Star Deaths - the Red Giants

by Jeanette Cain

Nothing lives forever, not even a star. As the hydrogen in a star lessens and the nuclear reactions inside the star's core begin to slow, the star will balloon out to create a red giant. This red giant may become more than a hundred times its original diameter. Those that can be seen in intergalactic space are the more massive stars that have become supergiants.

Nuclear reactions provide the heat source in red giants, but when the core shrinks to only one-tenth its normal size, it may be no larger than our planet of Earth. Energy is produced within the core from the gigantic pressures and temperatures to fuse helium and create the heavier elements, such as oxygen and helium. There remains a thin shell of hydrogen on the outside of the core that will continue to make helium.

Supergiants are those stars that have over eight times the mass of the Sun, and like the giants, the fusion of helium is their source of energy. Giant stars become giants when the hydrogen in their core is exhausted. The star will swell, cool, and move away from its main sequence. The stars with lower mass will become much brighter and head to the red giant region. The brightness of the high mass stars will be almost the same as the lower mass stars, but they will move into the supergiant region. The giant star's color will depend on the temperature of its surface. They may be red, yellow, white, or blue, which is the hottest. The outer atmosphere of the giant star may drift into space as a stellar wind, which can travel many light-years. A cloud of dust and gas given off by a giant star is the Toby Jug Nebula.

The most well-known giants include Aldebaran in the constellation of Taurus, and Arcturus in Bootes. Two giants that orbit one another is Capella in Auriga. Among the supergiants are Canopus in Carina, Rigel and Betelgeuse in the constellation of Orion, Deneb in Cygnus, and Antares in Scorpius. The Garnet Star, a.k.a. Mu Cephei, is one of the largest supergiants. Red giant sizes vary, but the normal star could become up to two hundred times larger than the Sun's diameter. As helium begins burning, the star will shrink between ten and one hundred times the Sun's diameter. The supergiants can become even larger, as the red supergiant Mu Cephei that is more than Saturn's orbit.

Earth's Sun will run out of hydrogen in about five billion years, but by then it will already be brighter than it is now. When it begins to turn into a red giant, it will expand so much that Mercury may be swallowed up and the sun will glow one thousand times brighter than it does now. The Sun will become stable when helium fusion begins, and for around two billion years it will exist as a giant star.

Sources:

1. Couper, Heather and Nigel Henbest. Space Encyclopedia DK Publishing, Inc.: NY 1999

2. Editors. Secrets of the Universe. International Master Publishing: US. 1999