Exposed. H1504+65 is the hottest white dwarf on record--and the only known star lacking helium and hydrogen.

Hot Star Bares All

Like a celestial exhibitionist, a star in the constellation Draco the Dragon has thrown off its mantle to expose its bare core, and it has taken astronomers aback. The surface of the star, known as H1504+65, is a sizzling 200,000 kelvin, making it the hottest white dwarf ever recorded, recent NASA satellite data reveal. What's more, the stellar core is almost entirely carbon and oxygen, with no trace of hydrogen and helium--the main constituents of stars. H1504+65 is the first known star that lacks helium altogether.

White dwarfs are the slowly cooling cores of sunlike stars in which nuclear fusion has stopped. Given its high temperature, H1504+65 must have shut down its fusion reactions very recently. According to Klaus Werner of the University of Tübingen, Germany, this may have happened as recently as a couple of centuries ago, in a process lasting at most a few decades. Werner led the team that observed the star with the Far Ultraviolet Spectroscopic Explorer and the Chandra X-ray Observatory.

But the team, which describes its findings in the July issue of Astronomy & Astrophysics, can't explain all the star's quirks, especially its absence of helium. Werner thinks the star's helium atmosphere may have been blown away by a sudden thermonuclear ignition of carbon--a theoretical possibility for relatively massive stars. Werner says such a flash could account for the surprising presence of magnesium in the star's outer layers. Magnesium is a by-product of carbon ignition, but it can also result from the nuclear burning of helium.

Other astronomers agree that the star is a real space oddity. The absence of hydrogen in a white dwarf's atmosphere is strange enough, says Gijs Nelemans of the University of Cambridge, U.K., but "finding no trace of helium is even crazier." However, Nelemans isn't so sure about the carbon flash scenario, which he says would require "rather extreme circumstances." Future observations of H1504+65 with the ultraviolet spectrograph on board the Hubble Space Telescope may settle the matter. Werner and colleagues hope to scan the spectra for the telltale presence of sodium, a product of a carbon flash that cannot be produced by helium burning.

Related sites
Abstract of paper, with link to full text
White dwarf primer
Far Ultraviolet Spectroscopic Explorer
Chandra X-ray Observatory