6 November: Supernovae

On this date in 1572 a supernova was observed in the constellation known as Cassiopeia. 10 things you didn't know about supernovae:

1. The word supernova was coined in 1931 by Fritz Zwicky and Walter Baade. In Latin, nova means "new", referring astronomically to what appears to be a new bright star. The prefix "super" is added to distinguish them from ordinary novae.
2. What causes them? They are basically exploding stars. Stars burn burn huge amounts of nuclear fuel at their cores, producing energy. For most of a star's life, there is balance between gravitational forces which try to compress the star into a tiny ball, and the outward pressure produced by the burning of the fuel. Supernovae happen when a large star runs out of fuel, so gravity wins. The star collapses and then explodes.
3. When supernovae explode, they jettison matter into space at around 9,000 to 25,000 miles (15,000 to 40,000 kilometres) per second. This can compress any clouds of molecules which happen to be in the vicinity, thus creating new stars.
4. Supernovae are element factories. The nuclear reactions in an average star can create elements through the periodic table as far as Iron. To create elements heavier than iron requires a supernova. So you could say that the Earth, and indeed our own bodies, have within them the remnants of ancient supernovae.
5. Somewhere in the universe, a star goes supernova every few seconds. However, in our galaxy there are only one or two every century. The last one to be directly observed in our galaxy was over 400 years ago by Johannes Kepler, who thought it was a new kind of star. It's now known as SN 1604.
6. Supernovae are given names by the International Astronomical Union. It will have the prefix SN, followed by the year it was observed. Historical supernovae like the one Kepler saw, need no other designation. Now, however, when we can look for them with more powerful Telescopes, several will be seen each year, so they get a letter as well, ie. SN 2021 A, SN 2021 B, and so on. If there are more than 26 observed in a year, then they will use two or even three letters.
7. When a supernova has finished exploding, what will be left is either a neutron star or a Black Hole, depending on how big the original star was. A neutron star is the rapidly spinning core of the star which emits radio waves. Nebulae are formed by the clouds of stuff emitted by the explosion.
8. A single supernova can easily outshine an entire galaxy of stars and release as much energy in a single burst as the Sun will in its entire, 10 billion-year lifespan.
9. There only been eight supernovas in recorded history which have been visible by the naked eye. Of these, only three have happened in the last 1,000 years. The first supernova to be observed by humans is believed to have been between 4500 and 1000 BC. The brightest supernova ever recorded was SN 1006 in 1006 AD, and could be seen in China, Europe, Egypt, Iraq and Japan.
10. What would happen to us if there was a supernova nearby? First of all, our sun won't become one, because it is too small. However, there are stars relatively close by which could. There is evidence that it has happened at some point because traces of radioactive iron-60, a strong indicator of supernova debris, is buried in the sea floor all over the planet. If the most dangerous type of supernova happened within 26 light years of us it could destroy half the ozone layer. However, there are no known stars likely to become that type of supernova within 500 light years.

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