White Dwarf: Dead Star Shining

At the end of a star's lifetime, when it's burned all there is inside it to burn, it often collapses into what's called a white dwarf star. These dense globes have roughly the mass of the sun packed tight into a ball the size of the Earth. Since there is no fuel left for fusion in white dwarfs, they shine only faintly, by emitting stored heat. A white dwarf is thought to be the end stage awaiting most of the stars in our galactic neighborhood, including the sun — only about 3 percent of nearby stars have masses so huge that they collapse even further in supernova explosions. Here are some especially interesting white dwarfs:

One of the hottest white dwarfs known to scientists lies in the heart of this nebula, called NGC 2440, about 4,000 light-years from Earth. The star (the bright dot near the photo's center), has a surface temperature of roughly 360,000 degrees Fahrenheit. Since white dwarfs cool as they age, this one must be relatively young. The glowing purple aura around it is the left-over material cast off by the star when it became a white dwarf at the end of its life.

Image: NASA/R. Ciardullo (PSU)/H. Bond (STScI)

By Clara Moskowitz