[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.17,0:00:10.03,Default,,0000,0000,0000,,Light is the fastest thing we know.
Dialogue: 0,0:00:10.03,0:00:13.11,Default,,0000,0000,0000,,It's so fast that we measure\Nenormous distances
Dialogue: 0,0:00:13.11,0:00:16.32,Default,,0000,0000,0000,,by how long it takes\Nfor light to travel them.
Dialogue: 0,0:00:16.32,0:00:20.40,Default,,0000,0000,0000,,In one year, light travels\Nabout 6,000,000,000,000 miles,
Dialogue: 0,0:00:20.40,0:00:22.92,Default,,0000,0000,0000,,a distance we call one light year.
Dialogue: 0,0:00:22.92,0:00:25.27,Default,,0000,0000,0000,,To give you an idea of just\Nhow far this is,
Dialogue: 0,0:00:25.27,0:00:29.20,Default,,0000,0000,0000,,the Moon, which took the Apollo astronauts\Nfour days to reach,
Dialogue: 0,0:00:29.20,0:00:32.28,Default,,0000,0000,0000,,is only one light-second from Earth.
Dialogue: 0,0:00:32.28,0:00:36.70,Default,,0000,0000,0000,,Meanwhile, the nearest star beyond\Nour own Sun is Proxima Centauri,
Dialogue: 0,0:00:36.70,0:00:39.73,Default,,0000,0000,0000,,4.24 light years away.
Dialogue: 0,0:00:39.73,0:00:44.28,Default,,0000,0000,0000,,Our Milky Way is on the order of\N100,000 light years across.
Dialogue: 0,0:00:44.28,0:00:46.88,Default,,0000,0000,0000,,The nearest galaxy to our own, Andromeda,
Dialogue: 0,0:00:46.88,0:00:49.86,Default,,0000,0000,0000,,is about 2.5 million light years away
Dialogue: 0,0:00:49.86,0:00:52.62,Default,,0000,0000,0000,,Space is mind-blowingly vast.
Dialogue: 0,0:00:52.62,0:00:56.96,Default,,0000,0000,0000,,But wait, how do we know how\Nfar away stars and galaxies are?
Dialogue: 0,0:00:56.96,0:01:01.23,Default,,0000,0000,0000,,After all, when we look at the sky,\Nwe have a flat, two-dimensional view.
Dialogue: 0,0:01:01.23,0:01:05.32,Default,,0000,0000,0000,,If you point you finger to one star,\Nyou can't tell how far the star is,
Dialogue: 0,0:01:05.32,0:01:08.68,Default,,0000,0000,0000,,so how do astrophysicists figure that out?
Dialogue: 0,0:01:08.68,0:01:10.92,Default,,0000,0000,0000,,For objects that are very close by,
Dialogue: 0,0:01:10.92,0:01:14.78,Default,,0000,0000,0000,,we can use a concept called\Ntrigonometric parallax.
Dialogue: 0,0:01:14.78,0:01:16.55,Default,,0000,0000,0000,,The idea is pretty simple.
Dialogue: 0,0:01:16.55,0:01:17.96,Default,,0000,0000,0000,,Let's do an experiment.
Dialogue: 0,0:01:17.96,0:01:21.29,Default,,0000,0000,0000,,Stick out your thumb and \Nclose your left eye.
Dialogue: 0,0:01:21.29,0:01:24.89,Default,,0000,0000,0000,,Now, open your left eye and\Nclose your right eye.
Dialogue: 0,0:01:24.89,0:01:26.88,Default,,0000,0000,0000,,It will look like your thumb has moved,
Dialogue: 0,0:01:26.88,0:01:31.07,Default,,0000,0000,0000,,while more distant background objects\Nhave remained in place.
Dialogue: 0,0:01:31.07,0:01:33.89,Default,,0000,0000,0000,,The same concept applies when\Nwe look at the stars,
Dialogue: 0,0:01:33.89,0:01:38.08,Default,,0000,0000,0000,,but distant stars are much, much \Nfarther away than the length of your arm,
Dialogue: 0,0:01:38.08,0:01:39.93,Default,,0000,0000,0000,,and the Earth isn't very large,
Dialogue: 0,0:01:39.93,0:01:43.08,Default,,0000,0000,0000,,so even if you had different telescopes\Nacross the equator,
Dialogue: 0,0:01:43.08,0:01:45.90,Default,,0000,0000,0000,,you'd not see much of a shift in position.
Dialogue: 0,0:01:45.90,0:01:51.23,Default,,0000,0000,0000,,Instead, we look at the change in the\Nstar's apparent location over six months,
Dialogue: 0,0:01:51.23,0:01:55.64,Default,,0000,0000,0000,,the halfway point of the Earth's\Nyearlong orbit around the Sun.
Dialogue: 0,0:01:55.64,0:01:58.81,Default,,0000,0000,0000,,When we measure the relative positions\Nof the stars in summer,
Dialogue: 0,0:01:58.81,0:02:02.84,Default,,0000,0000,0000,,and then again in winter,\Nit's like looking with your other eye.
Dialogue: 0,0:02:02.84,0:02:05.44,Default,,0000,0000,0000,,Nearby stars seem to have moved\Nagainst the background
Dialogue: 0,0:02:05.44,0:02:08.33,Default,,0000,0000,0000,,of the more distant stars and galaxies.
Dialogue: 0,0:02:08.33,0:02:13.09,Default,,0000,0000,0000,,But this method only works for objects no\Nmore than a few thousand light years away.
Dialogue: 0,0:02:13.09,0:02:15.78,Default,,0000,0000,0000,,Beyond our own galaxy,\Nthe distances are so great
Dialogue: 0,0:02:15.78,0:02:20.81,Default,,0000,0000,0000,,that the parallax is too small to detect\Nwith even our most sensitive instruments.
Dialogue: 0,0:02:20.81,0:02:23.72,Default,,0000,0000,0000,,So at this point we have to rely \Non a different method
Dialogue: 0,0:02:23.72,0:02:27.46,Default,,0000,0000,0000,,using indicators we call standard candles.
Dialogue: 0,0:02:27.46,0:02:32.08,Default,,0000,0000,0000,,Standard candles are objects whose\Nintrinsic brightness, or luminosity,
Dialogue: 0,0:02:32.08,0:02:34.38,Default,,0000,0000,0000,,we know really well.
Dialogue: 0,0:02:34.38,0:02:37.43,Default,,0000,0000,0000,,For example, if you know how bright\Nyour light bulb is,
Dialogue: 0,0:02:37.43,0:02:40.81,Default,,0000,0000,0000,,and you ask your friend to hold\Nthe light bulb and walk away from you,
Dialogue: 0,0:02:40.81,0:02:43.74,Default,,0000,0000,0000,,you know that the amount of light\Nyou receive from your friend
Dialogue: 0,0:02:43.74,0:02:47.15,Default,,0000,0000,0000,,will decrease by the distance squared.
Dialogue: 0,0:02:47.15,0:02:49.59,Default,,0000,0000,0000,,So by comparing the amount \Nof light you receive
Dialogue: 0,0:02:49.59,0:02:51.93,Default,,0000,0000,0000,,to the intrinsic brightness \Nof the light bulb,
Dialogue: 0,0:02:51.93,0:02:55.03,Default,,0000,0000,0000,,you can then tell how far away\Nyour friend is.
Dialogue: 0,0:02:55.03,0:02:58.28,Default,,0000,0000,0000,,In astronomy, our light bulb turns out to\Nbe a special type of star
Dialogue: 0,0:02:58.28,0:03:00.79,Default,,0000,0000,0000,,called a cepheid variable.
Dialogue: 0,0:03:00.79,0:03:03.03,Default,,0000,0000,0000,,These stars are internally unstable,
Dialogue: 0,0:03:03.03,0:03:06.100,Default,,0000,0000,0000,,like a constantly inflating \Nand deflating balloon.
Dialogue: 0,0:03:06.100,0:03:10.69,Default,,0000,0000,0000,,And because the expansion and contraction\Ncauses their brightness to vary,
Dialogue: 0,0:03:10.69,0:03:15.21,Default,,0000,0000,0000,,we can calculate their luminosity\Nby measuring the period of this cycle,
Dialogue: 0,0:03:15.21,0:03:19.16,Default,,0000,0000,0000,,with more luminous stars \Nchanging more slowly.
Dialogue: 0,0:03:19.16,0:03:21.53,Default,,0000,0000,0000,,By comparing the light\Nwe observe from these stars
Dialogue: 0,0:03:21.53,0:03:24.45,Default,,0000,0000,0000,,to the intrinsic brightness we've\Ncalculated this way,
Dialogue: 0,0:03:24.45,0:03:26.94,Default,,0000,0000,0000,,we can tell how far away they are.
Dialogue: 0,0:03:26.94,0:03:30.24,Default,,0000,0000,0000,,Unfortunately, this is still not \Nthe end of the story.
Dialogue: 0,0:03:30.24,0:03:34.80,Default,,0000,0000,0000,,We can only observe individual stars\Nup to about 40,000,000 light years away,
Dialogue: 0,0:03:34.80,0:03:37.89,Default,,0000,0000,0000,,after which they become \Ntoo blurry to resolve.
Dialogue: 0,0:03:37.89,0:03:41.08,Default,,0000,0000,0000,,But luckily we have another type\Nof standard candle:
Dialogue: 0,0:03:41.08,0:03:44.46,Default,,0000,0000,0000,,the famous type 1a supernova.
Dialogue: 0,0:03:44.46,0:03:49.75,Default,,0000,0000,0000,,Supernovae, giant stellar explosions\Nare one of the ways that stars die.
Dialogue: 0,0:03:49.75,0:03:51.58,Default,,0000,0000,0000,,These explosions are so bright,
Dialogue: 0,0:03:51.58,0:03:54.51,Default,,0000,0000,0000,,that they outshine the galaxies\Nwhere they occur.
Dialogue: 0,0:03:54.51,0:03:57.70,Default,,0000,0000,0000,,So even when we can't see \Nindividual stars in a galaxy,
Dialogue: 0,0:03:57.70,0:04:00.84,Default,,0000,0000,0000,,we can still see supernovae \Nwhen they happen.
Dialogue: 0,0:04:00.84,0:04:05.01,Default,,0000,0000,0000,,And type 1a supernovae turn out\Nto be usable as standard candles
Dialogue: 0,0:04:05.01,0:04:08.64,Default,,0000,0000,0000,,because intrinsically bright ones\Nfade slower than fainter ones.
Dialogue: 0,0:04:08.64,0:04:10.92,Default,,0000,0000,0000,,Through our understanding \Nof this relationship
Dialogue: 0,0:04:10.92,0:04:13.14,Default,,0000,0000,0000,,between brightness and decline rate,
Dialogue: 0,0:04:13.14,0:04:15.56,Default,,0000,0000,0000,,we can use these supernovae \Nto probe distances
Dialogue: 0,0:04:15.56,0:04:18.74,Default,,0000,0000,0000,,up to several billions of light years away.
Dialogue: 0,0:04:18.74,0:04:23.55,Default,,0000,0000,0000,,But why is it important to see\Nsuch distant objects anyway?
Dialogue: 0,0:04:23.55,0:04:26.66,Default,,0000,0000,0000,,Well, remember how fast light travels.
Dialogue: 0,0:04:26.66,0:04:30.62,Default,,0000,0000,0000,,For example, the light emitted by the Sun\Nwill take eight minutes to reach us,
Dialogue: 0,0:04:30.62,0:04:36.57,Default,,0000,0000,0000,,which means that the light we see now\Nis a picture of the Sun eight minutes ago.
Dialogue: 0,0:04:36.57,0:04:38.20,Default,,0000,0000,0000,,When you look at the Big Dipper,
Dialogue: 0,0:04:38.20,0:04:41.75,Default,,0000,0000,0000,,you're seeing what it looked like\N80 years ago.
Dialogue: 0,0:04:41.75,0:04:43.43,Default,,0000,0000,0000,,And those smudgy galaxies?
Dialogue: 0,0:04:43.43,0:04:45.68,Default,,0000,0000,0000,,They're millions of light years away.
Dialogue: 0,0:04:45.68,0:04:49.39,Default,,0000,0000,0000,,It has taken millions of years for\Nthat light to reach us.
Dialogue: 0,0:04:49.39,0:04:54.68,Default,,0000,0000,0000,,So the universe itself is in some sense\Nan inbuilt time machine.
Dialogue: 0,0:04:54.68,0:04:59.25,Default,,0000,0000,0000,,The further we can look back,\Nthe younger the universe we are probing.
Dialogue: 0,0:04:59.25,0:05:02.30,Default,,0000,0000,0000,,Astrophysicists try to read the history\Nof the universe,
Dialogue: 0,0:05:02.30,0:05:06.06,Default,,0000,0000,0000,,and understand how\Nand where we come from.
Dialogue: 0,0:05:06.06,0:05:10.87,Default,,0000,0000,0000,,The universe is constantly sending us\Ninformation in the form of light.
Dialogue: 0,0:05:10.87,0:05:13.74,Default,,0000,0000,0000,,All that remains if for us to decode it.