So since so many people have asked me what Cassiopeia A was and what a supernova was after I told them, I wanted to do this post (as many of you know, I would rather be Cassiopeia a, than a human being).
What is a supernova?
A blindingly bright star bursts into view in a corner of the night sky — it wasn’t there just a few hours ago, but now it burns like a beacon.
That bright star isn’t actually a star, at least not anymore. The brilliant point of light is the explosion of a star that has reached the end of its life, otherwise known as a supernova.
Supernovas can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They’re also the primary source of heavy elements in the universe.
On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe.
Exactly how a star dies depends in part on its mass. Our sun, for example, doesn’t have enough mass to explode as a supernova (though the news for Earth still isn’t good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf).
A star can go supernova in one of two ways:
- Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.
- Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.
Let’s look at the more exciting Type II first:
For a star to explode as a Type II supernova, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here’s what happens next:
- Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter towards the outside of the star.
- Once the star’s core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode (for this reason, these supernovas are also known as core-collapse supernovas).
- The core heats up and becomes denser.
- Eventually the implosion bounces back off the core, expelling the stellar material into space ? the supernova.