All about... supernovae
What are supernovae, and why are they important?
On 22 January 2014, it was announced that astronomers had found a new supernova in the galaxy M82, about 12 million lightyears away. The discovery is exciting astronomers since the last supernova observed within our own galaxy was seen by Kepler in 1604, but what is a supernova, and why do they matter?
The name was coined by Walter Baade and Fritz Zwicky in 1931 to distinguish the phenomenon from “ordinary” novae, which are nuclear explosions within white dwarf stars that cause a sudden brightening and often mean that the star become visible to the naked eye when it previously was not. (Hence nova, Latin for “new”.) Supernovae, as the name suggests, are much brighter. Occurring on average at a rate of three every hundred years they often outshine a whole galaxy, and can release as much radiation in a few weeks as the Sun will over its entire lifetime of billions of years. They’re caused by stars exploding, which can occur in two main ways.
One is that a white dwarf accretes material from a nearby companion and heats up. Once hot enough for carbon fusion to begin, a runaway nuclear reaction, blowing the star apart.
The other way is via core collapse – a very massive star becomes unable to support itself against gravitational collapse, usually by exhausting the nuclear fuel in its centre. The star begins to collapse, rapidly increases in temperature and blows off its outer layers. Remaining cores up to about three times the mass of the Sun will form neutron stars, while heavier ones will continue to collapse as a black hole.
The first type of supernovae can not only allow astronomers to investigate stellar structure and evolution, but importantly, because as the inherent brightness of these explosions are the same throughout the universe, we can accurately work out distances to stars and galaxies, helping to fix the scale of the cosmos.
Supernovae are a key source of heavy elements – distributing those heavier than oxygen around the galaxy and also producing those heavier than iron. The shockwaves generated during their explosions can trigger new star formation.
But they’re also potentially dangerous – a supernova even as far as 3000 lightyears away from the Earth could bombard our atmosphere with enough gamma rays to deplete the ozone layer, exposing us to harmful solar radiation. That a supernova that ended life here may help to trigger it elsewhere may provide some consolation.
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