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Humans have been looking up at the stars for thousands of years,
数千年来，人类一直在观察星星。
but it was only within the past century that we learned there's more than one galaxy out there.
不过直到上个世纪，人类才知道宇宙里不止一个星系。
Then, ever since the 1920s, astronomers have been finding more and more, farther and farther away.
此后，自从上世纪20年代开始，天文学家们发现的越来越多，探索的范围也越来越广阔。
But how many galaxies are there in the universe, really?
不过，讲真的，宇宙里到底有多少星系呢？
Well, we'll never have an exact number, but astronomers have ways of coming up with pretty good estimates.
确切的数字是永远也无法得知了，不过天文学家们倒是有估算个大概的方法。
It doesn't seem like it'd be all that hard to calculate the total number of galaxies.
统计星系的总数似乎并没那么难。
Just take a picture of the sky and count up all the ones you can see. Right? Well, it's more complex than that.
只要对着天空拍张照，然后数一下看到了多少颗星星就行了，是这样吗？可没这么简单哦。
How many galaxies your image will capture depends on how sensitive your equipment is,
你能捕捉到多少个星系取决于你的设备有多灵敏，
and how long you leave it running to catch all the super dim stuff. It also depends on what wavelengths of light it can detect.
以及你持续拍摄的时间有多久，这都取决于设备能捕捉到的光的波长。
Because the universe is expanding, light from distant galaxies gets stretched and shifted toward the longer end of the electromagnetic spectrum.
由于宇宙是不断延伸的，所以来自遥远星系的光会在受到拉伸后，偏移到电磁波谱较长的那段。
This effect is known as redshift.
这种现象就叫红移。
That means there could be galaxies whose light is redshifted so much that they're no longer visible to your eyes or your camera.
也就是说，有一些星系发出的光因受红移的强力作用而不再为肉眼可见，也无法被摄像机捕捉到。
So to see the oldest galaxies, with the most redshifted light, you need special, infrared telescopes.
所以，由于最古老的星系受到红移的作用最为明显，所以要想看到这样的星系就需要特殊的红外望远镜。
We've been using those for over a decade, but we've been counting galaxies for much longer.
虽然这种方式已经使用了十几年，但花在数星系上的时间反而更长了。
The first real step toward estimating the total number of them came back in 1996, when NASA published the Hubble Deep Field Image.
真正开始估算星系的总数要回溯到1996年，那时候美国宇航局发布了哈勃深场图像。
Over a period of 10 days, the Hubble Space Telescope imaged a portion of the sky 1/30th the diameter of the full Moon, using visible and ultraviolet light.
在十天的时间里，哈勃深场望远镜通过可见紫外线拍摄到了天空的一部分，这部分的长度是满月直径的1/30。
It was a portion you or I would say looked pretty darn empty. But it definitely wasn't.
这部分图像在我们常人看来似乎空无一物。但实际并非如此。
In fact, there were thousands of objects there, from pinpricks to large smudges.
实际上，那部分天空蕴含着无数的物体，大小不一。
Most were so faint that no telescope had ever seen them before.
不过大多数物体在图像上都很模糊，都是望远镜未曾观测过的。
Statistically speaking, the distribution of galaxies in the sky is uniform, so in each speck of sky you have roughly the same number of galaxies.
从统计学上讲，星系在天空中的分布是均匀的，所以每块相同大小的天空中有差不多一样多的星系。
Extrapolating from the Hubble Deep Field, that's 120 billion galaxies total.
根据哈勃深场图像的推断，总共有1200亿个星系。
A decade later, Hubble completed a new, deeper survey of a different part of the sky, producing the Hubble Ultra Deep Field.
十年后，哈勃又完成了对另一块天空的深度调研，这就有了哈勃超深场。
Thanks to the newer equipment, that upped the estimate to over 200 billion.
在这个更新的设备的帮助下，星系总数的估测值达到了2000多亿个。
Altogether, with the various fields Hubble and other telescopes have provided,
在哈勃等各种望远镜的帮助下，
astronomers estimated that there are between 100 and 200 billion galaxies in the observable universe.
天文学家估测在人类可观测的宇宙范围内，共有1000-2000亿个星系。
And they've been working with that number for the past two decades.
而过去二十年来，他们就在和这个总数值打交道。
But in 2016, a paper published in the Astrophysical Journal threw that number out the window.
不过，2016年，《天文物理期刊》上一篇文章推翻了这个总数。
It claimed a more accurate value was ten times that: 2 trillion galaxies.
这篇文章指出，更准确的总数是2万亿个星系，是此前总值的十倍。
To figure this out, the research team used deep space images from Hubble and other telescopes,
为了搞清楚这个问题，该研究组搜集了哈勃等望远镜观测到的深空图像，
as well as work published by other astronomers about the very early universe.
也参考了其他天文学家关于宇宙最初情况发表的作品。
See, nearby galaxies are relatively easy to count, because they're usually pretty bright.
我们都知道，地球附近星系的数量是相对容易数出来的，因为他们非常明亮。
But the farthest galaxies are mostly too dim to be detected. In this study, the team found a way around that.
但最遥远的星系大多非常模糊，不容易观测到，该小组在这项研究中心发现了一种应对这个问题的方法。
Because it takes light billions of years to travel from those galaxies to Earth, we can only see them as they were in the distant past.
由于光从各个星系抵达地球要走数十亿光年，所以我们看到它们的时候已经是它们很早之前的情况了。
So by using data about the early universe, the scientists developed a new mathematical model to estimate how many super far, dim galaxies are actually out there.
所以，有了宇宙初期的数据，天文学家就得以建立一个新的数学模型来估测无穷远的地方到底有多少个暗淡的星系。
The model could infer the presence of those dim galaxies that couldn't be visually detected.
这个模型可以推断出肉眼无法观测到的暗淡星系是否存在。
Even though they were too dim to show up in the image,
虽然他们太过暗淡而无法显示在图像上，
the computer could tell they were supposed to be there based on the number, mass, brightness, and distance of the galaxies it could detect.
但电脑可以根据观测到的数量、质量、亮度、各星系的距离来计算出那里是否存在星系。
And that's where 2 trillion came from.
通过这种方法得出了2万亿的数字。
As they looked back in time and counted those distant galaxies, the team also noticed something astronomers had seen before:
天文学家回看并计算过去这些遥远星系数量的时候，也注意到一些以前曾观测到的问题：
The farther back they looked, the more galaxies there were.
看起来离地球越远的地方，就有越多的星系。
In fact, when the universe was less than a billion years old, there were way more galaxies than there are today. That isn't actually that surprising.
实际上，宇宙诞生不到10亿年时的星系数量比今天多很多，这一点并不让人惊讶。
We've thought for a while that, over time, early galaxies merged to make a smaller number of physically larger galaxies.
我们思考了一段时间，发现随着时间的流逝，早期的一些星系会并合在一起，形成数量更少、但体量更大的星系。
This study just supported that.
这项研究证实了这一点。
But that also brings up a new problem: All those early galaxies don't actually exist anymore.
但这也带来了一个新问题：这些初期的所有星系实际上并不存在了。
They likely merged billions of years ago, but because it's taken light so much time to reach Earth, we can only see them from before those mergers happened.
它们很可能在几十亿年前并合了，但由于它们发出的光抵达地球需要很久的时间，所以我们看到的是它们并合之前的样子。
So we don't really know how many galaxies there are in those parts of the universe.
所以实际上我们并不真地知道那部分宇宙空间里有多少个星系。
This also means that there are galaxies so far away that their light will never reach us, because the universe is also expanding.
这也意味着会有一些离我们非常远的星系，它们发出的光甚至无法抵达地球，因为宇宙是不断延伸的。
They're beyond what astronomers call the cosmological horizon, or outside the observable universe.
它们已经超出了宇宙视界的范畴，或者说，它们不在人类可以观测到的宇宙范围内。
So the total number of galaxies in the entire universe is basically unknowable.
所以整个宇宙里星系的总数基本上是不可知的。
But that doesn't mean we won't stop learning, and that 2 trillion number isn't necessarily our final count.
但这并不意味着我们要停止学习，而且2万亿的数字也并不是我们得出的最终结果。
More powerful telescopes, like the James Webb Space Telescope, which will hopefully launch in 2019, will allow astronomers to collect more accurate data.
更强大的望远镜，比如将于2019年出世的詹姆斯·韦伯空间望远镜，可以让天文学家们收集到更多、更精确的数据。
The JWST is 100 times more powerful than the Hubble, and will be able to observe galaxies in those longer wavelengths.
詹姆斯·韦伯空间望远镜的观测能力是哈勃望远镜的100多倍，可以观测到波长更长的星系。
In the meantime, with at least 2 trillion galaxies out there, it's safe to say that every point in the sky is completely covered by galaxies.
而且，由于已经至少有2万亿个星系了，所以天空中每个点上都有星系的存在是毫不夸张的。
We definitely can't see all of them. But it's kind of comforting to know they're out there, bathing us in invisible starlight.
不过我们当然看不到所有星系，不过，知道它们在那里存在着，知道我们沐浴在无形的星光里，就足以抚慰人心了。
Thanks for watching this episode of SciShow Space!
感谢收看本期的《太空科学秀》！
If you've ever wondered how many stars there are in the universe, we made a video about that, too, and you can watch it on the main SciShow channel.
如果你从来没想过宇宙里有多少颗星星的话，我们也做了一个有关这个话题的视频，可以在《太空科学秀》的主频道上观看。
