天文学家的一项最新研究，找到了银河系最亮恒星头衔的一个新的有力竞争者。相关论文将发表在《天文学与天体物理学》（Astronomy and Astrophysics）杂志上。
通过美国宇航局斯匹策空间望远镜（Spitzer Space Telescope）和其他地基望远镜，科学家重新认识了这颗26000光年外、位于银河系灰尘中心的恒星----“牡丹星云恒星”（Peony nebula star）。研究结果表明，它发出的光线量相当于320万个太阳。
到目前为止，银河系最亮恒星的桂冠戴在7500光年外一颗名为Eta Carina的恒星头上，而它的发光功率相当惊人，为470万个太阳。不过，研究人员认为，对于这些极热的恒星而言，要精确确定它们的亮度和光度很难，因此，随着测量精度的提高，牡丹星云恒星和Eta Carina有可能拥有相似的发光量。
领导该项研究的德国波茨坦大学天文学家Lidia Oskinova表示，“牡丹星云恒星是我们目前所知的银河系内第二明亮的恒星，它深藏在我们的星系中心。银河系内还可能存在着其他同样明亮甚至更加明亮的恒星，只是它们还躲着我们的视线。”
实际上，天文学家曾经对牡丹星云恒星有所认识，但由于它被灰尘厚厚地遮蔽起来，它的极端光度一直没有得到揭示。而NASA斯匹策望远镜和欧洲南方天文台新技术望远镜（New Technology Telescope）所具备的红外眼能够刺穿灰尘，直接看到可见光无法透过的银河系中心。
值得一提的是，宇宙中最明亮的恒星往往也是最重的。天文学家估计，牡丹星云恒星在其诞生之初，质量大约为太阳的150至200倍。这一质量的恒星比较罕见，也困扰着天文学家。因为根据理论预测，如果一个恒星诞生时过重，它将无法自我维持，必定会分裂成两个或者多个恒星。
除重量外，牡丹星云恒星还有很宽大的“腰身”。该恒星属于一种特殊类型的蓝巨星（giant blue star）----沃尔夫-拉叶星（Wolf-Rayet star），它的直径大约为太阳的100倍。这意味着如果把该恒星放在太阳的位置，那么它可以达到水星轨道。
研究人员认为，由于该恒星的质量太大，它在接下来的数百万年里，随时都有可能发生超新星爆炸。这将有助于他们理解这样的大质量恒星在银河系进化中的作用。Oskinova说，“当它爆炸时，将蒸发掉附近任何有行星在轨运动的恒星，而离该恒星更远的地方，爆炸将触发新的恒星诞生。”
由于在该恒星的周围发现了灰尘和气体云，看上去很像牡丹花，研究人员就将其称为牡丹星云。论文第一作者Andreas Barniske表示，“该星云可能是由漏散出该大质量恒星的尘雾形成的。”（科学网 任霄鹏/编译）
（《天文学与天体物理学》（Astronomy and Astrophysics），Doi : 10.1051/0004-6361:200809568，A. Barniske, L. M. Oskinova, and W.-R. Hamann）Brightest Star in the Galaxy Has New Competition
07.15.08
Peony nebula star The 'Peony nebula' star, circled, is now the second-brightest star in our galaxy. Image credit: NASA/JPL-Caltech/Potsdam Univ.
Nicknamed the "Peony nebula star," the bright stellar bulb was revealed by NASA's Spitzer Space Telescope and other ground-based telescopes. It blazes with the light of an estimated 3.2 million suns.
The reigning "brightest star" champion is Eta Carina, with a whopping solar wattage of 4.7 million suns. But according to astronomers, it's hard to pin down an exact brightness, or luminosity, for these scorching stars, so they could potentially shine with a similar amount of light.
"The Peony nebula star is a fascinating creature. It appears to be the second-brightest star that we now know of in the galaxy, and it's located deep into the galaxy's center," said Lidia Oskinova of Potsdam University in Germany. "There are probably other stars just as bright if not brighter in our galaxy that remain hidden from view." Oskinova is principal investigator for the research and second author of a paper appearing in a future issue of the journal Astronomy and Astrophysics.
Scientists already knew about the Peony nebula star, but because of its sheltered location in the dusty central hub of our galaxy, its extreme luminosity was not revealed until now. Spitzer's dust-piercing infrared eyes can see straight into the heart of our galaxy, into regions impenetrable by visible light. Likewise, infrared data from the European Southern Observatory's New Technology Telescope in Chile were integral in calculating the Peony nebula star's luminosity.
"Infrared astronomy opens extraordinary views into the environment of the central region of our galaxy," said Oskinova.
The brightest stars in the universe are also the biggest. Astronomers estimate the Peony nebula star kicked off its life with a hefty mass of roughly 150 to 200 times that of our sun. Stars this massive are rare and puzzle astronomers because they push the limits required for stars to form. Theory predicts that if a star starts out too massive, it can't hold itself together and must break into a double or multiple stars instead.
Not only is the Peony nebula star hefty, it also has a wide girth. It is a type of giant blue star called a Wolf-Rayet star, with a diameter roughly 100 times that of our sun. That means this star, if placed where our sun is, would extend out to about the orbit of Mercury.
With so much mass, the star barely keeps itself together. It sheds an enormous amount of stellar matter in the form of strong winds over its relatively short lifetime of a few million years. This matter is pushed so hard by strong radiation from the star that the winds speed up to about 1.6 million kilometers per hour (one million miles per hour) in only a few hours.
Ultimately, the Peony nebula star will blow up in a fantastic explosion of cosmic proportions called a supernova. In fact, Oskinova and her colleagues say that the star is ripe for exploding soon, which in astronomical terms mean anytime from now to millions of years from now.
"When this star blows up, it will evaporate any planets orbiting stars in the vicinity," said Oskinova. "Farther out from the star, the explosion could actually trigger the birth of new stars."
In addition to the star itself, the astronomers noted a cloud of dust and gas, called a nebula, surrounding the star. The team nicknamed this cloud the Peony nebula because it resembles the ornate flower.
"The nebula was probably created from the spray of dust leaking off the massive Peony nebula star," said Andreas Barniske of Potsdam University, lead author of the study.
Wolf-Rainer Hamann, also of Potsdam University, is another co-author of the paper and the principal investigator of a Spitzer program enabling this research.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared spectrograph, which was used to determine the luminosity of the Peony nebula star, was built by Cornell University, Ithaca, N.Y. Its development was led by Jim Houck of Cornell. For more information about Spitzer, visit http://www.spitzer.caltech.edu/spitzer and http://www.nasa.gov/spitzer .
Media contact: Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
评论 (0)