WEBVTT

00:00:07.166 --> 00:00:10.034
Light is the fastest thing we know.

00:00:10.034 --> 00:00:13.113
It's so fast that we measure
enormous distances

00:00:13.113 --> 00:00:16.321
by how long it takes
for light to travel them.

00:00:16.321 --> 00:00:20.397
In one year, light travels
about 6,000,000,000,000 miles,

00:00:20.397 --> 00:00:22.915
a distance we call one light year.

00:00:22.915 --> 00:00:25.270
To give you an idea of just
how far this is,

00:00:25.270 --> 00:00:29.196
the Moon, which took the Apollo astronauts
four days to reach,

00:00:29.196 --> 00:00:32.276
is only one light-second from Earth.

00:00:32.276 --> 00:00:36.698
Meanwhile, the nearest star beyond
our own Sun is Proxima Centauri,

00:00:36.698 --> 00:00:39.731
4.24 light years away.

00:00:39.731 --> 00:00:44.276
Our Milky Way is on the order of
100,000 light years across.

00:00:44.276 --> 00:00:46.882
The nearest galaxy to our own, Andromeda,

00:00:46.882 --> 00:00:49.857
is about 2.5 million light years away

00:00:49.857 --> 00:00:52.616
Space is mind-blowingly vast.

00:00:52.616 --> 00:00:56.959
But wait, how do we know how
far away stars and galaxies are?

00:00:56.959 --> 00:01:01.234
After all, when we look at the sky,
we have a flat, two-dimensional view.

00:01:01.234 --> 00:01:05.321
If you point you finger to one star,
you can't tell how far the star is,

00:01:05.321 --> 00:01:08.684
so how do astrophysicists figure that out?

00:01:08.684 --> 00:01:10.915
For objects that are very close by,

00:01:10.915 --> 00:01:14.776
we can use a concept called
trigonometric parallax.

00:01:14.776 --> 00:01:16.550
The idea is pretty simple.

00:01:16.550 --> 00:01:17.962
Let's do an experiment.

00:01:17.962 --> 00:01:21.289
Stick out your thumb and 
close your left eye.

00:01:21.289 --> 00:01:24.894
Now, open your left eye and
close your right eye.

00:01:24.894 --> 00:01:26.882
It will look like your thumb has moved,

00:01:26.882 --> 00:01:31.069
while more distant background objects
have remained in place.

00:01:31.069 --> 00:01:33.890
The same concept applies when
we look at the stars,

00:01:33.890 --> 00:01:38.075
but distant stars are much, much 
farther away than the length of your arm,

00:01:38.075 --> 00:01:39.926
and the Earth isn't very large,

00:01:39.926 --> 00:01:43.079
so even if you had different telescopes
across the equator,

00:01:43.079 --> 00:01:45.902
you'd not see much of a shift in position.

00:01:45.902 --> 00:01:51.230
Instead, we look at the change in the
star's apparent location over six months,

00:01:51.230 --> 00:01:55.638
the halfway point of the Earth's
yearlong orbit around the Sun.

00:01:55.638 --> 00:01:58.809
When we measure the relative positions
of the stars in summer,

00:01:58.809 --> 00:02:02.839
and then again in winter,
it's like looking with your other eye.

00:02:02.839 --> 00:02:05.440
Nearby stars seem to have moved
against the background

00:02:05.440 --> 00:02:08.327
of the more distant stars and galaxies.

00:02:08.327 --> 00:02:13.090
But this method only works for objects no
more than a few thousand light years away.

00:02:13.090 --> 00:02:15.782
Beyond our own galaxy,
the distances are so great

00:02:15.782 --> 00:02:20.811
that the parallax is too small to detect
with even our most sensitive instruments.

00:02:20.811 --> 00:02:23.719
So at this point we have to rely 
on a different method

00:02:23.719 --> 00:02:27.459
using indicators we call standard candles.

00:02:27.459 --> 00:02:32.079
Standard candles are objects whose
intrinsic brightness, or luminosity,

00:02:32.079 --> 00:02:34.377
we know really well.

00:02:34.377 --> 00:02:37.434
For example, if you know how bright
your light bulb is,

00:02:37.434 --> 00:02:40.809
and you ask your friend to hold
the light bulb and walk away from you,

00:02:40.809 --> 00:02:43.736
you know that the amount of light
you receive from your friend

00:02:43.736 --> 00:02:47.153
will decrease by the distance squared.

00:02:47.153 --> 00:02:49.588
So by comparing the amount 
of light you receive

00:02:49.588 --> 00:02:51.932
to the intrinsic brightness 
of the light bulb,

00:02:51.932 --> 00:02:55.034
you can then tell how far away
your friend is.

00:02:55.034 --> 00:02:58.284
In astronomy, our light bulb turns out to
be a special type of star

00:02:58.284 --> 00:03:00.791
called a cepheid variable.

00:03:00.791 --> 00:03:03.028
These stars are internally unstable,

00:03:03.028 --> 00:03:06.997
like a constantly inflating 
and deflating balloon.

00:03:06.997 --> 00:03:10.689
And because the expansion and contraction
causes their brightness to vary,

00:03:10.689 --> 00:03:15.214
we can calculate their luminosity
by measuring the period of this cycle,

00:03:15.214 --> 00:03:19.159
with more luminous stars 
changing more slowly.

00:03:19.159 --> 00:03:21.534
By comparing the light
we observe from these stars

00:03:21.534 --> 00:03:24.450
to the intrinsic brightness we've
calculated this way,

00:03:24.450 --> 00:03:26.936
we can tell how far away they are.

00:03:26.936 --> 00:03:30.245
Unfortunately, this is still not 
the end of the story.

00:03:30.245 --> 00:03:34.796
We can only observe individual stars
up to about 40,000,000 light years away,

00:03:34.796 --> 00:03:37.893
after which they become 
too blurry to resolve.

00:03:37.893 --> 00:03:41.085
But luckily we have another type
of standard candle:

00:03:41.085 --> 00:03:44.465
the famous type 1a supernova.

00:03:44.465 --> 00:03:49.747
Supernovae, giant stellar explosions
are one of the ways that stars die.

00:03:49.747 --> 00:03:51.580
These explosions are so bright,

00:03:51.580 --> 00:03:54.512
that they outshine the galaxies
where they occur.

00:03:54.512 --> 00:03:57.701
So even when we can't see 
individual stars in a galaxy,

00:03:57.701 --> 00:04:00.843
we can still see supernovae 
when they happen.

00:04:00.843 --> 00:04:05.011
And type 1a supernovae turn out
to be usable as standard candles

00:04:05.011 --> 00:04:08.638
because intrinsically bright ones
fade slower than fainter ones.

00:04:08.638 --> 00:04:10.925
Through our understanding 
of this relationship

00:04:10.925 --> 00:04:13.143
between brightness and decline rate,

00:04:13.143 --> 00:04:15.562
we can use these supernovae 
to probe distances

00:04:15.562 --> 00:04:18.739
up to several billions of light years away.

00:04:18.739 --> 00:04:23.548
But why is it important to see
such distant objects anyway?

00:04:23.548 --> 00:04:26.662
Well, remember how fast light travels.

00:04:26.662 --> 00:04:30.621
For example, the light emitted by the Sun
will take eight minutes to reach us,

00:04:30.621 --> 00:04:36.568
which means that the light we see now
is a picture of the Sun eight minutes ago.

00:04:36.568 --> 00:04:38.198
When you look at the Big Dipper,

00:04:38.198 --> 00:04:41.746
you're seeing what it looked like
80 years ago.

00:04:41.746 --> 00:04:43.434
And those smudgy galaxies?

00:04:43.434 --> 00:04:45.681
They're millions of light years away.

00:04:45.681 --> 00:04:49.388
It has taken millions of years for
that light to reach us.

00:04:49.388 --> 00:04:54.676
So the universe itself is in some sense
an inbuilt time machine.

00:04:54.676 --> 00:04:59.248
The further we can look back,
the younger the universe we are probing.

00:04:59.248 --> 00:05:02.297
Astrophysicists try to read the history
of the universe,

00:05:02.297 --> 00:05:06.055
and understand how
and where we come from.

00:05:06.055 --> 00:05:10.870
The universe is constantly sending us
information in the form of light.

00:05:10.870 --> 00:05:13.745
All that remains if for us to decode it.