: Image: NASA/R. Ciardullo (PSU)/H. Bond (STScI)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:
Left: Blazing Young Dwarf
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, ESA, H. Bond (STScI) and M. Barstow (University of Leicester)The blazing blue star dominating this photo is not a white dwarf, but the faint white dot to the lower left is. The two stars are called Sirius A and Sirius B, respectively, and form a binary system. Sirius the Smaller is the nearest white dwarf to Earth (at a mere 8.6 light-years distant), so offers a particularly intriguing opportunity for research, as long as astronomers can separate its light out from the dominating brightness of its companion. Sirius B is smaller than the Earth, but with a mass many times greater, giving it a gravitational field about 350,000 times stronger than our planet's. Thus, if a 150-pound person were to stand on the dwarf's surface, he would weigh 50 million pounds.
: Image: : NASA/Tod Strohmayer (GSFC)/Dana Berry (Chandra X-Ray Observatory)This illustration shows the two white dwarf stars in the binary system J0806, which are frantically circling each other in an ever-quickening spiral. The two dwarfs orbit each other once every 321 seconds, and astronomers think this pace is speeding up, meaning they're destined to ultimately collide into each other and merge. The dense system, about 1,600 light-years away, is thought to be creating gravitational waves, or warps in the curvature of space-time, first predicted by Einstein's theory of general relativity.
: Credit: Casey ReedAnother strange binary system, called AE Aquarii, is composed of a normal star and a white dwarf. The smaller, denser white dwarf seems to be sucking material off its larger companion. While usually this would cause the dwarf to build up mass, the white dwarf in this case seems to be flinging the matter away from itself with great force. Astronomers think the white dwarf's rotation and strong magnetic field are behind the strange behavior, causing the star to propel a stream of material outward that emits a broad spectrum of radiation visible to us, 330 light-years across the galaxy.
: Image Credit: NASA/JPL-Caltech/T. Pyle (SSC)A white dwarf called G29-38 seems to be eating the comets orbiting around it, apparently leaving behind a cloud of shredded comet remains that were detected by NASA's Spitzer Space Telescope. The finding offered the first observational evidence that comets could outlive their suns. Scientists think G29-38 died and became a white dwarf about 500 million years ago, engulfing its inner planets as it did. The comets, however, which orbited farther out, may have survived. The dust Spitzer identified was probably created when a comet got knocked into the inner region of the system and was torn apart by the white dwarf's gravitational tides.
: Image: NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI), & T. A. Rector (NRAO)This photo of the Helix Nebula shows a star on its way toward becoming a white dwarf. The colorful rings of the nebula represent the cast-off outer gases of a star in the process of dying. The view of this star, close in mass to the sun, offers a glimpse of what our own parent star might look like one day as it nears the end of its life. The system lies about 650 light-years away toward the constellation Aquarius.
: Image: Garrelt Mellema (Leiden University) et al., HST, ESA, NASASpeeding winds send ripples of gas through the Red Spider Nebula, located about 3,000 light-years away in the constellation Sagittarius. The double-winged nebula houses a burning white dwarf inside it, whose stellar winds spew out at speeds of about 4 to 10 million miles per hour. These harsh winds send waves of gas through the nebula that can reach out 60 billion miles.
: Image: David A. Aguilar (CfA)The least-massive white dwarf known in our galaxy is about the size of Saturn, yet contains only one-fifth the mass of the Sun. This lightweight, designated SDSS J091709.55+463821.8, lies about 7,400 light-years from Earth. At first, astronomers were puzzled about how such a small white dwarf could form in the first place. Scientists now think the star used to be heavier, but a companion white dwarf star has been sucking mass off its sibling.
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