Type II Supernovae: Very Big Explosions

Supernovae, the explosive deaths of massive stars, are fairly rare events.  They occur just once every 50 years or so in the Milky Way.  But we owe our very existence to some long forgotten supernova billions of years ago which created most of the heavy elements in our solar system, including the elements that make life… and you… possible.

So it might be worth knowing a little more about these spectacular explosions…

As you learned before, when a big star has burned through its stock of hydrogen, helium, carbon, etc., it’s eventually left with iron and nickel in the core.  But iron and nickel can’t fuse together to release any more energy.  So the game is finally up… and when burning stops, there is no more energy to hold the star up against its own gravity, and the star’s hot core suddenly collapses into a neutron star or a black hole.  This releases a huge amount of gravitational energy.

At the same time, the gas in the star’s outer layers collapse, hits the dense core and bounces back outwards at high speed, releasing the energy of the supernovae.  The expanding shell of gas that bounces creates a shock wave in the interstellar medium that emits visible light; many such remnants of supernovae exist in our galaxy.  You can see one tonight in the constellation Cygnus.  It’s called the Veil Nebula.

Veil Nebula

The Veil Nebula in Cygnus, created by a supernova some 18,000 years ago.

During this violent physical event, there are particles and energy flying everywhere… atoms, gamma rays, neutrons, everything.  Many neutrons get captured by atomic nuclei in what’s called the r-process.  This is how most of the elements heavier than oxygen get made, right up to and including uranium.  The silicon in the chips of your computer and the iron in your blood were likely made in a supernova explosion.

The most recent nearby supernova was SN1987A in the Large Magellanic Cloud  (I confess that when I was an astronomy graduate student, I was at the observatory in Chile where this supernova was discovered, but I missed seeing it by 4 days; I used to catch rides to the telescope with the guy who discovered it).  Other famous supernovae include the supernova of 1054, which created the Crab Nebula you can see in Taurus, and Tycho’s star that blew in 1572 leaving a strong source of radio emission.

And there will be more supernovae in our skies.  Betelgeuse, Spica, Antares, and Eta Carina are well-known stars likely to end as supernovae in the next few thousand to few million years.  When they go, they’ll be the brightest object in our sky except for the Sun and Moon.  These exploding stars will be easily visible during the day and will cast shadows at night.  It will be quite a show.

What we’ve just described is called Type II supernova, the type caused by the sudden collapse of a massive star.  There is another type of supernova that’s immensely useful for for figuring out the distance scale of the universe.  But that’s for another issue…