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Light is the fastest thing we know.

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It's so fast that we measure[br]enormous distances

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by how long it takes[br]for light to travel them.

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In one year, light travels[br]about 6,000,000,000,000 miles,

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a distance we call one light year.

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

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the Moon, which took the Apollo astronauts[br]four days to reach,

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is only one light-second from Earth.

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Meanwhile, the nearest star beyond[br]our own Sun is Proxima Centauri,

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4.24 light years away.

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Our Milky Way is on the order of[br]100,000 light years across.

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The nearest galaxy to our own, Andromeda,

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is about 2.5 million light years away

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Space is mind-blowingly vast.

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But wait, how do we know how[br]far away stars and galaxies are?

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

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If you point you finger to one star,[br]you can't tell how far the star is,

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so how do astrophysicists figure that out?

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For objects that are very close by,

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we can use a concept called[br]trigonometric parallax.

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The idea is pretty simple.

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Let's do an experiment.

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Stick out your thumb and [br]close your left eye.

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Now, open your left eye and[br]close your right eye.

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It will look like your thumb has moved,

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while more distant background objects[br]have remained in place.

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The same concept applies when[br]we look at the stars,

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but distant stars are much, much [br]farther away than the length of your arm,

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and the Earth isn't very large,

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so even if you had different telescopes[br]across the equator,

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you'd not see much of a shift in position.

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Instead, we look at the change in the[br]star's apparent location over six months,

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the halfway point of the Earth's[br]yearlong orbit around the Sun.

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When we measure the relative positions[br]of the stars in summer,

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and then again in winter,[br]it's like looking with your other eye.

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Nearby stars seem to have moved[br]against the background

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of the more distant stars and galaxies.

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

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Beyond our own galaxy,[br]the distances are so great

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that the parallax is too small to detect[br]with even our most sensitive instruments.

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So at this point we have to rely [br]on a different method

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using indicators we call standard candles.

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Standard candles are objects whose[br]intrinsic brightness, or luminosity,

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we know really well.

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For example, if you know how bright[br]your light bulb is,

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and you ask your friend to hold[br]the light bulb and walk away from you,

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you know that the amount of light[br]you receive from your friend

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will decrease by the distance squared.

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So by comparing the amount [br]of light you receive

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to the intrinsic brightness [br]of the light bulb,

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you can then tell how far away[br]your friend is.

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In astronomy, our light bulb turns out to[br]be a special type of star

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called a cepheid variable.

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These stars are internally unstable,

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like a constantly inflating [br]and deflating balloon.

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

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we can calculate their luminosity[br]by measuring the period of this cycle,

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with more luminous stars [br]changing more slowly.

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By comparing the light[br]we observe from these stars

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to the intrinsic brightness we've[br]calculated this way,

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we can tell how far away they are.

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Unfortunately, this is still not [br]the end of the story.

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We can only observe individual stars[br]up to about 40,000,000 light years away,

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after which they become [br]too blurry to resolve.

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But luckily we have another type[br]of standard candle:

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the famous type 1a supernova.

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

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These explosions are so bright,

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that they outshine the galaxies[br]where they occur.

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So even when we can't see [br]individual stars in a galaxy,

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we can still see supernovae [br]when they happen.

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And type 1a supernovae turn out[br]to be usable as standard candles

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because intrinsically bright ones[br]fade slower than fainter ones.

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Through our understanding [br]of this relationship

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between brightness and decline rate,

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we can use these supernovae [br]to probe distances

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up to several billions of light years away.

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

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Well, remember how fast light travels.

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For example, the light emitted by the Sun[br]will take eight minutes to reach us,

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which means that the light we see now[br]is a picture of the Sun eight minutes ago.

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When you look at the Big Dipper,

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you're seeing what it looked like[br]80 years ago.

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And those smudgy galaxies?

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They're millions of light years away.

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

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

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

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Astrophysicists try to read the history[br]of the universe,

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and understand how[br]and where we come from.

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

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All that remains if for us to decode it.