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Light is the fastest thing we know.
光是我们所知道的传播速度最快的东西。
It's so fast that we measure enormous distances by how long it takes for light to travel them.
正因为光如此之快的传播速度，我们就用光走过的时间来描述那些十分遥远的距离的。
In one year, light travels about 6,000,000,000,000 miles, a distance we call one light year.
光在一年中传播的距离大概是六万亿英里，我们称这个距离为一光年。
To give you an idea of just how far this is,
现在我们来举例说明一光年的距离究竟有多远，
the Moon, which took the Apollo astronauts four days to reach, is only one light-second from Earth.
阿波罗宇航员用时四天登上了月球，而光从月亮到地球只需要一秒钟。
Meanwhile, the nearest star beyond our own Sun is Proxima Centauri, 4.24 light years away.
另外，比邻星--离太阳系最近的恒星，离我们有4.24光年远。
Our Milky Way is on the order of 100,000 light years across.
我们所在的银河系的直径大概是十万光年。
The nearest galaxy to our own, Andromeda, is about 2.5 million light years away Space is mind-blowingly vast.
离我们最近的星系，仙女座星系，离我们有250万光年。我们根本无法想象宇宙之大。
But wait, how do we know how far away stars and galaxies are?
但是，我们是如何知道恒星和星系的距离的呢？
After all, when we look at the sky, we have a flat, two-dimensional view.
每当我们抬头看天空，我们所见的只是一个二维平面视图。
If you point you finger to one star, you can't tell how far the star is, so how do astrophysicists figure that out?
当你伸手指向某一颗星星时，你无法得知这颗星星离你到底有多远，那么天体物理学家们如何得知距离呢？
For objects that are very close by, we can use a concept called trigonometric parallax.
对于离我们比较近的星体，我们只需要用三角视差来估算距离。
The idea is pretty simple. Let's do an experiment.
这个理论很简单。只需要做一个小实验就可以说明。
Stick out your thumb and close your left eye.
伸出你的大拇指，然后闭上你的左眼。
Now, open your left eye and close your right eye.
现在，闭上你的左眼，同时睁开你的右眼。
It will look like your thumb has moved, while more distant background objects have remained in place.
你会发现你的大拇指好像移动了。但是相对遥远的背景里的物体却没有动。
The same concept applies when we look at the stars,
这个理论同样适用于看恒星的时候，
but distant stars are much, much farther away than the length of your arm, and the Earth isn't very large,
但是恒星离我们的距离相比于我们胳膊的长度不知道长了多少倍，而且相对来说，地球也不是很大的星体，
so even if you had different telescopes across the equator, you'd not see much of a shift in position.
所以即使你在赤道两边用不同的望远镜观测同一颗星体，你也很难看到这颗星体位置的移动。
Instead, we look at the change in the star's apparent location over six months,
为了解决这个问题，我们改为观察六个月内星体位置的移动，
the halfway point of the Earth's yearlong orbit around the Sun.
这个时间刚好是地球绕太阳轨道旋转半周的时间。
When we measure the relative positions of the stars in summer,
我们在夏天观测恒星的相对位置，
and then again in winter, it's like looking with your other eye.
等到了冬天再观测时，就像我们在用另外一只眼睛看它。
Nearby stars seem to have moved against the background of the more distant stars and galaxies.
离我们近的恒星似乎移动了位置，而遥远距离的恒星和星系保持不动。
But this method only works for objects no more than a few thousand light years away.
但是此方法只适用于距离不超过几千光年的天体。
Beyond our own galaxy, the distances are so great
在我们的星系之外，其他的天体如此之远，
that the parallax is too small to detect with even our most sensitive instruments.
以至于视差太小了，连最精密的仪器也无法测得。
So at this point we have to rely on a different method using indicators we call standard candles.
所以，我们必须找到别的办法，这个办法叫标准烛光法。
Standard candles are objects whose intrinsic brightness, or luminosity, we know really well.
标准烛光是天文学中已经知道光度的天体。
For example, if you know how bright your light bulb is,
打个比方，如果你知道你自家灯泡的亮度，
and you ask your friend to hold the light bulb and walk away from you,
然后你让别人拿着那只灯泡向远离你的方向走去，
you know that the amount of light you receive from your friend will decrease by the distance squared.
你知道你看到的灯泡的亮度是以他走的距离的平方在减弱的。
So by comparing the amount of light you receive to the intrinsic brightness of the light bulb,
所以通过比较你看到的灯泡的亮度和灯泡的原始亮度，
you can then tell how far away your friend is.
你可以计算出他距你有多远。
In astronomy, our light bulb turns out to be a special type of star called a cepheid variable.
应用到天文学中，你的灯泡就变成了一些特殊的天体--造父变星。
These stars are internally unstable, like a constantly inflating and deflating balloon.
这些星星的内部不是很稳定，就像一只一会儿鼓起来一会儿扁下去的气球。
And because the expansion and contraction causes their brightness to vary,
它们的亮度随着膨胀和收缩而变化，
we can calculate their luminosity by measuring the period of this cycle, with more luminous stars changing more slowly.
我们可以通过它们膨胀收缩的周期来计算它们的亮度，越亮的星星，这个周期越长。
By comparing the light we observe from these stars to the intrinsic brightness we've calculated this way,
通过比较观测到的这些恒星的亮度和我们计算出来的它们原始的亮度，
we can tell how far away they are.
我们就可以知道它们距离我们有多远。
Unfortunately, this is still not the end of the story.
可惜，这个方法也有它的局限性。
We can only observe individual stars up to about 40,000,000 light years away,
用这个方法，我们只能测量到距离我们不超过四千万光年的独立的恒星，
after which they become too blurry to resolve.
超过这个距离的恒星会变得太模糊而无法分辨。
But luckily we have another type of standard candle: the famous type 1a supernova.
不过幸运的是，我们还有另一种标准烛光，著名的Ia型超新星。
Supernovae, giant stellar explosions are one of the ways that stars die.
超新星爆发，也就是巨型恒星爆炸，是恒星死亡的方式之一。
These explosions are so bright, that they outshine the galaxies where they occur.
这些爆炸是非常亮的，它发生的时候可以照亮整个星系。
So even when we can't see individual stars in a galaxy, we can still see supernovae when they happen.
所以即使我们无法分辨星系中独立的恒星，我们还是可以看到超新星爆发。
And type 1a supernovae turn out to be usable as standard candles
Ia型超新星被证明是可用的标准烛光，
because intrinsically bright ones fade slower than fainter ones.
因为本征亮度较亮的超新星，其亮度衰减的速率较慢。
Through our understanding of this relationship between brightness and decline rate,
凭借我们对超新星的亮度和衰减速率的关系的了解，
we can use these supernovae to probe distances up to several billions of light years away.
我们可以用这些超新星来测量离我们几十亿光年远的天体。
But why is it important to see such distant objects anyway?
可是我们为什么要观测这么遥远的天体呢？
Well, remember how fast light travels.
回答这个问题要回到光的传播速度上。
For example, the light emitted by the Sun will take eight minutes to reach us,
比如说，光从太阳传播到地球需要八分钟，
which means that the light we see now is a picture of the Sun eight minutes ago.
这就意味着我们看到的太阳是八分钟前太阳的样子。
When you look at the Big Dipper, you're seeing what it looked like 80 years ago. And those smudgy galaxies?
当我们看北斗星时，我们看到的是北斗星80年前的样子。那些朦胧的星系呢？
They're millions of light years away.
它们距离我们数百万光年。
It has taken millions of years for that light to reach us.
来自它们的光需要传播数百万年才能到达地球。
So the universe itself is in some sense an inbuilt time machine.
所以我们的宇宙从某种程度上来说是一个内置时光机。
The further we can look back, the younger the universe we are probing.
我们看得越远，我们越接近宇宙刚开始的样子。
Astrophysicists try to read the history of the universe, and understand how and where we come from.
天体物理学家们试图研究宇宙的历史来解答我们如何而来以及我们从哪里来。
The universe is constantly sending us information in the form of light.
宇宙不断地以光的形式给我们发送信息。
All that remains if for us to decode it.
剩下的就等我们来解读。
