image © Australian Astronomical Observatory, photograph by David Malin; image source; larger image
Here is a before-and-after view of a part of the sky where a supernova appeared in 1987. A supernova is a catastrophic explosion in a large star. Two hours before this supernova was seen through telescopes, it was announced by a spike in the count of neutrinos in several detectors on Earth. The arrow points to the star before it exploded.
Neutrinos are tiny, uncharged, nearly-massless particles that travel at almost the speed of light. A supernova produces a vast number of neutrinos; in fact, most of the energy of a supernova is given off in neutrinos. To learn more about neutrinos, see Physics at Home and Worth a Look just below.
To learn about Supernova 1987a, visit Hyperphysics and Physics Central, and for much more detail see AAVSO.
The Brightest Supernova Ever
Speaking of supernovas...here's the biggest supernova explosion ever observed. It was the death of an extraordinarily massive star, but one located in a galaxy about 240,000,000 light-years away.
In fact, we have a similar star in our own galaxy, and only 7500 light-years away. You can see an image of this monster here.