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Light is the fastest thing we know.

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It's so fast that we measure
enormous distances

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by how long it takes
for light to travel them.

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In one year, light travels
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
how far this is,

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the Moon, which took the Apollo astronauts
four days to reach,

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is only one light-second from Earth.

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Meanwhile, the nearest star beyond
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
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
far away stars and galaxies are?

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

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If you point you finger to one star,
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
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 
close your left eye.

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Now, open your left eye and
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
have remained in place.

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The same concept applies when
we look at the stars,

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but distant stars are much, much 
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
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
star's apparent location over six months,

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

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When we measure the relative positions
of the stars in summer,

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

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Nearby stars seem to have moved
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
more than a few thousand light years away.

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Beyond our own galaxy,
the distances are so great

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

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

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using indicators we call standard candles.

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Standard candles are objects whose
intrinsic brightness, or luminosity,

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we know really well.

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

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

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you know that the amount of light
you receive from your friend

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will decrease by the distance squared.

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So by comparing the amount 
of light you receive

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to the intrinsic brightness 
of the light bulb,

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you can then tell how far away
your friend is.

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In astronomy, our light bulb turns out to
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 
and deflating balloon.

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

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

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with more luminous stars 
changing more slowly.

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By comparing the light
we observe from these stars

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to the intrinsic brightness we've
calculated this way,

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we can tell how far away they are.

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

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

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after which they become 
too blurry to resolve.

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But luckily we have another type
of standard candle:

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the famous type 1a supernova.

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Supernovae, giant stellar explosions
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
where they occur.

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

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we can still see supernovae 
when they happen.

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

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because intrinsically bright ones
fade slower than fainter ones.

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Through our understanding 
of this relationship

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between brightness and decline rate,

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we can use these supernovae 
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
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
will take eight minutes to reach us,

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which means that the light we see now
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
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
that light to reach us.

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

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

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Astrophysicists try to read the history
of the universe,

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and understand how
and where we come from.

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

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All that remains if for us to decode it.