哈勃发现一块天体化石
Image Credit: ESA/Hubble & NASA, A. Sarajedini
This densely populated group of stars is the globular cluster NGC 1841, which is part of the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way galaxy that lies about 162,000 light-years away. Satellite galaxies are bound by gravity in orbits around a more massive host galaxy. We typically think of the Andromeda Galaxy as our galaxy’s nearest galactic companion, but it is more accurate to say that Andromeda is the nearest galaxy that is not in orbit around the Milky Way galaxy. In fact, dozens of satellite galaxies orbit our galaxy and they are far closer than Andromeda. The largest and brightest of these is the LMC, which is easily visible to the unaided eye from the southern hemisphere under dark sky conditions away from light pollution.
The LMC is home to many globular clusters. These celestial bodies fall somewhere between open clusters – which are much less dense and tightly bound – and small, compact galaxies. Increasingly sophisticated observations reveal the stellar populations and characteristics of globular clusters are varied and complex, and we have yet to fully understand how these tightly packed groups of stars form. However, there are certain consistencies across all globular clusters: they are very stable and hold their shape for a long time, which means they are generally very old and contain large numbers of very old stars. Globular clusters are akin to celestial ‘fossils.’ Just as fossils provide insight into the early development of life on Earth, globular clusters such as NGC 1841 can provide insights into very early star formation in galaxies.
Text credit: European Space Agency (ESA)
影像来源：ESA/Hubble & NASA, A. Sarajedini
这个密集的恒星群是球状星团NGC 1841，它是大麦哲伦星云（LMC）的一部分，LMC是银河系的一个卫星星系，距离我们大约162,000光年。卫星星系受到引力的束缚，绕着质量更大的宿主星系运行。我们通常认为仙女座星系是离我们星系最近的伴星系，但更准确的说法是，仙女座星系是离我们最近的、不在银河系轨道上的星系。事实上，有几十个卫星星系围绕着银河系运行，它们比仙女座星系近得多。其中最大和最亮的是LMC，在远离光污染的黑暗天空条件下，从南半球用肉眼很容易看到它。
LMC是许多球状星团的家园。这些天体介于疏散星团（密度小得多，束缚松散）和小而紧凑的星系之间。越来越复杂的观测表明，球状星团的恒星数量和特征是多样而复杂的，我们还没有完全理解这些紧密聚集的恒星群是如何形成的。然而，，所有球状星团都有一定的相似性：它们非常稳定，并在很长一段时间内保持其形状，这意味着它们通常非常古老，并且包含大量非常古老的恒星。就像化石可以让我们了解地球上生命的早期发展一样，像NGC 1841这样的球状星团可以让我们了解星系中非常早期的恒星形成。
文字来源：欧洲航天局（ESA）