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We do not see many changes in the sky from night to night. Over the course of a human lifespan, only the planets, the moon, and an occasional comet are likely to move and change appearance significantly.
But sometimes the sky changes in a flash. Sometimes, stars die a violent death called a supernova. A supernova occurs when a very large star exhausts its nuclear fuel. The star collapses, triggering a violent explosion. The light from a supernova can be as bright as an entire galaxy for a short period of time. Some supernovae in our galaxy have been so bright they have been visible during the day!
Unfortunately, supernovae visible to the naked eye are rare. One occurs in our galaxy every few hundred years, so there is no guarantee you will ever see one in our galaxy in your lifetime. In 1987, a supernova called 1987A was visible in a nearby galaxy called the Large Magellanic Cloud. It was visible to the naked eye to people in the southern hemisphere. Astronomers are still closely studying 1987A's blast wave as it collides with interstellar gas and dust.
The SDSS has discovered several supernovae. Three of them are in the SDSS's Early Data Release (EDR). In the next exercise, you are going to try to find them by comparing SDSS EDR images with those in the Palomar Sky Survey.
The two images are below. The image on the left is from POSS I, and the image on the right is from the SDSS. Click on either image for a larger view.