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5.2 Supernovas [Astronomy: State of the Art]


What happens in the situation where there
is sufficient mass in the stellar core to overcome the electron degeneracy pressure
that leads to leads to a white dwarf? (Music)
“In a Champagne supernova in the sky…February starts.. That hot dismembered constellation…
Well she said she’s from a quasar 40 thousand million light years away… Astronomy…” What happens is an extraordinary phenomenon.
The electrons and the protons are forced to fuse in a reverse version of beta decay, creating
pure neutron material. Neutrons have no electric charge so this matter can for nuclear densities,
in other words the entire star can take the density of an atomic nucleus, but of huge
mass. This neutron star collapses until it’s stopped by the pressure of degeneracy
of neutrons themselves. That pressure leads the core to bounce, and material heads outward at relativistic speeds. This core collapse takes place in less than a second, the bounce phase, in perhaps
a millisecond, and the result of this enormous energy deposition outward is something we
see as a supernova. Computer simulations are now good enough to simulate the extraordinary
and extreme astrophysics that takes place in a collapsing massive star core. The neutron
material can be simulated, the bounce phase, and the blast wave. There are other results—not easy to observe. A torrent of neutrinos is produced by the inverse beta decay reaction
the leads to the neutron material. Those neutrinos were actually observed for the first time
in 1987, verifying our models of this process. In fact, close to an Earth-mass of neutrinos
in their rest mass can be generated instantaneously in a supernova event. There’s also a prediction
that gravity waves will be produced, and we’re waiting for LIGO to confirm that prediction.
The simulations also show very clearly how much the gas is heated in the blast wave that
moves out. The temperatures momentarily reach billions of Kelvin which is easily sufficient
to generate by nucleosynthesis in a blast wave all the elements of the periodic table
including the uranium elements and the transuranics, radioactive elements that will in fact subsequently
decay, but all the stable elements too. The amount of energy released is so great that
when the blast wave expands and cools to visible light, its dying star can reach a brightness
equivalent to that of an entire galaxy, billions of times brighter than the star that led to
it. We know how many massive star there are in the galaxy so its fairly easy to calculate
how many supernovas should occur. Because massive stars are extremely rare, and their life times are
still long compared to a human lifetime, the event rate is low. It’s predicted to be
about once every 100 years. Intriguingly there hasn’t been a supernova visible in our galaxy
since 1604. By poisson statistics, or the average event rate, we are definitely due for a supernova.
The last 4 known to have occurred in the galaxy, occurred in 1006, 1054, 1572
(that was known as Brahe’s supernova), and 1604 (known as Kepler’s supernova). Tycho
Brahe was lucky enough to observe two supernovae in his life and actually used them to draw
inferences that the supernovae were distant events occurring beyond the realm of the planets.
What happens when a nearby dying star brightens to the brightness of an entire galaxy? Well
in these four instances the event becomes visible in the daytime sky. So if a supernova
went of relatively near us it would be a phenomenal event. It would actually be so bright that
astronomers would have trouble studying it because it would saturate or burn-out the
detectors on their large telescopes. Amatuer astronomers would have a field day and anyone
with a telescope smaller than a meter. Evidence in particular of the 1054 event is found around
the world in petroglyphs, rock art, and records of old civilizations. The Chinese called it
a guest star and it was clearly visible in the sky for weeks. This is the brightest of
the known supernova that have occurred in human history, but statistically there must’ve
been many more in the times before people kept written records. We can only guess what
ancient civilizations or cultures or hunter gathers must’ve thought when a star suddenly
brightened enough to rival the Sun. Astronomers of the modern era haven’t been lucky enough
to find a supernova in our galaxy. On the other hand be careful what you wish for. If
a supernova went off within 10 or 15 lightyears of the Earth there would be bad news for life
on Earth. But at a safe distance of perhaps 100 lightyears or 200 lightyears it would
be a spectacular event. In the history of modern astronomical detectors only 1 supernova
has been extremely easy to view, and that was the supernova that occurred in 1987 in
the Large Magellanic Cloud the nearest galaxy neighbor to the Milky Way Galaxy. The Magellanic Cloud is a 170,000 lightyears away, so that’s the time it took the signal to reach us, so
you can image the situation. A star died in our nearest neighbor galaxy before humans
left the plains of africa to fan off around the world in their migration. That light wave
passed through space, deep space, reaching the edge of our galaxy as human civilizations
started to form and then, 20 or so years ago, the light wave swept past the Earth. This
was an event where the neutrinos slightly preceded the light and a handful of neutrino
events were seen in a Japanese detector called Kamiokande. It was a wonderful kick start to the idea of Neutrino Astronomy, which is now rapidly maturing. The 1987 A supernova allowed us
to test our models of how stars dies, confirming some of the ideas, and disproving others.
It’s, so far, the best test bet for theories of supernovae…….on the dark material there.
Supernovas form in two different ways called type 1 or type 2, although subclasses have
now been defined In a type 2 supernova and isolated, massive
star dies explosively. In a type 1 supernova the system is a binary system, where the star is
triggered to supernova event by the passage of material in a binary orbit. Material can
siphon from one star to another when they are fairly close, within say a hundred or a thousand
times the radius of either of the stars. This is a particularly tight orbit, most binary
stars are in much wider orbits. The supernovae that occur in a binary star system are particularly
valuable to astronomers. Because of the regulated way the mass is transferred, the explosion
has a very well regulated and typical amount of energy constant between systems to within
15%. This means that it acts as a standard bomb, or a standard lightbulb, in the sky allowing
astronomers to use it to calculate distances and do cosmology. With no current or very
recently dying stars to observe we’re left looking as supernova remnants. The enormous
illuminated shrouds of gas and glowing dust that are seen distances of tens or hundreds
of lightyears away, and times of centuries or millennia after the supernova went off.
The sky is littered with evidence of dying stars in previous times. The Veil Nebula is
a beautiful form that’s been imaged with the Hubble Space Telescope, and Tycho’s supernovae has been imaged with the Chandra X-Ray Observatory. It’s recent enough that it’s still glowing
in x-rays. In this animation we observe a supernova explosion as a pure animation, but
it ends with the actual Hubble Space Telescope and Chandra data, showing the illumination
of the gas and the incredibly complex patterns that are formed in the interstellar medium. We’re now not only in a positionto explain the cosmic abundance of the elements in the periodic table, but element by element to say where those elements were produced
and how. It’s easy to see. there are the primordial elements hydrogen and helium, and then a set
of heavier elements produced in massive stars, but the vast majority of the heavy elements
throughout the periodic table are produced in the more evanescent late stages of a massive star’s life. Either by the slow ingestion of neutrons, gradually bumping up the atomic number by
1, or by the violent and explosive addition of nuclear material in a supernova blast wave.
At the end of a star’s life, if the massive core is too big to be supported by degeneracy pressure
of electrons, the star collapses, bounces, and detonates in a enormous blast wave going
off into space and carrying much of the stars total material, called a supernova. The average
rate of supernovae in our galaxy should be about one a century, and since the last one
occurred over four centuries ago, we’re due. If a supernova occurred nearby it would be
a spectacular event, rivalling the Sun in the daytime sky. Supernovae are responsible
for creating many, if not most, of the heavy elements in the universe, including gold,
silver, platinum, the precious elements, and even the transuranics.

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