Observing Supernovae

Okay, so last time we got to the point of core collapse, rebound, and blast wave -- the explosion. This type of supernova is called a Type II supernova. (Type Ia supernova come from exploding white dwarfs -- we'll get to those soon...).

Historical naked eye supernovae:

SN 1006 (Type Ia)
The Crab, 1054 AD (Type II)
Tycho's supernova (Type Ia)
Kepler's supernova (Type Ia)
SN 1987a (Type II)

Think about this: You study supernovae for a living. Where do you want to look for them? How do you know when a star is about to go supernova?

Type II Supernova Light Curves

Let's look at the brightness of a supernova as a function of time -- a light curve.

Why is it shining? What provides the energy of a supernova?

Radioactive decay

Remember that atomic nuclei can have both protons and neutrons. The number of protons defines the element (ie 1=hydrogen, 2=helium, 6=carbon, etc). But a given element can have different numbers of neutrons - for example ³He, which has only one neutron instead of the normal two. These are called isotopes. Many isotopes are radioactive, meaning they will beta-decay into a different element, giving off energy.

When a supernova goes off, we see this energy emitted by radioactive decay.

First we have

Then we get

A short review of radioactive decay

Radioactive decay means that the number of isotopic nuclei drops like

where the half-life is defined as

So, in a homework you will show that since the luminosity must be related to dN/dt,

So the rate of decline of the light curve depends on the isotopic decay rate.

Decay	Half-life	SN Decline Rate
⁵⁶Ni -> ⁵⁶Co	6.1 days	0.11 mag/day
⁵⁶Co -> ⁵⁶Fe	77.7 days	0.009 mag/day

Supernovae Remnants

The Crab Nebula

By age dating this nebula, we find that it is about 950 years old. The supernova was observed and noted by the Chinese (on July 4, 1054 AD), and rock paintings found in New Mexico suggest the Anasazi Indians also saw it. There is some debate as to whether or not the Europeans saw it, although it was so bright that it should have been observable in the day time!

The Crab has been found to be very rich in metals, like oxygen, carbon, iron, etc -- the products of nucleosynthesis. More interstellar recycling.

What powers the Crab -- why does it shine? Certainly shock excitation plays a role, but that can't explain all the energy we see coming from the Crab. The supernova energy itself has been radiated away, so what else could be giving the Crab energy??

Supernova 1987A:

The nearest (d ~ 50 kpc) supernova in modern times:

It gives us a chance to observe a very young supernova remnant.

We see a very young remnant, surrounded by dust and gas (about 1/2 parsec in diameter) which has been lit up by the supernova.

We now see evidence that this ring is brightening, evidence that supernova ejecta is colliding with the ring.