Welcome to my blog "Mystery of Galaxy".
Today our topic is Supernova. 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 goodnight. Instead
they go out with a bang. We're going to talk about how
the most massive stars in the universe evolve and die.
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| Source: ESO/M. Kornmesser / CC BY (https://creativecommons.org/licenses/by/4.0) |
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.
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| Sirius Source: Hubble European Space Agency Credit: Akira Fujii / Public domain |
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| PI Andromeda Source: David Ritter / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) |
PI Andromeda is 6.5 times the mass of our Sun but it is 800 times
brighter.
And mu Columbae is16 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 super giant 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 case of our sun we see 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.
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| Rigel Source: Haktarfone at English Wikipedia / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) |
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 atone-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.
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| white dwarf Source: European Southern Observatory / CC BY (https://creativecommons.org/licenses/by/2.0) |
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.
With 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 250,000Earth'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 58neutrinos 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 12kilometers 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.
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| Crab Nebula in the constellation Taurus. Source:ESO / CC BY (https://creativecommons.org/licenses/by/4.0) |
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".
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| Hypernova Source:NASA/GSFC/Dana Berry / Public domain |
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| Black hole |
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 blackhole 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.
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| Gamma ray bursts Source: NASA/GSFC / Public domain |
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 off150 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.
that's all thank you.
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