How do Astronomers figure out how old a supernova is?

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How old? I'd imagine by knowing the distance. They can determine how long it took the light of the supernova to get to us.

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Yes, but wouldn't that be only a fraction of the supernova's age? How long does it take a star to become a supernova? How do astronomer's figure that out?

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There are different types of supernova. Type Ia supernova are thought to result from a buildup of hydrogen gas on the surface of a white dwarf, while Type II supernova are thought to result from the explosion of a massive star star at the end of its life.

For type II supernova the collapse of the star is thought to be very rapid. A massive star will go through a number of stages of fusion in the core - each stage shorter than the previous. The final stage is the formation of an iron core from a silicon oxygen core. This stage is very fast - about a week. The collapse of the iron core only takes a few seconds and results in the supernova.

If we want to know how long ago a supernova happened, there are a
few methods. In a few cases, we have records of humans witnessing
the supernova on a certain date -- so that's easy!

Second, we can take pictures of the remains of the star (like the
Crab Nebula) several years apart and see the material expanding
outward. We then know how fast it is expanding outward and just calculate
how long ago the material would have been at the central point. Then
we know how long ago the supernova happened.

Another method is to measure how far away a supernova remnant is
(this is hard), measure how far across it is, and use the Doppler shift
to tell us at what speed the material is moving. Then we know how far
the material has travelled and the speed at which it has travelled, so we
can calculate how long it took to traverse the distance.

Of course, there are complications to these methods, but you probably
don't care to know about those.

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I thought it was even shorter than that. Like a tiny fraction of a second or something.

Before the final cataclysmic core collapse into a neutron star, is the core of a type-II-supernova-to-be already electron degenerate? I.e. is it already in the same state of matter as a white dwarf?

Do you mean how old is the star? Generally they can tell the age of a star by its mass and color. The more massive the star, the faster and hotter it burns - making it bluer. And the faster it burns, the sooner it goes supernova.

Speaking of which -- are there any Population II stars (what you young whippersnappers nowadays call "halo population" stars, you know, the metal-poor ones) that are known to have gone supernova? My understanding is that a main sequence spectral class A or B star that's metal poor can last a lot longer than one that's metal rich, due to its lack of access to the CNO cycle.

Actually yes I suppose for a clearer understanding of the issue I'd need to know how Astronomers figure out how old a star is. How do astronomers assign an age to a certain color sun?

As you probably can tell I'm a bit confused...

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The ages are based on very complex computer models of stars. These
models include a lot of physics: details of nuclear fusion, how light is
transported inside the star, convection (or "boiling") inside the star, the
chemical composition of the star, the mass of the star, how the star loses mass
over its life, and other such things. These models are then matched to
observed objects (the sun being the best one, as we have been able to test
model predictions against the real thing).

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