White Dwarfs

As we have seen in the preceding discussion of red giant evolution, red giants will eventually consume all their nuclear fuel. After ejection of the envelope, the cores of these stars shrink to the very hot, very dense objects that we call white dwarfs. Technically, white dwarfs are not stars, because their energy comes from gravitational contraction and radiation of leftover heat, not fusion reactions.

The Age of M4

The globular cluster M4 is very old. Typical estimates for its age give about 14 billion years. For a cluster this old, all stars with mass more than about 0.8 solar masses in the cluster should have evolved through the red giant to the white dwarf stage by now (except for the few most massive stars, which would have exploded as supernovae early in the life of the cluster).

White Dwarfs in M4
The following figure shows a high density of white dwarfs found near the center of the globular cluster M4, which is the nearest globular cluster to Earth at a distance of 7000 light years in the constellation Scorpius. An image of M4 taken with a ground-based telescope is shown in the left frame and the right frame shows the blown up region marked by the lines, as imaged by Hubble. The white dwarfs in the field are circled; as expected, they are quite faint compared with the other stars. Even the brightest white dwarfs in this image are as faint as a 100 Watt lightbulb at the distance of the Moon. Although this is an unusual concentration of white dwarfs, we expect to find many of them in old clusters because they represent the endpoint of stellar evolution for solar-mass stars (see the box).

Sirius B
The first white dwarf star to be discovered was the faint companion of the bright star Sirius, in Canis Major. The companion star is known as Sirius B, and although not especially faint (visual magnitude 8.5, slightly fainter than Neptune), it is difficult to observe because it is so close to Sirius A, which is the brightest star in Earth's sky. Sirius B was first observed by the American astronomer Alvan Clark in 1862, but its presence was inferred by the German astronomer and mathematician Friedrich Wilhelm Bessel in 1834 (see the right panel).