It’s so fast that we measure enormous distances by how long it takes for light to travel them.

因為它很快速，所以我們用光速來測量龐大的距離

In one year, light travels about 6 trillion miles,

光一年可行進6百萬兆英里

A distance we call one light year

我們稱這段距離為光年

To give you an idea of just how far this is,

為了讓大家知道一光年到底有多遠

the Moon, which took the Apollo astronauts four days to reach, is only one light second from Earth

太空人搭阿波蘿號花四天時間到達月球，實際距離卻只有一光秒

Meanwhile, the nearest star beyond our own Sun is Proxima Centauri, 4.24 light years away

同時，南門二，最靠近太陽的恆星是4.24光年

Our Milky Way is on the order of 100,000 light years across

銀河系大約是10萬光年的量級

The nearest galaxy to our own, Andromeda, is about 2.5 million light years away

距離我們最近的星系，仙女座，大約距離250萬光年

Space is mind-blowingly vast

宇宙非常浩瀚

But wait, how do you know how far away stars and galaxies are?

但等一下，你怎麼知道恆星和銀河距離多遠呢？

After all, when we look at the sky, we have a flat, two-dimensional view

總之，看天空時，我們有平面二維視圖

If you point your finger to one star, you can’t tell how far the star is,

如果用手指著星星，你不知道它到底距離多遠

So how do astrophysicists figure that out?

所以天體物理學家到底怎麼算出距離呢？

For objects that are very close by, we can use a concept called trigonometry parallax

距離近的物體，我們用三角視差法來了解

The idea is pretty simple. Let’s do an experiment.

三角視差法的觀念非常簡單，我們來做個實驗

Stick out your thumb and close your left eye.

伸出你的大拇指，把它放在靠近左眼的地方

Now open your left eye and close your right eye

現在，張開左眼，閉上右眼

It will look like your thumb has moved while more distant background objects have remained in place

你會覺得大拇指移位了，而遠處的物體卻停留在原位

The same concept applies when we look at the stars

我們用同樣的概念來看恆星

But distant stars are much, much farther away than the length of your arm

恆星的距離比你的手臂還要來的遠

And the Earth isn’t very large, so even if you had different telescopes across the equator,

而且地球沒有很大，所以即使你跨越赤道，使用不同的望遠鏡

You’d not see much of a shift in position

你也看不到多大的改變

Instead, we look at the change in the star’s apparent location over 6 months,

相反地，我們利用地球中心點一整年的運行軌道來環繞太陽

The halfway point of the Earth’s yearlong orbit around the Sun.

以觀察恆星超過六週的位置

When we measure the relative positions of the stars in summer and then again in winter, it’s like looking with your other eye.

我們測量恆星在夏季的相對位置，也測量在冬季的相對位置，它就像你的右眼

Nearby stars seemed to have moved against the background of the more distant stars and galaxies.

附近的恆星看似比遙遠的恆星及銀河系更遙遠

But this method only works for objects no more than a few thousand light years away.

但這種方法只適用於距離幾千光年內的物體

Beyond our own galaxy, the distances are so great that the parallax is too small to detect with even our most sensitive instruments.

在我們的銀河系外，距離太大，視差太小，我們也無法用最敏銳的儀器來觀察

So at this point we have to rely on a different method using indicators we call standard candles.

所以我們必須依賴其他的方法，利用指標，我們稱之為標準燭光

Standard candles are objects whose intrinsic brightness, or luminosity, we know really well.

我們都了解標準燭光的固定亮度和光度

For example, if you know how bright your light bulb is, and you ask your friend to hold the light bulb and walk away from you,

舉例來說，如果你想知道燈泡多亮，就請朋友手拿燈泡，慢慢遠離你

You know that the amount of light you receive from your friend will decrease by the distance squared

從燈泡得到的光量會隨距離增加而減少

So by comparing the amount of light you receive to the intrinsic brightness of the light bulb,

比較燈泡的光量和固定亮度

You can then tell how far away your friend is.

你就會知道你朋友距離多遠

In Astronomy, our light bulb turns out to be a special type of star called a Cepheid variable.

在天體學中，燈泡成為一種特殊形式的恆星，造父變星

These stars are internally unstable, like a constantly inflating and deflating balloon.

這些恆星的內部不穩定，就像個不斷充氣和放氣的氣球

And because the expansion and contraction causes their brightness to vary,

因為膨脹和收縮所導致的亮度變化

We can calculate their luminosity by measuring the period of this cycle,

發光的恆星改變較慢

With more luminous stars changing more slowly.

我們可以透過這段週期來計算該恆星的亮度

By comparing the light we observe from these stars to the intrinsic brightness we’ve calculated this way,

比較我們已經算出的恆星固定亮度

We can tell how far away they are.

我們就知道它們距離多遠

Unfortunately, this is still not the end of the story.

不幸的是，這還不是故事的結局

We can only observe individual stars up to about 40 million light years away,

我們最遠只能觀察到距離4千萬光年的個體恆星

After which they become too blurry to resolve

更遠的恆星就會變得模糊

But luckily we have another type of standard candle

但慶幸的是，我們還有另外一種標準燭光

The famous Type 1a Supernova

有名的1a型超新星

Supernovae, giant stellar explosions are one of the ways that stars die.

超新星，巨大恆星的爆炸是恆星死亡的一種方式

These explosions are so bright that they outshine the galaxies where they occur.

這種恆星的爆炸非常明亮，所以只會發生在所屬的銀河系中

So even when we can’t see individual stars in a galaxy,

甚至我們看不到銀河系中的個體恆星

We can still see supernovae when they happen.

我們仍能看到爆炸的超新星

And Type 1a supernovae turn out to be usable as standard candles,

1a型超新星就會變成有用的標準燭光

Because intrinsically bright ones fade slower than fainter ones.

因為本質會發亮的恆星褪色比昏暗的恆星慢

Through our understanding of this relationship between brightness and decline rate,

透過我們對亮度和下降率的了解

We can use these supernovae to probe distances up to several billions of light years away.

我們可以用這些超新星來探測遠至幾十億光年的距離

But why is it important to see such distant objects anyway?

但觀察遙遠的物體為什麼那麼重要呢？

Well, remember how fast light travels.

記住光有多快

For example, the light emitted by the Sun will take 8 minutes to reach us,

例如：我們花8分鐘時間才能接受到太陽光

Which means that the light we see now is a picture of the sun 8 minutes ago.

那就表示我們現在所看到的光，是太陽8分鐘前所顯示的影像

When you look at the Big Dipper, you’re seeing what it looked like 80 years ago.

如果你看北斗七星，你是在看它八十年前的影像

And those smudgy galaxies?

而那些模糊不清的銀河系呢？

They’re millions of light years away.

他們距離我們幾百萬光年

It has taken millions of years for that light to reach us.

它要花幾百萬年的光，我們才能看到

So the universe itself is in some sense an inbuilt time machine.

所以在某種意義上，宇宙是個內置的時間機器

The further we can look back, the younger the universe we are probing.

我們可以回顧一下宇宙年輕時的樣貌

Astrophysicists try to read the history of the universe and understand how and where we come from

天體物理學家嘗試閱讀宇宙的歷史，並了解我們如何而來、從何而來

The universe is constantly sending us information in the form of light.

宇宙持續用光的形式來傳送訊息給我們

All that remains is for us to decode it.

而剩下的就是我們自己要解碼的東西了