What about supernovae?

I know that I've talked about supernovae before, but I want to be clearer about what they really are. What I find amazing is how much astronomers and physicists know about these truly awesome cosmic events.

There are two types of supernovae: Type I and Type II, and they each have sub types. This definition is based on the spectra from the event. The most common supernova is Type 1a. This type of supernova occurs when a larger star and a smaller white dwarf star orbit each other as a binary system. The white dwarf star (because of its intense gravity) begins to steal matter (hydrogen and helium) from the larger star's atmosphere, and when the amount of the stolen gases makes the white dwarf reach the Chandrasekhar (named for a famous Indian astrophysicist) limit of 1.44 solar masses, it collapses in a humongous explosion because of a runaway nuclear fusion reaction. In other words it acts like an enormous hydrogen bomb. Type 1a supernovae act like standard candles for astronomers to measure the distance to far-flung galaxies. They always put out the same luminosity and since brightness decreases by the square of the distance, measuring a 1a type supernova in a distant galaxy tells an astronomer how far away it is, and if the spectra is shifted toward the red because of it moving away from us. The astronomer can tell how fast it's moving away. This is how astronomers discovered that the universe is expanding at an accelerated rate.

Type II supernovae are mostly associated with a large star's core collapse. This happens when the star begins trying to work after iron is produced in the core. Iron is poison to the star and the fusion in its core stops, causing the core to suddenly collapse. The result is a violent explosion, actually more like an implosion.
A core of large star forms layers where higher elements are being fused. The outer layer is where hydrogen is fused, the next layer is where helium is fused, followed by carbon, neon, oxygen, silicon and then the last layer is where iron tries to fuse, but it won't. This iron at the very center of the star's core is what causes the collapse when the core exceeds the Chandrasekhar limit. At this point the inner core is compressed into neutrons, which causes the stuff falling down to bounce and create a shock wave, better known as an explosion.

In order for a star to undergo a supernova explosion when its core collapses it has to be at least eight times the sun's mass and its core must collapse while the star still has hydrogen in the outer layer of its core.
The most viewed type of supernova is Type II-P, which involve massive stars that are 90 times more massive than our sun. You can imagine how huge these babies are. They would make our sun look like a dinky moon.

When a supernova occurs it creates a very bright object, much brighter than any star. In fact, supernovae can outshine the entire galaxy that it's in. Supernovae take anywhere from 15 to 20 days to reach peek luminosity and then 40 to 100 days to drop down to 10% luminosity. This depends on the supernova type.

The amount of energy released by a supernova is in the 1.5 to 100 foe range. A foe is ten to fifty one power of ergs. That's a lot of power, many times more than if all of the world's nuclear weapons were set off at once.

Most supernovae result in nebulas, some of which are very beautiful astronomical objects in their own right. In many cases these nebulae begin to form new stars. As I've said, the elements in our bodies came from a supernova, so they are important to not only the structure of the universe but also the formation of life.

Supernovae have been recorded going all the way back to the ancient Chinese, Arabs and Japanese astronomers. The most recent supernova is our Milky Way galaxy was observed on October 9, 1604. There are many candidate stars to go supernova in our galaxy and some of them are pretty close to us. The closest, IK Pegasi, is only 150 light years away, but it's believed that it will be several million years before the white dwarf star orbiting a larger star gets to the Chandrasekhar limit, so don't sweat it.

Thanks for reading.