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Baby star near the black hole in the middle of our Milky Way: It exists after all
An international team of researchers under the leadership of Dr Florian Peißker at the University of Cologne's Institute of Astrophysics has discovered a very young star in its formation phase near the supermassive black hole Sagittarius A* (Sgr A*) at the centre of our Milky Way. The star is only several tens of thousands of years old, making it younger than humanity. The special thing about baby star X3a is that theoretically it should not be able to exist so close to the supermassive black hole in the first place. However, the team believes that it formed in a dust cloud orbiting the giant black hole and sank to its current orbit only after it had formed. The study "X3: a high-mass Young Stellar Object close to the supermassive black hole Sgr A*" has been published in The Astrophysical Journal.
The vicinity of the black hole at the centre of our Galaxy is generally considered to be a region characterized by highly dynamic processes and hard X-ray and UV radiation. Precisely these conditions act against the formation of stars like our Sun. Therefore, for a long time scientists had assumed that over periods of billions of years, only old, evolved stars can settle by dynamical friction in the vicinity of the supermassive black hole. However, quite surprisingly, already twenty years ago very young stars were found in the immediate vicinity of Sgr A*. It is still not clear how these stars got there or where they formed. The occurrence of very young stars very close to the supermassive black hole has been referred to as "the paradox of youth."
The baby star X3a -- which is ten times as big and fifteen times as heavy as our Sun -- could now close the gap between star formation and the young stars in the immediate vicinity of Sgr A*. X3a needs special conditions to form in the immediate vicinity of the black hole. First author Dr Florian Peißker explained: "It turns out that there is a region at a distance of a few light years from the black hole which fulfils the conditions for star formation. This region, a ring of gas and dust, is sufficiently cold and shielded against destructive radiation." Low temperatures and high densities create an environment in which clouds of hundreds of solar masses can form. These clouds can in principle move very fast towards the direction of the black hole due to cloud-cloud collisions and scattering that remove the angular momentum.
In addition, very hot clumps formed in close proximity to the baby star which could then be accreted by X3a. These clumps could thus also contribute to X3a reaching such a high mass in the first place. However, these clumps are only a part of the formation history of X3a. They still do not explain its "birth."
The scientists assume the following scenario to be possible: shielded from the gravitational influence of Sgr A* and intense radiation, a dense enough cloud could have formed in the outer gas and dust ring around the centre of the Galaxy. This cloud had a mass of about one hundred suns and collapsed under its own gravity to one or more protostars. "This so-called fall time approximately corresponds to the age of X3a," Peißker added. Observations have shown that there are many of these clouds that can interact with each other. It is therefore likely that a cloud falls towards the black hole from time to time.
This scenario would also fit X3a's stellar development phase, which is currently evolving into a mature star. It is therefore quite plausible that the gas and dust ring acts as the birthplace of the young stars in the centre of our Galaxy. Dr Michal Zaja?ek at Masaryk University in Brno (Czech Republic), a co-author of the study, clarified: "With its high mass of about ten times the Solar mass, X3a is a giant among stars, and these giants evolve very quickly towards maturity. We have been lucky to spot the massive star in the midst of the comet-shaped circumstellar envelope. Subsequently, we identified key features associated with a young age, such as the compact circumstellar envelope rotating around it."
Since similar dust and gas rings can be found in other galaxies, the described mechanism could apply there as well. Many galaxies can therefore host very young stars in their very centres. Planned observations with NASA's James Webb Space Telescope or the European Southern Observatory's Extremely Large Telescope in Chile will test this star formation model for our Galaxy as well as others.
注释：
Dynamical：adj
表示“动力的；有生气的 ”，means "dynamic;characterized by action or forcefulness or force of personality “，如：Its dynamical capability is not good enough. 它的动力性能过不了关。
Clump: n
表示“ 丛；块 ”，means "a grouping of a number of similar things “，如：held a bit of the hearty earth in my hand to form a hard clump and added it to the bowl.我用手捧起一点肥沃的土壤，并使其成为硬实的一整块，把它放进碗里。
Accrete：v
表示“ 共生 ”，means "grow together (of plants and organs); “，如：The accrete effects of concrete slab, wood board, iron plate and plastic board were better than other material. 比较不同试验材料的生物附着效果,混凝土板、木板、铁板和塑料板的附着效果较好。
gravitational: adj
表示“重力的 ”，means "of or relating to or caused by gravitation “，如：It is a spontaneous gravitational instability. 原因在于其自发的重力不稳定性。
protostar: n
表示“原恒星 ”，如：Thus, the infant science of protostar observation has a variety of tools at its disposal. 这样一来，观测原恒星这门年轻的科学，就拥有许多种可以利用的工具了。
plausible: adj
表示“似真实合理的；动听的；”，means "seeming to be true or reasonable “，如：Such a theory seems very plausible. 这一理论貌似十分有理。
circumstellar: adj
表示“ 环绕恒星的”，如：Another expectation, which Hubble overturned, was that the circumstellar disks would be deeply embedded within their parent clouds and would therefore be impossible to see.另外有一个被哈伯推翻的猜测，是因为环绕恒星的气盘深藏在其母云气之中，所以无从窥其究竟。
中文简要说明：
由科隆大学天文物理研究所（University of Cologne's Institute of Astrophysics）费斯克（Florian Peißker）博士领导的一个国际研究小组在我们的银河系（Milky Way）中心的人马座A*（Sagittarius A*）超大质量黑洞附，发现了一颗处于形成阶段的非常年轻的恒星，它甚至比我们人类还要年轻。
据美国《每日科学》（ScienceDaily）2月28日报导，这项研究 《X3：靠近超大质量黑洞人马座A*的高质量年轻恒星》现已经发表在《天文物理期刊》（The Astrophysical Journal）上。这颗恒星目前被称为X3a，它只有几万年的历史，这比我们人类（智人）还要年轻。婴儿星X3a（Baby Star）的特殊之处在于，理论上它不应该能够存在于这么靠近超大质量黑洞的地方。
不过，研究小组认为，它是围绕在巨大黑洞外的尘埃云（Dust Cloud）所形成的，并且在形成之后才沉降到目前的轨道上。
我们银河系中心的黑洞附近通常被认为是一个极度活跃的区域，并不利于像我们太阳这样的恒星的形成。因此长久以来，科学家们认为在数十亿年的时间里，只有一些古老的的恒星才可能因为一些机缘巧合被吸引到超大质量黑洞附近轨道运行。
然而，相当令人惊讶的是，早在20年前科学家就已经在人马座A*附近发现了非常年轻的恒星。目前还不清楚这些恒星是如何到达那里的，或者它们是在哪里形成的。非常靠近超大质量黑洞的非常年轻的恒星的出现，被称为「年轻悖论」（the paradox of youth）
婴儿恒星X3a的体积是我们太阳的10倍大，质量则是太阳的15倍。X3a需要特殊条件才能在黑洞附近形成。费斯克博士解释说。「事实证明，在距离黑洞几光年的地方，有一个区域满足了恒星形成的条件。这个区域是由气体和尘埃组成的环形区域，足够冷，并且屏蔽了吸积盘（accretion disk）极具破坏性的辐射。
低温且高密度创造出了得天独厚的一个环境，其中包含了可以形成数百个太阳的气体云。而云与云的碰撞和散射消除了角动量（angular momentum），这些气体云原则上可以非常快速地朝黑洞的方向移动。
此外，在靠近幼年恒星的地方形成了非常热的团块，然后可能被X3a吸纳。因此，这些团块也可能有助于X3a首先达到如此高的质量。然而，这些团块只是X3a形成历史的一部分。它们仍然不能解释它的「诞生」。
科学家们假设了以下的成因：「在恰巧避开人马座A*引力影响和强烈辐射的情况下，在银河系中心周围的外围气体和尘埃环中可能形成了足够稠密的云。这片云的质量约为一百个太阳，它们可以相互作用，并在其自身的引力下坍缩为一颗或多颗原星。这个所谓的坍缩时间大约与X3a的年龄相符，」费斯克博士补充说。观察表明，除了发生这种巧合之外，无时无刻都会有气体、尘埃云被黑洞拉走。
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