神秘的环状星系一直困扰着天文学家
Of all the galaxies observed by astronomers, barely any are as strange or geometrically unique as Hoag's Object. This bizarre galactic circle, categorized as a "ring galaxy," is located some 600 million light-years from Earth in the constellation Serpens and spans about 120,000 light-years across. Even more befuddling is that Hoag's Object is not one, but two galaxies — a cosmic arrangement that has baffled researchers since its discovery by American astronomer Arthur Hoag in 1950.
在天文学家观测到的所有星系中，几乎没有一个像霍格星系那样奇特或在几何上独一无二。这个奇异的星系圈被归类为“环状星系”，位于巨蛇星座，距地球约6亿光年，跨度约12万光年。更令人困惑的是，霍格的目标不是一个，而是两个星系——自1950年美国天文学家阿瑟·霍格发现霍格星系以来，这种宇宙布局一直困扰着研究人员。
Hoag's Object, a rare ring galaxy located some 600 million light-years from Earth. (Photo: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: Ray A. Lucas (STScI/AURA) [public domain]/Wikimedia Commons)
At first, astronomers believed the unusual arrangement of Hoag's Object was a trick of the eye caused by gravitational lensing. This phenomenon, first proposed by Einstein's Theory of General Relativity, occurs when the gravitational mass of one object can bend light in such a way as to magnify the appearance of a more distant object. Astronomers have used such cosmic lenses before to peer into the distant hearts of other galaxies that would otherwise be impossible to detect with modern instruments. This idea was later disproved after observations of Hoag's Object in 1974 showed it weighed far too little (about the mass of 700 billion suns) to cause gravitational lensing of any magnitude.
起初，天文学家们认为霍格的物体的不同寻常的排列是由引力透镜效应引起的。这一现象最初是由爱因斯坦的广义相对论提出的，当一个物体的引力质量可以使光弯曲，从而放大远处物体的外观时，就会出现这种现象。天文学家以前曾使用过这样的宇宙透镜来窥视其他星系遥远的中心，否则用现代仪器是无法探测到的。这个想法后来被证明是错误的，因为1974年对霍格星的观测表明它的质量太小了(大约7000亿个太阳的质量)，不足以产生任何量级的引力透镜效应。
Instead, Hoag's Objects features what appear to be two distinct galaxies, with young bright blue stars encircling a center core of older reddish-stars. Between them lies a perceptible moat of darkness.
相反，霍格星系的特征似乎是两个截然不同的星系，年轻的亮蓝色恒星环绕着较老的红色恒星的中心核心。在他们之间有一条可以看见的黑暗的护城河。
"It's one of these weird little objects you point at without fully understanding what they mean," François Schweizer of the Carnegie Observatories in Pasadena, California, told New Scientist in 2011.
加州帕萨迪纳卡耐基天文台的弗朗索瓦·施韦泽在2011年接受《新科学家》采访时表示:“这是一种奇怪的小东西，你指着它却不完全明白它的意思。”
The Vela ring galaxy, visible as a bright core surrounded by a baby blue halo. (Photo: European Space Agency/CC 2.0)
So how did these incredibly rare cosmic anomalies, accounting for only .01% of all the galaxies discovered, ever come to be? The most popular theory presently is that Hoag's Object was once a common disc-shaped galaxy that suffered a direct hit from a smaller, neighboring galaxy. The resulting collision, which would have happened billions of years ago, wrapped the original galaxy's gravitational pull and formed the beautiful symmetry we see today.
那么，这些罕见得令人难以置信的宇宙异常是如何形成的呢?目前最流行的理论是，霍格的目标曾经是一个普通的圆盘状星系，它直接受到了邻近较小星系的撞击。由此产生的碰撞可能发生在数十亿年前，它包裹了原始星系的引力，形成了我们今天看到的美丽的对称。
Another theory posits that the galaxy has simply sucked in enough intergalactic mass over time to form the beautiful ring we see today.
另一种理论认为，随着时间的推移，星系只是吸收了足够的星系间质量，形成了我们今天看到的美丽的环。
At the one o'clock position in this image, you can see yet another ring galaxy in the distance behind Hoag's Object. (Photo: NASA/Hubble Space Telescope)
Formation theories aside, astronomers noted something else incredible about Hoag's Object when they trained the Hubble Telescope's sensitive optics on it back in 2002. Hiding behind this cosmic wonder at the one o'clock position in the above photo is yet another rare ring galaxy –– making this an image of a galaxy within a galaxy within a galaxy within a galaxy!
撇开形成理论不谈，2002年，天文学家们将哈勃望远镜的灵敏光学仪器对准霍格的天体时，还发现了其他一些不可思议的现象。上图中1点钟位置的宇宙奇观背后，隐藏着另一个罕见的环状星系——这是一张星系中的星系!
