HUNTING BLACK HOLES, EXPLODING STARS AND MORE
The Universe is full of action and excitement, however peaceful it is to look up into a starry sky. In far away galaxies, whole star systems are swallowed by gigantic black holes that are millions of times heavier than the Sun. The biggest stars live fast, die young and leave a good looking corpse (a neutron star or a black hole), but only after a spectacular explosion that releases more energy in a few moments than the Sun will emit in its whole life. Most of the time, this cosmic violence results in the production of lots of high-energy radiation (e.g. X-rays and gamma rays) and exotic particles like neutrinos, but not much that we can readily see here on Earth. Most of the time.

The X-ray view of the heart of the Crab Nebula, the remains of a supernova that was seen night and day 950 years ago.
(NASA/ CXC/ASU/ J. Hester et al.)

Nearly 1000 years ago, in May 1006, a new star suddenly appeared, and in a few days became the brightest star ever recorded - in fact it was a supernova, and for about three months was visible even in daylight. It was visible to the naked eye at night for another three years after its dramatic appearance.

In all, there have been seven historical supernovae over the last 2000 years that were close enough for everyone to see. You might remember the last one that exploded in 1987 if you were lucky enough to be in the Southern Hemisphere at the time. All were easily seen at night for months, and the brightest few were also visible during the day. However, only a tiny fraction of the supernova energy is radiated as visible light, and most of the energy goes into ejecting the gases in the outer layers of the star. The gas is blown out at up to 15,000 km/s (about 30 million m.p.h.!) and is heated to over one million degrees as it plunges into the gas in the environment, producing a huge amount of X-rays.