevent its complete and total collapse
into oblivion. The core collapses until
it reaches a mathematical volume of zero,
crushing itself out of existence. The
core is now a black hole. As the black
hole forms inside the collapsing star,
matter rapidly funnels into it. This sets
up powerful jets along the black hole
spin axis and we see these jets bursting
through in powerful gamma
ray bursts. These explosions are the most
luminous and violent in the entire
universe; they are literally the biggest
bangs since the Big one.
Whether it is a gamma-ray burst, a
hypernova, or even a "mere" supernova, there
is a tremendous amount of material
blasting into the interstellar medium.
This gas is loaded with heavy elements
and when it slams into a nearby
interstellar cloud, it shocks the cloud
into collapse. Suddenly hundreds of stars
are able to form in a single blow. Over
the next tens of millions of years ,that
dead star stuff rearranges itself to
become new stars and rocky planets that
surround them. And on at least one of
those rocky planets, some of that dead
star stuff rearranged itself again to
become life. The iron in our blood, the
calcium in our bones... virtually
everything that makes up you and me was
created in a supernova explosion. We owe
our existence to supernovae but that
doesn't mean we want to be anywhere near
one of these things when they go off.
A supernova 50 light years away would
sterilize all life here on Earth. Even
100 light years away would dramatically
raise global temperature and radiation
levels. In fact, a supernova that went off
150 light years away is probably
responsible for a mass extinction that
occurred 2.6 million years ago. Luckily,
these massive stars are rare. In fact the
closest star that could possibly go
supernova is Spica in the constellation
Virgo.
It's 250 light-years away and it hasn't
even begun to evolve yet so we're safe...
...for now at least. As always, I need to
thank my Patreon supporters for helping
to make this channel possible, and I
really want to give a special shout out
to my newest patrons - and I'm just gonna
read their names here really quick so I
don't mess it up too badly -
it's Justin McNutt, Alexander Primyak,
Zach Peterson, The ANNO, and I want
to welcome my first ever cosmological
sponsor, Steven J. Morgan. Thank you all so
much from the
bottom of my heart and let me just also
add a personal note here...I've been a
little bit absent lately and that's for
a reason.
You see matter rearranges itself to
produce life but unfortunately it also
rearranges itself to produce other
things. My mother has returned to the
universe.
This past summer she was very ill in her
fifth battle with cancer. When I
wasn't working I was visiting her and
that left very little time for videos.
Then the semester began and I had to
start teaching classes again among which
was a new class I hadn't taught before
and so I had to put a lot more work into
it than I expected.
Well anyway when I wasn't teaching I was
with mom and then mom passed on
September 24th. Her name was Cecilia
Reddy and she is my first-ever inspirer.
She gave me my first telescope, she
always encouraged me to pursue what made
me happy and what brought me joy and she
was so proud of this channel and I know
she was proud of me too. She was a
professor of English and I guess she
named me after herself because my
students call me Professor Ready at
school so it's kind of funny. But mom,
thank you for everything and just know
that you're always gonna be with me and
I'm always with you and even though
matter rearranges itself doesn't mean
they can't rearrange the memories. So
anyway, thank you so much for all of your
support and your patience as I've been a
little bit busy in the last couple of
months just getting everything ready and
settling matters. But now I'm back and
I'm so glad to be back with you. Thank
you again and if you would like to learn
how neutron stars and black holes work
I'll be making some videos on them very
soon and I also have a video that talks
about how stars work and how our Sun
will die if you'd like to go back and
just kind of see how we got to this
point in our discussion. Until then thank
you so much,
make sure you subscribe and ring that
notification bell so that you don't miss
out on any new videos. And until next
time, stay curious my friends.
