00:00Astronomers using NASA's Hubble Space Telescope have found a rare, ultra-massive white dwarf
00:06formed from merging stars.
00:09The discovery was made possible by Hubble's sensitive ultraviolet observations and suggests
00:14these unusual white dwarfs may be more common than once thought.
00:19The white dwarf is 128 light-years away and 20% more massive than the Sun.
00:24In visible light, it looked like a typical white dwarf, but Hubble's ultraviolet data
00:29revealed something unusual, evidence of carbon in its atmosphere.
00:34A white dwarf is what stars like the Sun become after they exhaust their nuclear fuel.
00:40Near the end of its life, this type of star expels most of its outer material, creating
00:45a planetary nebula.
00:47Only the hot core of the star remains.
00:49White dwarfs that form from a single star usually have atmospheres made of hydrogen and helium.
00:56These thick layers cover its carbon and oxygen, or oxygen and neon, surface, preventing their
01:02detection in the star's spectrum.
01:04When carbon shows up in a white dwarf spectrum, it can point to a more violent origin, such
01:10as a collision between two white dwarfs or between a white dwarf and a subgiant star.
01:14These collisions can burn off most of the hydrogen and helium, leaving only a thin layer behind,
01:21allowing astronomers to detect carbon on the white dwarf's surface.
01:25This white dwarf's merger origin would be impossible to identify without Hubble's ultraviolet sensitivity.
01:32In hotter white dwarfs, spectral lines from elements heavier than helium fade in visible light,
01:38and stay bright and ultraviolet, where Hubble can detect them.
01:42Researchers discovered this white dwarf's unusual origin by looking at its ultraviolet spectrum,
01:48suggesting that other white dwarfs with a typical visible light spectrum may also be the result
01:54of cosmic collisions.
01:55The team plans to explore this possibility further, and hopes to continue this research by studying
02:02how common carbon stars are among similar white dwarfs, and how many stellar mergers might
02:08be hiding among the normal white dwarf family.
02:11This research will help improve our understanding of the environments in which stars evolve and
02:16the paths that lead to supernova explosions.
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