科学家发现最冷恒星 表面温度像地球夏天

　　The temperature of the surface is like a hot summer's day in Arizona or Seville - and would be quite pleasant for humans. It's the coldest object ever photographed outside our solar system.

　　But WD 0806-661 B is not a planet - it's a very small star. Its mass is just six to nine times the gas giant Jupiter.

　　Penn State Associate Professor of Astronomy and Astrophysics Kevin Luhman, who discovered the object said, "It is a very small star with an atmospheric temperature about as cool as the Earth's."

　　The astronomers used Nasa's Spitzer space telescope - the most sensitive space telescope on Earth - to find the cold star, scanning 600 near-earth stars for objects orbiting them. Infrared telescopes are used because cold objects "shine out" brightly in the images. The record-breaking star is 63 million light years from Earth and orbits a dense, collapsed "white dwarf" star.

　　The star is a brown dwarf - and formed, like other stars, out of a cloud of dust and gas.

　　But because it failed to accumulate enough mass from the dust cloud, the thermonuclear reaction that "lights up" normal stars fail to ignite.

　　The new brown dwarf's surface is between 27 and 80 degrees centigrade - so in places, it's a temperature humans might enjoy.

　　Ever since brown dwarfs first were discovered in 1995, astronomers have been trying to find new record holders for the coldest brown dwarfs - the objects are seen as valuable laboratories to help us understand the atmospheres of extrasolar planets with Earth-like temperatures.

　　Astronomers have named the brown dwarf "WD 0806-661 B". It orbits the "white dwarf" core of a star . The white dwarf would have shone like our sun until its outer layers were expelled into space in the final stages of a star's life.

　　"The distance of this white dwarf from the Sun is 63 light-years, which is very near our solar system compared with most stars in our galaxy", Luhman said.

　　The orbit of the brown dwarf is huge, about 2,500 times the distance between Earth and the sun.