Saturday, September 6, 2014

What is a Supernova?

Have you ever seen an especially bright star bursting into view in a corner of the night sky?  If you have been watching the sky for a period of time and wondered why you missed seeing that bright star before, maybe it wasn’t really there. Well, at least not as bright as it now appears.

“What happened,” you ask? That bright star is no longer a star. Most likely it was a red giant star or supergiant red star, depending on its size. That brilliant point of light you now see is the explosion of that star, which has reached the end of its life. Supernovas can temporarily outshine entire galaxies and radiate more energy than our sun in its entire lifetime. Once every fifty years in a galaxy the size of the Milky Way a supernova will occur. To put this in perspective, a supernova happens once every second or so somewhere in the universe.  

This series of events happens when a red giant star has started the nuclear process and has a lot of gravity pulling it inward. The star normally doesn’t collapse inward because there is a lot of nuclear fusion happening inside it; this forces the energy outward. When the fusion stops, the force of gravity pulls the star inward. When the inner shell hits the iron core, a huge shock wave is created, and the star explodes. It has gone supernova. 

Exactly how a star dies depends on its mass. For instance, our sun doesn’t have enough mass to explode as a supernova. That’s good news and bad news for Earth. When our sun does die—in a few billion years—it will swell into a red giant star that will vaporize Earth and all the planets around it. After it destroys most of our solar system it will gradually cool into a white dwarf.

Astronomers have been observing supernovas for quite some time. The Chinese have records dating back to 1054 for one occurring in the constellation we call Taurus. Tycho Brahe and Johannes Kepler observed bright supernovas occurring in the Milky Way Galaxy in 1572 and again in 1604. Today we can see remnants of all three of those supernovas, which appear as expanding clouds of gas where each one was originally discovered.

Although astronomers are observing, studying, and cataloging the occurrences of supernovas, there has not been an occurrence in the Milky Way since 1604. Do you think the Milky Way is due for another occurrence? Are the supergiant red stars, Betelgeuse in Orion (about 600 light years away) and Antares in Scorpius (about 550 light years away) ready to go supernova?  

Visit the Chandra X-Ray Images site for images of some incredible supernovas: 

My sources:,, Science for Kids,,, and national geographic science.