This World Wide Web page written by Dan Styer, Oberlin College Physics Department;;
last updated 2 November 2000.

A supernova is an exploding star. A high-mass main sequence star evolves into a supergiant, which switchbacks back and forth on the Hertzsprung-Russell diagram as its core grows ever hotter and its nuclear furnace produces ever heavier elements. Soon after its innermost core begins producing iron, the most stable nucleus, a crisis occurs. The star produces so much energy that it explodes with a burst that sends its outer layers flying off into space at about 850 miles per second. During this so-called supernova explosion the star shines, for about a month, as brightly as 600 million suns. Because so much happens so quickly, supernova explosions are not well-understood.

Supernovae are rare. In the last millenium, humanity has recorded only four in our galaxy.

The 1054 Supernova

Chinese records show that in July or August of 1054 (probably on 4 July) a "guest star" appeared in the constellation Taurus. This star was as bright as the full moon and visible during the daytime for a month. There is also some evidence that this supernova was noticed by European and Native American cultures. (A general history is available here.)

If, today, we train our telescopes at the site of the 1054 supernova, we find the Crab Nebula. (This is also known as M1: the first item on Messier's list of "fuzzy objects".)

The Hubble Space Telescope has given us detailed views of the Crab Nebula. Here are the edges:

And here is the center:

The little star surrounded by ovals is all that remains of the supernova's core. We will see later that this is a neutron star.

The 1572 Supernova

On 11 November 1572, Danish astronomer Tycho Brahe observed an "extra star", about as bright as Jupiter, in the constellation Cassiopeia. Today, this is often called "Tycho's supernova". Its remnant is hardly noticeable using visible light, but its X-ray image is remarkable:

The 1604 Supernova

This supernova was noticed on 9 October 1604, when it was already brighter than any other star in the sky. Johannes Kepler first saw it on 17 October, but he studied it extensively so it is called "Kepler's supernova". As with Tycho's supernova, the remnant is not spectacular in the visible domain, but it has a nice X-ray image:

The timing of the closely spaced 1572 and 1604 supernovae was in one sense unfortunate. The telescope was invented in 1608: If these supernovae had just held off a few years, they could have been studied much more intensely using telescopes!

The 1987 Supernova

We had to wait 383 years to see a nearby supernova through a telescope. on 24 February 1987, Ian Shelton, of the University of Toronto, noticed a supernova in the Large Magellanic Cloud while he was on an observing trip at the Las Campanas Observatory in Chile. (The Large Magellanic Cloud is a satellite galaxy to the disk of our own Milky Way Galaxy, so some might disagree when I say that SN1987A was ``in our galaxy".) As the first supernova detected in any galaxy in 1987, it was designated SN1987A. Here are before and after pictures of the supernova:

In contrast to the 1572 and 1604 supernovae, the 1987 supernova came at a particularly fortunate time. The supernova was visible only in the southern hemisphere, and most large telescopes are located in the northern hemisphere. However, two high-quality telescopes had just been built in the south: at the Anglo-Australian Observatory in Australian (opened in 1971), and at the Cerro Tololo Inter-American Observatory in Chile (opened in 1974). Both observatories have been carefully following the progress of the supernova. So has the Hubble Space Telescope. Here is a Hubble image of the ring of material ejected from the supernova explosion: