We tend to think of stars as being very, very hot, but what about a star that you could use as a deep freeze? Astronomers using NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope have discovered a dim star-like object with the catchy title WISE J085510.83-071442.5.0 that is colder than the North Pole. Lying about 7.2 light years from Earth, it is not only the coldest brown dwarf on record, but the fourth-closest star system to the Solar System.
A brown dwarf is a substellar object between about 13 and 80 times the size of Jupiter. That may seem large, but it’s still not enough mass to start a fusion reaction, so they tend to be on the cool side, giving off only infrared light. However, brown dwarfs aren't brown. A red dwarf is a small star that can sustain nuclear fusion, giving off a reddish light, while a black dwarf is the name given to white dwarf stars that can no longer sustain fusion and have gone cold. The name "brown dwarf" was coined because brown was suggested as being halfway between red and black.
WISE J085510.83-071442.5.0 is the coldest brown dwarf yet found with a temperature between -54 and 9⁰ F (-48 to -13⁰ C). According to NASA, the previous record holders were “about room temperature.” It’s 3 to 10 times the size of Jupiter, which means that it could be a gas giant planet floating in interstellar space, but NASA says that since brown dwarfs are very common, scientists regard it as one of the smallest brown dwarfs found.
The object was first discovered by the WISE space telescope, which is designed to work in the infrared band of the spectrum. A cold brown dwarf like WISE J085510.83-071442.5.0 would normally be very difficult to see, but being only 7.2 light years away and floating in interstellar space made it stand out. Being so close also means that it seems to move across the sky very quickly, which caught the attention of Kevin Luhman, an astronomer at Pennsylvania State University's Center for Exoplanets and Habitable Worlds, University Park, in March 2013
"This object appeared to move really fast in the WISE data," says Luhman. "That told us it was something special."
Luhman combined the WISE data with images taken by the Spitzer space telescope and the Gemini South telescope on Cerro Pachon in Chile, which allowed him to determine the star’s temperature and distance by measuring its parallax. That is, how much the star shifts across the sky as the telescopes orbit around the Sun.
"It's very exciting to discover a new neighbor of our solar system that is so close," says Luhman. "And given its extreme temperature, it should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures."
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