第三类 Population III
从1904年开始，Walter Baade将星系中观测到的恒星分为两类，‘第一类’和‘第二类’。从那之后又建立了更多的分类术语，用于更精细的分类，主要用于形容恒星的形状(如薄盘、厚盘、星系晕或突起)，天文学家今天仍然在使用‘第一类’和‘第二类’进行分类。
Starting the 1940s by Walter Baade, stars observed in galaxies were divided into two categories, Population I and Population II. Although many more classifying terms has since been established for much more refined categories, mainly focusing on the stars’ shapes (e.g. thin disk, thick disk, halo or bulge of the galaxies), astronomers today still are using the old terms.
第一类，也被称为Pop I，是富含金属的一类，而第二类 (Pop II)是金属含量较少的一类。但是，即使是金属含量最低的恒星，其金属含量也远高于大爆炸时遗留下来的气体。这意味着，可能有一些恒星会不属于这两类中的一类。
Population I, also known as Pop I, are the metal-rich ones, while Population II (Pop II) are the metal-poor ones. However, even the most metal poor stars have metallicitiesfar above the level for gases left over from the big bang. This means, there could be stars not fitting into one of the two categories.
因此，出现了第三类 (Pop III) 来解决这个问题。它们完全由原始气体 — 氢、氦和极少量的锂和铍组成。也就是说，Pop III恒星的气体从来没有被循环利用过 (从上一代恒星的爆炸中分解)，它们是来自大爆炸的原始材料。
So, Population III (Pop III) was introduced to resolve this issue. They are the ones composed entirely of primordial gas – hydrogen, helium and very small amounts of lithium and beryllium. This means, the gas from Pop III stars would have never been recycles (decomposed from previous generation stars’ explosion), they were the original materials from the Big Bang.
图为NASA的哈勃望远镜的M80球状星团中的第二类恒星们
Population II stars in the globular cluster M80 in an image taken by the Hubble Space Telescope
最终，Pop III恒星的聚变会慢慢产生金属，并成为Pop II恒星，金属的比例逐渐增加。
Eventually, the fusion in Pop III stars would produce metals and become Pop IIs, and gradually increase in its metallic proportion.
然而，Pop III的唯一问题是它们仍然是假设的一类恒星，尽管科学家们对这个问题已经进行了大量的研究，但至今还没有发现任何一个Pop III。
However, the only problem with Pop III is that they are still hypothetical, despite intense research on the topic, there has been no Pop III discovered yet.
其中一个主要原因可能是大多数Pop III至今已经耗尽了它们的燃料，现在能观察到只有它们的残骸，例如：白矮星、中子星或黑洞。而从那些天体中，我们是基本不可能看到它们最初的组织的。
One of the main reasons is that the majority of Pop III have long exhausted their fuel, and can now only be observed as remanent, for example: white dwarfs, neutron stars or black holes. It would not be possible to see their original composition today.
尽管这是有可能的，但单凭这一点并不能完全解释Pop III恒星的消失，因为质量最低的恒星今天应该仍然存在于星系中。
Though that is possible, it alone cannot explain the absence of Pop III stars, as the ones with the lowest masses should still be present in the galaxy population today.
当然也有可能是一些其他的原因，但无论如何，我们观测到Pop III的可能性是非常小的，因此它们永远停留在假设状态。尽管如此，仍有一些天文学家在寻找它们。
There are also some reasons, but whatever it may be, it is extremely unlikely that we will ever observe a Pop III star and therefore remain hypothetical forever. Nevertheless, there are still some astronomers out there on the hunt for them.
由此可见科学精神的伟大！
This just shows the great spirits of science!
图为用ESO的超大望远镜发现的CR7 星系，是迄今为止观测到的早期宇宙中最亮的星系之一，其中很有可能含有Pop III 恒星
The CR7 galaxy discovered by the Very Large Telescope from ESO. It is one of the brightest galaxies discovered yet in the early universe and is likely to contain Pop IIIs in there
图片来自 ESO, NASA官网
Pictures from official website of ESO, NASA.
