為什麼恆星周圍的星塵三年內全部消失

06-26

Why did all the planet-forming dust around this star disappear in only three years?

為什麼恆星周圍的星塵三年內全部消失

Back in 1983, astronomers discovered a star 450 light-years away that was surrounded by a cloud of dust — a textbook example of a solar system in the making. Now, nearly thirty years later, the cloud has suddenly and mysteriously disappeared.

在1983年，天文學家發現了一顆被星塵環繞距離我們450光年的恆星，這顆恆星可以作為太陽系如何形成的教科書。現在，幾乎三十年過去了，恆星周圍的星塵突然神秘的消失了。

This discovery could mean that everything we thought we knew about the birth of planets is wrong.

這項發現可能會否定我們所有對於行星誕生的感知。

This is one of those observations that, on the face of it, doesn"t make a lot of sense. Astronomers have always believed that it takes hundreds of thousands of years — if not millions — for the dust orbiting a young star to congeal and form planets. This recent observation, however, indicates that planets may form significantly faster than we thought. Or alternatively, that these dust clouds behave in ways unknown to us.

雖然這項發現從某種角度來說沒有什麼意義。天文學家總是相信要形成一顆行星，需要星塵在其周圍環繞上百年才可以形成。但是這最近的發現卻暗示行星可能超出我們想像的速度形成。換句話說，這些塵雲以我們不知道的方式運動著。

Making sure the data is correct

確認數據正確。

To confirm and cross-reference this strange discovery, researchers were pooled from a number of institutions, including universities from Georgia, San Diego, Los Angeles, California-Polytechnic, and Australia. This was a team effort.

要確認並互相參照這項發現，研究人員被從各項機構里選出包括喬治亞、聖迭戈、洛杉磯、澳大利亞、加州理工大學。這需要團隊合作。

What troubled these scientists was that the cloud of dust circling a star in the Scorpius-Centaurus stellar nursery simply disappeared within the span of three years.

困惑這些科學家的是來自天蠍星系中一顆行星被發現其周圍的星塵子在三年內消失不見。

The researchers used data from a wide array of sources, including a mid-infrared imager at the Gemini South Observatory in Chile, NASA"s Wide-field Infrared Survey Explorer, the Japanese AKARI telescope, and the European Space Agency"s Photodetector Array Camera and Spectrometer. Looking at all the data provided by these instruments, the scientists could not help but conclude that, from the period 2008 to 2009 the infrared emissions dropped by nearly two-thirds (indicating a dramatic decrease in dust), and that by 2010 the dust cloud had all but vanished.

調查人員運用了各種途徑的數據，包括來自智利的南雙子星天文台的中紅外線成像器，美國航天局的廣角紅外線探測研究器，日本的星野望遠鏡和歐洲空間局的探測器陣列相機和光譜。仔細研究這些儀器提供的數據，科學家不得不承認從2008到2009年紅外線輻射下降了近二分之三（代表星塵的銳減），然後到2010年所有的星塵就此消失。

Okay, so what happened to all the dust?

好吧。那到底發生了什麼？

With the data largely confirmed, the scientists were forced to come up with theories explaining the anomaly.

由於這些確鑿的證據，科學家們不得不為此提供一個解釋。

One suggestion, as absurd as it sounds, is that it doesn"t take long for planets to form given certain conditions — what is dubbed "runaway accretion." In such a scenario, the growth of dust and rocks snowballs quickly, leading to the rapid, nearly instantaneous formation of planets. If true, this would completely reset our notions of just how quick and easy it is for planets to form around a star.

其中一個荒謬的假設是在某些特定的條件下行星可以迅速形成，因此被稱作「失控吸積」。在這種情況下，星塵的成長和石頭的形成很快，導致行星迅速形成。如果這是真的，那麼這大概會徹底重置我們對於星球形成的速度的觀念。

But three years? Really? It would be nice to see computer models prove this idea. Moreover, if this theory is true, a new planet should be orbiting this star. Hopefully, using more advanced telescopes than we have now, we might someday be able to find it.

但是真的只需三年嗎？希望電腦模型能證明這一點。進一步來講，如果這項理論是真實的話，一個新的行星需要來環繞這個恆星。希望今後運用更加先進的望遠鏡我們能發現它們。

Another possibility is what the scientists call "collisional avalanche." This theory suggests that the dust was expelled from the sun"s orbit. Because these dust particles are so small — smaller than a grain of sand — the constant stream of photons emanating from the sun could push them away and into each other until they leave the sun"s orbit. The problem with this theory, however, is that it makes planetary formation sound rare and difficult, which we know it certainly is not.

另一個可能的假設是被科學家們稱作「碰撞雪崩」。這項理論說明星塵可能被從太陽運動軌道中拋出。由於這些顆粒太小，比普通的沙子還要小，從太陽中釋放出來的質子流可以使它們相互碰撞直到它們離開太陽的軌道。但這項理論的問題是，這使得行星的形成變得奇怪和困難。我們知道行星的形成不可能這樣。

And yet another theory suggests that the sun swallowed all the dust in one gigantic gulp. Again, another unsatisfying answer. As Nola Taylor-Redd noted in a recent Space.com article, "[This] theory, known as the collisional cascade model, would likely require time scales longer than two years to clear the dust from the system."

然後另一項理論表示這些星塵被太陽吸收了。然而再次這不是個滿意的答案。就像 Nola Taylor-Redd在最近一篇space.con上發表的文章中提到的那樣「我認為這個被稱作碰撞瀑布模型的理論，需要至少兩年的時間才可能將星塵從系統中清除。

Clearly more work needs to be done, and the researchers are planning their next move. Specifically, they want to analyze old observations like the one made in 1983 and compare them to new data pulled from modern telescopic techniques, to make a systematic search for other stars that may have recently gone through a similar phase.

很明顯更多的工作需要做，研究人員現在正在計划下一步的工作。具體的，他們希望能分析在1983年所做的研究並將他們和新的來自現代望遠鏡的數據進行對比，做個對其他有著相似周期恆星的系統性研究。

These results were published today (July 5) in the journal Nature.

這些結果在今天（7.5）的自然雜誌上刊登。

Sources: Daily Mail, Space.com.

參考文獻：每日郵報 space.com