Hubble reveals weighty secrets of white dwarf

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Hubble reveals weighty secrets of white dwarf

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White dwarf

White dwarfs are the burned-out remains of stars like our Sun and tend to be small because they have exhausted their nuclear fuel, collapsing into dense embers that cool over billi

The Hubble Space Telescope has studied the nearest and brightest white dwarf in unprecedented detail, confirming the stellar ember is about as massive as the Sun but smaller than the Earth. The research should help astronomers understand how stars evolve.

White dwarfs are the burned-out remains of stars like our Sun and tend to be small because they have exhausted their nuclear fuel, collapsing into dense embers that cool over billions of years.

Because of their size, they can be difficult to study - particularly if their light is drowned out by that of a brighter companion. Astronomers have long battled this problem with the nearest white dwarf, which is 10,000 times fainter than its partner, Sirius - the brightest star in the sky. Both lie about 9 light years from Earth.

Now researchers led by Martin Barstow of the University of Leicester, UK, have analysed data taken in 2003 by Hubble's Space Telescope Imaging Spectrograph (STIS), separating and then analysing only the light from the white dwarf, called Sirius B.

Stretched light

The small star has a surface temperature of about 25,200 C - hotter than Sirius, which is about 10,500 C. It is also so dense that its gravity slows down the light leaving its surface, stretching it into longer, redder wavelengths. STIS measured this "gravitational redshift" to calculate the white dwarf's mass, which is about the same as the Sun's.

But all of that mass is squeezed into an ember just 12,000 kilometres across - slightly smaller than the Earth. The white dwarf is so densely packed that an average person standing on its surface would weigh the equivalent of 23,000 tonnes.

"Accurately determining the masses of white dwarfs is fundamentally important to understanding stellar evolution," says Barstow.

White dwarfs that consume matter from their companions can explode as type 1a supernovae, which are used as "standard candles" to measure cosmic distances. In 1988, these supernovae provided the evidence for the accelerating expansion of the universe when they were found to be dimmer than expected.

Journal reference: Monthly Notices of the Royal Astronomical Society (vol 362, p 1134)



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