Hubble’s latest image is of the spiral galaxy NGC 4051, which is located 45 million light years from Earth. This particular galaxy is notable for having played host to a large number of supernovae over the years: the first seen in 1983 (SN 1983I), the second in 2003 (SN 2003ie), and the most recent in 2010 (SN 2010br).
Supernovae are epic cosmic events which occur in the final stages of the life of a star. Stars burn brightly because in their core hydrogen is undergoing fusion and producing huge amounts of energy. But eventually a star will burn through all of its hydrogen and start fusing helium instead. And once it runs out of helium, if the star is big enough it can start working through other elements like carbon or neon.
As the star eats through its fuel the fusion process will slow, and the gravitational forces which push inward on the star are no longer kept in check by the energy of photons created by the fusion pushing out. The core of the star will shrink and become more dense and hot.
Eventually a huge star will have used up the elements inside it until the core is mostly composed of iron, which requires more energy for fusion than the reaction produces, and it will run out of fuel completely. When this happens, the star collapses inward very rapidly — in a matter of microseconds — and the core will reach a temperature of billions of degrees Celsius.
In this extremely hot, extremely dense situation, the iron atoms are shoved together with tremendous force until they “bounce” back outward in a huge explosion. This explosion causes an enormous flash of light brighter than billions of our Sun and sends out a shockwave that travels across the galaxy.
When a supernova detonates, it shines in the sky so brightly that it looks like a bright new star. But it gradually fades over weeks and leaves behind a tiny, dense neutron star which gives off radio waves in bursts, called a pulsar. Or if the original star was big enough (at least ten times as large as our Sun), then the dense core can collapse under its own gravity and become a black hole.
