Astronomers have discovered a mysterious stream of ancient stars at the distant edges of the galaxy: a strange stellar breed so unlike any we've seen before, they may very well be the last of their kind.
在銀河系遙遠邊緣，天文學家們業已發現一處，神秘的遠古恆星流：這是一種極不像任何人們，先前曾發現的奇怪恆星群，它們很可能是其同種類的最後一群。
This unusual collection of stars – called the 'Phoenix stream', after the Phoenix constellation in which they are visible – is what's known as a stellar stream: an elongated chain of stars that used to exist in a spherical form, known as a globular cluster.
該以其在鳳凰星座可見，被稱為'鳳凰流'的異常恆星聚集，是所謂的恆星流：這是先前以球體形狀存在，被通稱為球狀星團的細長恆星鏈。
圖1. 藝術家想像之包圍銀河系的恆星流 (圖援用自原文)
Such clusters can be torn asunder by a galaxy's gravitational forces, in which case their globular form becomes warped, stretching out into a ghostly caravan of stars, fated to distantly orbit a faraway galactic core.
此類星團會遭星系重力零亂地拉扯。在這種情況下，其球體形狀變得扭曲、伸展成一個，注定要在遠處，圍繞遙遠星系核心運行的幽靈般恆星旅隊。
Neither stellar streams nor globular clusters are new to science, but there's something about the Phoenix stream that is. Its chemistry is different to any globular cluster we've ever seen, almost like it doesn't belong here.
恆星流及球狀星團都不是科學新鮮事。不過，有關鳳凰流，那是有值得重視的事。其化學物質不同於，人們曾發現的任何球狀星團，幾乎就像它不屬於這裡。
"We can trace the lineage of stars by measuring the different types of chemical elements we detect in them, much like we can trace a person's connection to their ancestors through their DNA," explains astronomer Kyler Kuehn from the Lowell Observatory in Arizona.
來自美國亞利桑那州，洛厄爾天文台的天文學家，Kyler Kuehn解釋：「藉由測量恆星中，發現之不同種類的化學元素，能探究恆星所屬種系。這極像透過DNA，能追蹤一個人與其祖先的關係。」
There are around 150 known globular clusters in the Milky Way, all of which exist in what's called the galactic halo – a tenuous spherical structure that envelops the relatively flat galactic disk, where most of a galaxy's stars otherwise congregate.
在銀河系中，大約有150個已知的球狀星團，全數存在於所謂的星系暈中。這是一種包圍相對平坦之星系盤的稀薄球形結構物，在此大多數的星系恆星，以另外方式聚集。
Out in the fringes of the halo, though, there are still plenty of stars assembled inside the globular clusters. Each cluster can contain hundreds of thousands of stars, and observations of the clusters in the Milky Way have shown that all clusters demonstrate a certain consistency in their stellar chemistry: the stars in the clusters are enriched with 'heavier' chemical elements that are more massive than hydrogen and helium.
不過，在此暈外圍，仍然有大量聚集於球狀星團內部的恆星。每一星團能具有數十萬顆恆星，且銀河系中的星團觀測已經證實，所有星團在其恆星的化學物質上，皆顯示了一定的一致性：在此些星團中的恆星，富含比氫及氦更大質量的'較重'化學元素。
After the Big Bang, theory holds that all the gas in the Universe was made up of either hydrogen or helium, which in turn formed the Universe's first stars. Other elements, such as oxygen, carbon, and magnesium, only became possible much later via the fusion mechanisms of subsequent generations of stars.
理論認為，在宇宙大爆炸之後，宇宙中的所有氣體是由，依序形成宇宙最早恆星的氫或氦組成。諸如氧、碳及鎂等其他元素，很久之後經由隨後數代恆星的融合機制才成為可能。
The chemical legacy of those later fusion mechanisms is all around us, as a certain proportion of heavier elements has been observed in all our galaxy's globular customers. That is, until now. This chemical threshold – called the metallicity floor – is not obeyed by the Phoenix stream, which demonstrates less heavy elements in its stars than we thought was theoretically possible for such a celestial structure.
後來那些融合機制的化學遺產物，全在咱們身邊。因為，在咱們所有銀河系的球狀星團中，皆曾被觀測到一定比例的較重元素。也就是說，直到目前。鳳凰流(顯示在其恆星中，重元素比咱們認為，理論上此類天體結構物可能有的還少)並未依循，這種被稱為金屬性底限的化學門檻。
"This stream comes from a cluster that, by our understanding, shouldn't have existed," explains astronomer Daniel Zucker from Macquarie University in Australia. Or at least, it shouldn't exist now, could be another way of putting it.
來自澳大利亞麥格理大學的天文學家，Daniel Zucker解釋：「根據他們的瞭解，此恆星流來自一處，不應該曾存在的星團。或至少(其目前不應該存在)可能是來自另一種產生的方式。」
Observations of the Phoenix stream conducted by an international team of researchers as part of the Southern Stellar Stream Spectroscopic Survey Collaboration have revealed that its "metal abundance is substantially below the empirical metallicity floor", the authors explain in their new study.
此些撰文者們，在他們的新研究中解釋，該由一支國際研究人員團隊所進行，作為南方恒星流光譜勘測合作(這是暗能量勘測(DES： Dark Energy Survey)銀河系研究團隊成員與一支澳大利亞天文學家團隊之間的一項合作)一部分的鳳凰流諸多觀測，已經揭露其"金屬豐度事，實上低於經驗上的金屬性底限"。
Up until now, the metallicity floor was a useful way of classifying a scientific constant seen across all present-day globular clusters. It still is, as it happens – but the Phoenix stream is no present-day globular cluster.
到目前為止，該金屬性底限是分類，在當前所有球狀星團中，所發現科學常數的一種有效方法。碰巧，這仍然是。不過，鳳凰流不是當今的球狀星團。
The team thinks it may be a sole survivor: a celestial relic of a bygone age in the early Universe, when stars made their light in different ways.
該團隊認為，它可能是唯一倖存者：也就是，它是在初期宇宙中，一個消逝之時期(當恆星以不同方式產生光的時)的天體殘餘物。
"One possible explanation is that the Phoenix stream represents the last of its kind, the remnant of a population of globular clusters that was born in radically different environments to those we see today," says astronomer Ting Li from Carnegie Observatories in Pasadena.
來自卡內基天文台(位於美國加州帕薩迪納市)的天文學家，Ting Li宣稱：「一種可能的解釋是，鳳凰流代表了，其種類的最後恆星流。也就是，在與咱們目前所看到，完全不同的環境中，一群球狀星團遭到壓迫的殘餘物。」
Lots of questions remain, of course. If the Phoenix stream is a remnant of a relic from the early Universe, is it the only one? Do others also exist, hidden in the vastness of the galactic halo?
當然，仍有很多問題。倘若此鳳凰流是來自，早期宇宙一種遺物的殘餘物，這是唯一的？也存在其他隱藏於，此浩瀚銀河之星系暈中的殘餘物？
"In astronomy, when we find a new kind of object, it suggests that there are more of them out there," says astronomer Jeffrey Simpson from the University of New South Wales (UNSW) in Australia.
來自澳大利亞新南威爾斯大學(UNSW)的天文學家，Jeffrey Simpson宣稱：「在天文學中，當人們發現新種類的天體時，這暗示，在那裡有更多的那種天體。」
If other old travellers are still on the trail, we don't have forever to find them. Like globular clusters, stellar streams are not immortal things. Once they're stretched out into a string of stars, it's only a matter of time before they disband, and disperse throughout the galaxy.
倘若在此蹤跡上，仍有其他古老的移動恆星群，則人們無需不斷地尋找它們。與球狀星團一樣，恆星流並非不朽的天體。一旦它們伸展成一串恆星，在它們解體、分散在整個銀河系之前，這只是時間問題。
"Who knows how many relics like the Phoenix stream might be hiding in the Milky Way's halo?" wonders German astronomer J. M. Diederik Kruijssen from Heidelberg University, who wasn't involved in the study but has authored a commentary on it. "Now that the first one has been found, the hunt is on."
德國海德堡大學，未涉及、不過已經針對該項研究，撰寫一篇評論的天文學家，J. M. Diederik Kruijssen懷疑，「誰能知曉多少類似此鳳凰流的遺物，可能隱藏於銀河的星系暈中？既然已經發現第一個，就開始搜尋吧！」
The findings are reported in Nature.
此些研究發現發表於《自然》期刊。
原文網址：https://www.sciencealert.com/mysterious-stellar-steam-unlike-any-we-ve-seen-may-be-lone-relic-of-early-universe
翻譯：許東榮
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