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Astronomers using NASA’s Hubble Space Telescope

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have found a rare ultramassive white dwarf

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formed from merging stars.

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The discovery was made possible by Hubble’s 

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sensitive ultraviolet observations

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and suggests these unusual white dwarfs

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may be more common than once thought.

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The white dwarf is 128 light-years away

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and 20 percent more massive than the Sun.

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In visible light it looked like a typical white dwarf,

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but Hubble’s ultraviolet data revealed something unusual:

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evidence of carbon in its atmosphere.

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A white dwarf is what stars like the Sun become

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after they exhaust their nuclear fuel.

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Near the end of its life,

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this type of star expels most 
of its outer material,

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creating a planetary nebula,

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only the hot core of the star remains.

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White dwarfs that form from a single star usually

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have atmospheres made of hydrogen and helium.

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These thick layers cover its carbon and oxygen,

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or oxygen and neon surface,

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preventing their detection in the star’s spectrum.

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When carbon shows up in a white dwarf’s spectrum,

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it can point to a more violent origin,

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such as a collision between two white dwarfs

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or between a white dwarf and a subgiant star. 

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These collisions can burn off

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most of the hydrogen and helium,

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leaving only a thin layer behind,

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allowing astronomers to detect carbon

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on the white dwarf’s surface.

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This white dwarf’s merger origin would be

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impossible to identify without Hubble’s 
ultraviolet sensitivity.

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In hotter white dwarfs, spectral lines from

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elements heavier than 
helium fade in visible light

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but stay bright in ultraviolet,

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where Hubble can detect them.

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Researchers discovered this white dwarf’s

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unusual origin by looking at its ultraviolet spectrum,

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suggesting that other white dwarfs

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with a typical visible light spectrum

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may also be the result of cosmic collisions.

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The team plans to explore this possibility further,

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and hopes to continue this research by

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studying how common carbon stars are

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among similar white dwarfs

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and how many stellar mergers might be hiding

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among the normal white dwarf family.

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This research will help improve our understanding of  

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the environments in which stars evolve

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and the paths that lead to supernova explosions.

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