科學家發現最冷恆星 表面溫度像地球夏天

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.據英國《每日郵報》10月21日報道，這顆星球表面的溫度就像是在亞利桑那州或是塞利維亞的夏天——非常宜人。這也是目前拍攝到的太陽系外溫度最低的星體。

但是WD 0806-661 B號星球並不是一顆行星——而是一顆非常小的恆星。它的質量僅僅是巨大的氣體行星木星的6-9倍。

美國賓夕法尼亞州立大學天文與天體物理學副教授凱文·盧邁發現了這一星體，他說：“這是一顆非常小的恆星，它的大氣溫度和地球的一樣涼爽。”

科學家們利用美國宇航局斯皮策太空望遠鏡——世界上靈敏度最高的太空望遠鏡——來尋找較冷的恆星，通過環繞的天體掃描到了600顆恆星。運用紅外望遠鏡是因爲寒冷的星體在圖像中會明亮地“閃耀”。這顆破紀錄的恆星距離地球6300萬光年，圍繞着一顆高密度的白矮星運動。

這顆恆星是褐矮星——像其他恆星一樣，外部由塵埃和氣體雲構成。

但是因爲它沒能通過塵埃雲累積足夠的質量，所以也沒有激起“點亮”一般恆星的核聚變。

這顆新發現的褐矮星地表溫度在27到80度之間——在這些地方，溫度很適宜人類居住。

褐矮星第一次被發現是在1995年，天文學家一直在試圖尋找最冷褐矮星的新紀錄保持者——這一星體被視爲是有價值的實驗點，可以幫助我們瞭解太陽系外與地球溫度類似的星球的大氣狀況。

天文學家將這顆“褐矮星”命名爲“WD 0806-661 B”號。它環繞着一顆恆星的白矮星內核運動。這顆白矮星就像太陽照射地球一樣照射這顆星球，直到它走到生命盡頭，外層逐漸冷卻。

“這顆白矮星距離太陽63光年，與銀河系中大多數星球相比，它距離我們的太陽系非常近。”盧邁說。

這顆褐矮星的運行軌道相當龐大，大約是地球到太陽距離的2500倍。