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In the beginning,
there was darkness...
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and then, bang...
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giving birth to an endless
expanding existence...
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00:00:09,384 --> 00:00:11,927
of time, space, and matter.
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Now, see further
than we've ever imagined...
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beyond the limits of our existence...
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in a place we call "The Universe. "
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It's a crucible
of creation and destruction.
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The name "Milky Way"
sounds like something...
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kind of comforting and sweet.
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But the Milky Way galaxy
is a monster.
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It's just one galaxy among billions,
and we're living on the edge.
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Just recently, we've discovered
that there are two small galaxies...
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colliding
with the Milky Way right now.
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It's a tapestry of brilliant
suns and blinding dust.
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It's surprising
how little of the light...
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from our rather luminous
Milky Way galaxy reaches us.
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It's a place of extremes,
where stars can drift lazily...
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or be flung out at more
than a million miles per hour.
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Now scientists have pierced
the galaxy's heart of darkness...
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to find our way
through the Milky Way.
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It's 100,000 light-years in diameter...
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has a trillion times
more mass than our Sun.
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It began about
thirteen billion years ago...
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and is still under construction.
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It's our galaxy, the Milky Way.
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The Milky Way galaxy
is an extremely active place.
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It's like a construction project.
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There's things going on
all the time.
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You have old stars
dying and torn down...
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and then that material gets used
to build brand-new stars.
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In the midst of this work zone...
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lies our little solar system
and a whole lot more.
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I suppose the best way to think
of the Milky Way galaxy...
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is our family of stars.
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These are the stars that
we travel through the universe with...
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in a clump,
all orbiting a common center.
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Within its far-reaching spiral arms...
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lie clues to where we started...
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and how it all will end.
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On a clear summer night,
the stars of the Milky Way...
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unfurl like a shimmering banner
across the sky.
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Ancient Egyptians
saw this river of stars...
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as a pathway to the afterlife...
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but the Greeks
were the first to name it.
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The Milky Way comes
from the word "galacos"...
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which is Greek for milk.
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And though we once believed
we lived at the center of the universe...
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we now know
that we don't even live...
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in the center
of our own galaxy.
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When we look up
into the night sky...
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and we see
this milky swath of stars...
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that we call the Milky Way...
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what we're actually seeing
is a spiral arm of the galaxy...
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that's closer to the center
of the galaxy than we are.
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We can't really see the center
of the galaxy from here...
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but what we can see is
one of the spiral arms...
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that's a few thousand light-years
closer to the center than we are.
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As we gaze at the Milky Way
from our earthbound position...
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it's like looking
at the edge of a coin.
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We get no sense
of the galaxy's real shape.
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If, however, you look
at a galaxy from the top down...
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it's a disk, remember...
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and it's like looking
at a Frisbee from the top down.
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You can see its full glory.
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Even though it's thin,
you don't see how thin it is...
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but you see its full structure.
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You can make the analogy
of the Milky Way...
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as being very much like a city.
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There's a central region...
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there's big buildings...
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there's a lot of action
in the middle...
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and that's certainly true
of our galaxy.
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Then you move out
to the suburbs...
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where life is a little bit
more comfortable...
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a little more relaxing.
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It's a better place
to raise a family.
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This is where we reside.
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Our solar system is among
the Milky Way's spiral arms...
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26,000 light-years
from the bustling center.
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Our galaxy is so large...
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that it takes Earth more than
200 million years to make one lap.
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The Sun is located
in what would be...
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just an average neighborhood
around the city center.
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But, again, if you stand
in the middle of this neighborhood...
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you don't really know
what the neighborhood...
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on the opposite side
of the city looks like...
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because you can't see it directly.
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00:05:04,429 --> 00:05:07,597
As we move outward
beyond the suburbs...
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the population
becomes more sparse.
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And, yeah, you've even got
sort of the boondocks.
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In our galaxy,
you've got the halo...
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where you have very old stars...
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in sort of wide orbits
around the galaxy.
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The galactic sprawl doesn't stop...
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at the Milky Way's
loose and undefined halo.
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It reaches far beyond
our neighborhood...
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and out to a group of galaxies
called the Local Group.
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If you think
of the Milky Way galaxy...
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as a city like Los Angeles...
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then you can think of these as counties
all within the state of California.
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And together, this Local Group
makes up the entire state.
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Besides our huge Milky Way...
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and the even larger
Andromeda galaxy...
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the Local Group consists of close
to fifty smaller galaxies...
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the closest of which are roughly
40,000 light-years away.
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There are two relatively
nearby dwarf galaxies...
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the Large and Small
Clouds of Magellan...
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that orbit
our Milky Way galaxy...
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and are easily seen
in the southern hemisphere.
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They're small, little galaxies.
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But our Local Group has
several dozen such galaxies...
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sort of wandering around inside it.
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The big galaxies are the minority.
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One advantage of actually being
in the outer edges of our galaxy...
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we have a clearer view
of outside of our Milky Way.
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We're able to see
the rest of the universe...
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our neighboring galaxies
and galaxy clusters...
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beyond our own
Local Group of galaxies.
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Inside the Milky Way's halo
lie massive globular clusters.
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Globular clusters are densely
packed regions of stars...
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that are all of similar composition.
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These are like the ethnic neighborhoods
of the Milky Way galaxy.
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Indeed, these globular clusters formed
when the galaxy was very young.
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They are among the first stars
to have formed.
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Some globular clusters are
twelve or thirteen billion years old.
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They contain a hundred thousand
or even a million stars.
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Globular clusters,
almost as old as the universe itself...
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gave us the first clue
to our place in the galaxy.
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We saw these globular
clusters in the sky...
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but their center was somewhere
far away from us.
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And that was one
of our first measurements...
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of where the middle
of the galaxy should be.
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They weren't orbiting around us...
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but around an area somewhere
about 27,000 light-years away.
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We can never get far enough
away from our own galaxy...
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to actually see it.
138
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But by looking at other galaxies...
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00:07:56,767 --> 00:07:59,352
and comparing what we know
about our own galaxy...
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we've got a pretty good picture
of the Milky Way.
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00:08:04,108 --> 00:08:06,818
The very first time we saw
galaxies through telescopes...
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we didn't even know
they were galaxies.
143
00:08:10,156 --> 00:08:12,908
We thought they were just
nebulae in our own galaxy...
144
00:08:12,950 --> 00:08:15,702
and we were just amazed
by the beautiful spiral shape.
145
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So it was sort of natural
for astronomers...
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to classify galaxies according
to their shape, what we see.
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Astronomers recognize
four basic galactic shapes:
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Elliptical, built of old stars
and which doesn't spin...
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lenticular, consisting
of a bulge and a disc...
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00:08:34,764 --> 00:08:37,098
and little
or no new star formation...
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irregular, which
has no real shape at all...
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like the Magellanic Clouds
in our own Local Group...
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and spiral,
which includes our Milky Way.
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It's a pinwheel
of young and old stars...
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spinning gracefully
through space.
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A long time ago, people thought
that maybe an elliptical galaxy
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eventually
collapses down into a spiral...
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or maybe eventually
spirals all come together...
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and form an elliptical.
160
00:09:06,128 --> 00:09:09,422
And it was sort of hard to figure out
exactly what the sequence is.
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One thing we know is that elliptical
galaxies tend to be very large.
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Centaurus A,
a misshapen elliptical galaxy...
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about thirteen million
light-years away...
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suggests why
elliptical galaxies get so big.
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There's a lot of evidence that there's
a spiral galaxy in there somewhere...
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that got absorbed
by a larger galaxy.
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00:09:33,614 --> 00:09:35,740
So, right now, it's possible...
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00:09:35,783 --> 00:09:39,411
that these really big
elliptical galaxies we see...
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may be the mergers of several spirals
and other types of galaxies.
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Our Milky Way isn't in danger
of being absorbed any time soon.
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But the heart of our spiral galaxy
has revealed a secret of its own.
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Only recently, we discovered
that the Milky Way...
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is not a classic spiral,
but a barred spiral.
174
00:10:03,894 --> 00:10:06,104
There's a bar of stars
going through the center...
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and the spiral arms sort of
attach off from that bar.
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Spanning 27,000 light-years...
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it's the most popular bar
in the galaxy.
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Thirty million stars gravitate to it.
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The bar of our galaxy is
a natural result of gravity...
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00:10:24,123 --> 00:10:27,667
the mutual gravitational interactions
of the individual stars...
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00:10:27,710 --> 00:10:32,464
that form the disk of our galaxy
and the bulge of our galaxy.
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It causes sometimes the stars
to sort of bunch up...
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into this bar configuration.
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Surrounding the middle
of the Milky Way...
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is a huge central bulge.
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It's mostly composed of stars between
ten and eleven billion years old.
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The size of the bulge...
188
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is linked to the Milky Way's
most gripping feature of all...
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a supermassive black hole.
190
00:10:58,741 --> 00:11:00,033
Though it would easily fit...
191
00:11:00,076 --> 00:11:02,786
in the space
between the Earth and our Sun...
192
00:11:02,828 --> 00:11:06,206
it's four million times
more massive than our Sun.
193
00:11:07,291 --> 00:11:08,708
Now, that sounds like a lot...
194
00:11:08,751 --> 00:11:10,877
but other galaxies
have central black holes...
195
00:11:10,920 --> 00:11:14,297
which extend up to a billion
times the mass of our Sun.
196
00:11:14,340 --> 00:11:18,385
So, whereas we call
our black hole...
197
00:11:18,427 --> 00:11:22,013
at the center of our Milky Way
a supermassive black hole...
198
00:11:22,056 --> 00:11:24,974
among supermassive black holes,
it's kind of a runt.
199
00:11:28,187 --> 00:11:32,524
Black holes can't be seen directly
because light can't escape them.
200
00:11:33,442 --> 00:11:35,985
Astronomers have located
galactic ground zero...
201
00:11:36,028 --> 00:11:39,239
through a radio source
in the constellation Sagittarius...
202
00:11:39,281 --> 00:11:42,701
known as Sagittarius A star.
203
00:11:42,743 --> 00:11:44,786
It's creating quite a stir.
204
00:11:45,746 --> 00:11:48,790
The black hole in the middle
of our galaxy is spinning...
205
00:11:48,833 --> 00:11:50,500
and it appears to be spinning...
206
00:11:50,543 --> 00:11:54,421
at a rate of about one spin
per eleven minutes.
207
00:11:56,465 --> 00:12:00,301
As it spins, central region stars
caught in its gravity...
208
00:12:00,344 --> 00:12:02,637
get swept along for the ride...
209
00:12:02,680 --> 00:12:06,141
orbiting it at about
three million miles per hour.
210
00:12:08,352 --> 00:12:11,354
The black hole affects
the central region the most.
211
00:12:12,773 --> 00:12:15,024
But we can't feel
its tug on Earth...
212
00:12:15,067 --> 00:12:18,403
since we orbit the galaxy
far away from the center.
213
00:12:19,488 --> 00:12:22,240
The colossal forces
at the galaxy's heart...
214
00:12:22,283 --> 00:12:26,161
are negated by the Milky Way's
unimaginable size.
215
00:12:27,121 --> 00:12:28,788
To me, the name "Milky Way"
sounds like something...
216
00:12:28,831 --> 00:12:30,707
kind of comforting,
sweet, you know...
217
00:12:30,750 --> 00:12:32,083
a candy bar,
that sort of thing.
218
00:12:32,126 --> 00:12:34,461
But the Milky Way galaxy
is a monster.
219
00:12:35,463 --> 00:12:37,338
It is incredibly huge.
220
00:12:37,381 --> 00:12:40,258
So, when you think about
how vast our solar system is...
221
00:12:40,301 --> 00:12:42,093
the fact that it takes years...
222
00:12:42,136 --> 00:12:45,722
for the fastest spacecraft to get
out to Saturn or Jupiter...
223
00:12:45,765 --> 00:12:48,391
if the entire solar system
were the size of a CD...
224
00:12:48,434 --> 00:12:50,977
the Earth would be comparable
to the Milky Way.
225
00:12:51,020 --> 00:12:52,353
That's huge.
226
00:12:52,396 --> 00:12:55,398
That's absolutely mind-blowing,
and it never ceases to amaze me.
227
00:12:57,193 --> 00:13:00,111
And the amazement
goes on and on.
228
00:13:00,154 --> 00:13:02,322
The galaxy's bustle
and commotion...
229
00:13:02,364 --> 00:13:04,157
may be concentrated
in the center...
230
00:13:04,200 --> 00:13:08,828
but the spectacular spiral arms
have action of their own.
231
00:13:08,871 --> 00:13:11,748
It's here that stellar
neighborhoods are being built...
232
00:13:11,791 --> 00:13:14,334
and stars are being born.
233
00:13:17,505 --> 00:13:19,714
This is our galaxy.
234
00:13:19,757 --> 00:13:21,132
It's a lot to take in...
235
00:13:21,175 --> 00:13:23,968
and we're just beginning
to probe its depths.
236
00:13:25,930 --> 00:13:29,599
Our suburban location makes
it difficult to get the big picture...
237
00:13:30,351 --> 00:13:34,395
and the hazy clouds of cosmic dust
only block our view.
238
00:13:35,606 --> 00:13:38,191
Even the most powerful
optical telescope...
239
00:13:38,234 --> 00:13:40,652
can't pierce the darkness.
240
00:13:40,694 --> 00:13:43,154
So, if we want to learn more
about the Milky Way...
241
00:13:43,197 --> 00:13:46,115
we need to look beyond
what our eyes can see.
242
00:13:50,120 --> 00:13:52,664
For all its vastness
and empty space...
243
00:13:52,706 --> 00:13:55,375
the Milky Way is
tremendously active...
244
00:13:55,417 --> 00:13:58,461
and populated with some
astonishing phenomena:
245
00:13:59,713 --> 00:14:00,964
Star clusters...
246
00:14:01,006 --> 00:14:02,674
nebulas...
247
00:14:02,716 --> 00:14:05,969
blazing invaders
from other galaxies.
248
00:14:06,887 --> 00:14:10,348
Technology is making us
rethink old beliefs...
249
00:14:10,391 --> 00:14:14,686
and is showing us things
we've never even considered before.
250
00:14:14,728 --> 00:14:16,688
We have ignition...
and liftoff.
251
00:14:20,901 --> 00:14:22,652
T-plus fifteen seconds.
252
00:14:22,695 --> 00:14:25,280
We're sending
the New Horizon spacecraft...
253
00:14:25,322 --> 00:14:28,241
on its way to the very edge
of our solar system.
254
00:14:31,453 --> 00:14:35,039
We live on a dusty planet
in a dusty galaxy...
255
00:14:35,082 --> 00:14:37,292
in a dusty universe.
256
00:14:37,334 --> 00:14:39,586
Empty space isn't so empty.
257
00:14:41,046 --> 00:14:45,383
Optical telescopes can see
only as far as the nearest dust cloud...
258
00:14:46,594 --> 00:14:48,094
which isn't far at all.
259
00:14:49,471 --> 00:14:51,681
Most of our galaxy
is invisible to us, however...
260
00:14:51,724 --> 00:14:54,434
and that's because the galaxy
is full of dust...
261
00:14:54,476 --> 00:14:55,894
dust clouds, if you will...
262
00:14:55,936 --> 00:14:58,062
and these dust clouds
block the light...
263
00:14:58,105 --> 00:15:00,523
from most of the stars
in our galaxy.
264
00:15:00,566 --> 00:15:02,609
It's surprising
how little of the light...
265
00:15:02,651 --> 00:15:06,905
from our rather luminous
Milky Way galaxy reaches us.
266
00:15:06,947 --> 00:15:09,908
The reason for that is simply
because the dust blocks it.
267
00:15:10,826 --> 00:15:15,079
But visible light is just a small sliver
of the energy spectrum...
268
00:15:15,122 --> 00:15:20,126
and radio waves rush in where
visible light beams fear to tread.
269
00:15:21,128 --> 00:15:24,297
On a foggy day, you might
not be able to see very far...
270
00:15:24,340 --> 00:15:26,925
at visible wavelengths,
which your eye can see...
271
00:15:26,967 --> 00:15:31,137
but you can still listen
to your radio or watch your TV.
272
00:15:32,097 --> 00:15:35,183
The ability of radio waves
to penetrate space dust...
273
00:15:35,225 --> 00:15:38,436
is crucial
to the study of the stars...
274
00:15:38,479 --> 00:15:41,481
but that use
was discovered by accident.
275
00:15:43,484 --> 00:15:49,030
In 1933, Karl Jansky, an engineer
at Bell Labs in New Jersey...
276
00:15:49,073 --> 00:15:52,200
built an antenna to track down
the source of static...
277
00:15:52,242 --> 00:15:54,786
on transatlantic
telephone lines.
278
00:15:54,828 --> 00:15:57,538
He was surprised to discover
the interference...
279
00:15:57,581 --> 00:16:00,083
was raining down
from the center of our galaxy...
280
00:16:00,125 --> 00:16:02,543
the constellation Sagittarius.
281
00:16:02,586 --> 00:16:04,253
It took several decades...
282
00:16:04,296 --> 00:16:07,799
for scientists to realize
that Jansky was on to something.
283
00:16:07,841 --> 00:16:11,636
Celestial bodies emit
electromagnetic radiation...
284
00:16:11,679 --> 00:16:15,306
and, thus,
radio astronomy was born.
285
00:16:15,349 --> 00:16:18,685
But radio waves
were just the beginning.
286
00:16:18,727 --> 00:16:21,646
Because the human eye can't
see all the light that's available...
287
00:16:21,689 --> 00:16:23,523
we have to resort
to technology.
288
00:16:23,565 --> 00:16:25,483
And one of the best ways
of seeing the universe...
289
00:16:25,526 --> 00:16:28,486
in a very different way
is with an infrared camera.
290
00:16:28,529 --> 00:16:32,824
An infrared camera sees only
the wavelengths generated by heat...
291
00:16:32,866 --> 00:16:36,202
then converts it to something
the human eye can see.
292
00:16:37,663 --> 00:16:39,247
There actually is
no visible light at all...
293
00:16:39,289 --> 00:16:40,540
that passes through this lens.
294
00:16:40,582 --> 00:16:42,542
This lens only lets
heat light through.
295
00:16:42,584 --> 00:16:43,626
And then you can see...
296
00:16:43,669 --> 00:16:45,712
what everything looks like
in infrared light.
297
00:16:46,547 --> 00:16:48,506
Three, two...
298
00:16:48,549 --> 00:16:49,590
main engine start...
299
00:16:49,633 --> 00:16:52,802
In 2003,
the Spitzer Space Telescope...
300
00:16:52,845 --> 00:16:55,346
equipped with a battery
of infrared cameras...
301
00:16:55,389 --> 00:16:56,931
was launched into space.
302
00:16:56,974 --> 00:16:59,017
...and the evolution
of our universe.
303
00:16:59,059 --> 00:17:02,270
Its mission is to explore
some of the youngest stars...
304
00:17:02,312 --> 00:17:05,189
and farthest galaxies
in the universe.
305
00:17:06,275 --> 00:17:07,525
The instruments
that Spitzer has on it...
306
00:17:07,568 --> 00:17:08,735
are actually many times...
307
00:17:08,777 --> 00:17:11,821
millions of times more sensitive
than these cameras...
308
00:17:11,864 --> 00:17:14,449
but they're basically
the same thing.
309
00:17:14,491 --> 00:17:16,826
This camera doesn't
peer into the heavens...
310
00:17:19,413 --> 00:17:23,416
but is used by firefighters
to save lives right here on Earth.
311
00:17:27,713 --> 00:17:30,214
The camera can see
through a smoke-filled room...
312
00:17:30,257 --> 00:17:34,052
to read the body heat from
an unconscious or immobilized victim.
313
00:17:37,931 --> 00:17:41,768
This same technology has enabled
astronomer Susan Stolovy...
314
00:17:41,810 --> 00:17:45,104
to see 26,000 light-years away...
315
00:17:45,147 --> 00:17:49,692
visualizing the center of
the galaxy as never before.
316
00:17:49,735 --> 00:17:51,736
This high-resolution mosaic...
317
00:17:51,779 --> 00:17:55,865
was assembled from roughly
12,000 individual images...
318
00:17:55,908 --> 00:17:57,617
taken from the Spitzer
Space Telescope.
319
00:17:58,660 --> 00:18:01,370
Even though that sounds like
a lot of data, and it is...
320
00:18:01,413 --> 00:18:04,415
it only took about sixteen hours
of telescope time.
321
00:18:04,458 --> 00:18:06,918
This particular region
of the galactic center...
322
00:18:06,960 --> 00:18:10,338
spans an area of the sky
that's equivalent to four full Moons...
323
00:18:10,380 --> 00:18:12,882
in one direction
and three full Moons in the other.
324
00:18:14,635 --> 00:18:19,806
That's the equivalent of 900 light-years
across by 700 light-years high...
325
00:18:20,974 --> 00:18:23,518
a small fraction
of what's out there to see...
326
00:18:23,560 --> 00:18:26,145
but a phenomenal achievement
nonetheless.
327
00:18:27,272 --> 00:18:28,606
Just a few decades ago...
328
00:18:28,649 --> 00:18:31,609
the galactic center
was not a subject of study...
329
00:18:31,652 --> 00:18:33,319
because you couldn't see it.
330
00:18:33,362 --> 00:18:34,862
Visually, nothing gets through.
331
00:18:34,905 --> 00:18:38,533
Only one light wave in a trillion
can penetrate the dust.
332
00:18:38,575 --> 00:18:41,744
But if you go into the infrared...
333
00:18:41,787 --> 00:18:44,914
or use radio astronomy
or X-ray astronomy...
334
00:18:44,957 --> 00:18:46,582
you can see
what's going on there.
335
00:18:47,835 --> 00:18:50,503
Different wavelengths of
the electromagnetic spectrum...
336
00:18:50,546 --> 00:18:53,798
can reveal different aspects
of the galaxy...
337
00:18:53,841 --> 00:18:56,843
because they penetrate
celestial objects differently.
338
00:18:59,930 --> 00:19:03,975
Radio waves have the lowest energy
or longest wavelength...
339
00:19:04,017 --> 00:19:06,018
but most celestial objects
emit them.
340
00:19:07,354 --> 00:19:11,941
Then comes infrared,
visible light, ultraviolet.
341
00:19:12,818 --> 00:19:16,988
High energy X-rays with wavelengths
about the size of an atom...
342
00:19:17,030 --> 00:19:20,241
are emitted by black holes
and supernovas.
343
00:19:20,284 --> 00:19:22,201
The highest energy, gamma rays...
344
00:19:22,244 --> 00:19:26,289
come from the collision or decay
of subatomic particles...
345
00:19:26,331 --> 00:19:29,458
like when stars explode
at billions of degrees.
346
00:19:30,586 --> 00:19:32,879
Together, these waves
give astronomers...
347
00:19:32,921 --> 00:19:36,924
a more complete picture of the activity
and shape of our galaxy.
348
00:19:39,511 --> 00:19:41,262
Many, if not all,
of the wavelengths...
349
00:19:41,305 --> 00:19:43,306
are needed to study the cosmos.
350
00:19:44,308 --> 00:19:47,185
The various wavelengths
of the electromagnetic spectrum...
351
00:19:47,227 --> 00:19:49,687
are similar
to the strings on a violin.
352
00:19:54,443 --> 00:19:56,694
In music,
many wavelengths of sound...
353
00:19:56,737 --> 00:19:59,572
are used to communicate
a musical idea.
354
00:19:59,615 --> 00:20:01,240
This piece
has a very large range...
355
00:20:01,283 --> 00:20:03,242
from a low note
to a high note.
356
00:20:10,459 --> 00:20:13,711
If we were to restrict ourselves
to the visible light spectrum...
357
00:20:13,754 --> 00:20:16,047
it's almost as if we
were only to hear two notes...
358
00:20:16,089 --> 00:20:17,673
in the middle of that piece.
359
00:20:22,095 --> 00:20:23,888
Using alternate wavelengths...
360
00:20:23,931 --> 00:20:26,599
the more we look,
the more we're discovering.
361
00:20:29,478 --> 00:20:32,021
Just recently, we've discovered
that there are two small galaxies...
362
00:20:32,064 --> 00:20:34,523
colliding with the Milky Way
right now...
363
00:20:34,566 --> 00:20:36,776
and the only reason
we didn't know they were there...
364
00:20:36,818 --> 00:20:38,736
is because
there was so much dust...
365
00:20:38,779 --> 00:20:41,072
in the disk of our galaxy,
we couldn't see them.
366
00:20:41,114 --> 00:20:43,741
We're living
inside this cloud...
367
00:20:43,784 --> 00:20:45,076
and it's something
we're not aware of.
368
00:20:45,118 --> 00:20:48,454
But with infrared light,
you can cut through that dust.
369
00:20:49,248 --> 00:20:52,083
And the minute we turned
infrared telescopes to the sky...
370
00:20:52,125 --> 00:20:55,503
we saw these little galaxies
up there, coming right at us.
371
00:20:59,132 --> 00:21:01,676
From our cockeyed position
in the Milky Way...
372
00:21:01,718 --> 00:21:05,054
it's difficult to gauge
what our galaxy really looks like.
373
00:21:06,556 --> 00:21:10,518
Radio and optical astronomy
give us a glimpse of its features...
374
00:21:10,560 --> 00:21:14,355
but to get the big picture,
we need to look outward.
375
00:21:15,774 --> 00:21:18,317
So the way we gain some
understanding of our own galaxy...
376
00:21:18,360 --> 00:21:20,486
is, first of all,
by looking at other galaxies...
377
00:21:20,529 --> 00:21:24,031
and seeing what they look like
and seeing things in other galaxies...
378
00:21:24,074 --> 00:21:27,618
that correspond to things
that we see in our own galaxy...
379
00:21:27,661 --> 00:21:30,037
like clouds of gas
that are kind of lined up...
380
00:21:30,080 --> 00:21:32,206
along what looks
like a spiral arm.
381
00:21:32,249 --> 00:21:35,001
You know, we can see parts
of spiral arms in our own galaxy...
382
00:21:35,043 --> 00:21:37,003
and we figure
that they're all connected...
383
00:21:37,045 --> 00:21:39,380
kind of like the spiral arms
of other galaxies...
384
00:21:39,423 --> 00:21:41,173
that we can see
from the outside.
385
00:21:42,509 --> 00:21:45,970
We know that the Milky Way's
four main spiral arms...
386
00:21:46,013 --> 00:21:50,433
swing out from the downtown
center like wide streets.
387
00:21:50,475 --> 00:21:55,313
From the inside out,
they're named Norma...
388
00:21:55,355 --> 00:21:56,355
Scutum-Crux...
389
00:21:57,274 --> 00:21:58,524
Sagittarius...
390
00:21:59,526 --> 00:22:00,693
and Perseus.
391
00:22:04,072 --> 00:22:06,657
If the arms
are the galaxy's suburbs...
392
00:22:06,700 --> 00:22:10,244
then our solar system lives
on a quiet, dead-end street...
393
00:22:10,287 --> 00:22:15,207
between Sagittarius and Perseus
on what's called the Orion Spur.
394
00:22:16,877 --> 00:22:18,502
All the stars in the Milky Way...
395
00:22:18,545 --> 00:22:22,798
add up to a community
of about 200 to 400 billion...
396
00:22:22,841 --> 00:22:25,176
and they're on the move.
397
00:22:25,218 --> 00:22:26,802
We are orbiting
around the galaxy.
398
00:22:26,845 --> 00:22:28,179
We change our position.
399
00:22:28,221 --> 00:22:30,014
So far, we think
the Sun has always been...
400
00:22:30,057 --> 00:22:32,141
about the same distance
away from the center...
401
00:22:32,184 --> 00:22:33,184
but we've been in and out...
402
00:22:33,226 --> 00:22:35,853
of pretty much every spiral arm
that there is.
403
00:22:37,731 --> 00:22:40,649
The spirals
are called density waves...
404
00:22:40,692 --> 00:22:43,694
areas where the stars and gas
get pushed together.
405
00:22:44,946 --> 00:22:47,406
As the density waves
spiral around...
406
00:22:47,449 --> 00:22:50,451
the billions of stars ride
over and through them.
407
00:22:52,496 --> 00:22:54,372
When you think about watching
the Tour de France...
408
00:22:54,414 --> 00:22:55,873
and you see
all of these bicycles.
409
00:22:55,916 --> 00:22:57,208
They're all moving forward.
410
00:22:57,250 --> 00:23:00,169
Sometimes they kind of clump up
around one bicyclist.
411
00:23:00,212 --> 00:23:01,921
And sometimes
they're stretched out.
412
00:23:01,963 --> 00:23:03,714
That's sort of what
the spiral arms are like...
413
00:23:03,757 --> 00:23:06,384
that the stars are going around
like the bicyclists...
414
00:23:06,426 --> 00:23:07,843
sometimes in clumpy areas...
415
00:23:07,886 --> 00:23:10,054
and sometimes
in more spread-out areas...
416
00:23:10,097 --> 00:23:12,098
but they keep going
around the center.
417
00:23:14,559 --> 00:23:17,770
Stars don't
usually travel alone.
418
00:23:17,813 --> 00:23:22,066
While giant globular clusters
populate the galaxy's halo...
419
00:23:22,109 --> 00:23:26,237
the galactic disk has open
or galactic clusters.
420
00:23:26,279 --> 00:23:27,947
These bundles of young stars...
421
00:23:27,989 --> 00:23:31,117
are barely held together
by their mutual gravity.
422
00:23:32,285 --> 00:23:36,414
Now, "open cluster" implies that
the stars are actually free to go.
423
00:23:36,456 --> 00:23:37,498
It is open.
424
00:23:37,541 --> 00:23:38,874
So this is a cluster, usually...
425
00:23:38,917 --> 00:23:41,085
of many stars
that have formed together...
426
00:23:41,128 --> 00:23:44,130
all from one of these
giant clouds of dust and gas.
427
00:23:44,172 --> 00:23:46,841
Over time, they're going
to move away from each other...
428
00:23:46,883 --> 00:23:48,551
distribute themselves
around the galaxy.
429
00:23:50,595 --> 00:23:55,474
Astronomers have counted about
20,000 open clusters in the galaxy.
430
00:23:56,059 --> 00:23:59,270
The Pleiades is the one
found nearest to the Earth.
431
00:23:59,312 --> 00:24:01,939
It formed
a hundred million years ago...
432
00:24:01,982 --> 00:24:04,442
and will be around
at least twice that long...
433
00:24:04,484 --> 00:24:07,736
before the galaxy's spiral arms
tear it apart.
434
00:24:09,865 --> 00:24:13,367
Closer to home, our own Sun,
orbiting in solitude...
435
00:24:13,410 --> 00:24:16,454
may have once been part
of an open-cluster star...
436
00:24:16,496 --> 00:24:18,330
that struck out on its own.
437
00:24:19,249 --> 00:24:23,210
The Sun, the star clusters,
our own planet...
438
00:24:23,253 --> 00:24:27,047
in fact, the entire galaxy
and the universe beyond...
439
00:24:27,090 --> 00:24:29,633
are built of dust and gases.
440
00:24:30,469 --> 00:24:33,429
These particles
that now block our view...
441
00:24:33,472 --> 00:24:36,432
are what got us here
in the first place...
442
00:24:36,475 --> 00:24:40,519
and the star-making machinery
is still cranking.
443
00:24:50,530 --> 00:24:54,366
Nothing beats the spectacle
of a glorious sunset...
444
00:24:55,702 --> 00:24:59,246
but we owe it all
to dust and gas.
445
00:25:00,332 --> 00:25:05,002
The setting Sun appears yellow,
orange, or even red for two reasons.
446
00:25:05,045 --> 00:25:08,088
First of all, the molecules of air
in the atmosphere...
447
00:25:08,131 --> 00:25:11,008
are scattering the violet,
blue, and green light...
448
00:25:11,051 --> 00:25:12,676
out of our line of sight...
449
00:25:12,719 --> 00:25:16,680
leaving the yellows, oranges,
and reds to reach our eyes.
450
00:25:16,723 --> 00:25:19,683
And second of all,
particles like dust or smoke...
451
00:25:19,726 --> 00:25:23,354
or smog in the atmosphere
absorb blue light...
452
00:25:23,396 --> 00:25:25,105
more than they do red light.
453
00:25:26,691 --> 00:25:32,696
Even the grandeur of a blue sky
is really an optical illusion.
454
00:25:32,739 --> 00:25:34,031
Why is the sky blue?
455
00:25:34,074 --> 00:25:36,742
There's nothing blue about
the gases of our atmosphere...
456
00:25:36,785 --> 00:25:39,745
but as sunlight comes
through our atmosphere...
457
00:25:39,788 --> 00:25:42,122
the shorter wavelengths,
the blue light...
458
00:25:42,165 --> 00:25:44,917
get scattered more
than the longer wavelengths do.
459
00:25:44,960 --> 00:25:46,877
So if you look
at any particular part of the sky...
460
00:25:46,920 --> 00:25:48,379
you're more
likely to see blue light...
461
00:25:48,421 --> 00:25:50,172
being scattered
towards your eye.
462
00:25:55,929 --> 00:26:00,015
Space is dark because there aren't
enough gas or dust particles...
463
00:26:00,058 --> 00:26:02,601
to reflect the light
of a billion stars.
464
00:26:04,271 --> 00:26:07,982
And though space may be
a vacuum, it's not perfect.
465
00:26:09,526 --> 00:26:13,487
The galactic disk,
the largest portion of the Milky Way...
466
00:26:13,530 --> 00:26:17,783
owes about fifteen percent
of its mass to dust and gas.
467
00:26:19,202 --> 00:26:23,789
Gas clouds can span hundreds
or even thousands of light-years...
468
00:26:23,832 --> 00:26:27,585
providing the raw material
that fires the galaxy.
469
00:26:27,627 --> 00:26:31,922
These regions of cosmic dust
and gas are called nebulae...
470
00:26:31,965 --> 00:26:35,676
and they produce effects
rivaling anything seen on Earth.
471
00:26:38,722 --> 00:26:40,931
A good example
is the Orion Nebula...
472
00:26:40,974 --> 00:26:43,267
in the constellation Orion.
473
00:26:43,310 --> 00:26:46,395
This region is active
with stellar formation...
474
00:26:46,438 --> 00:26:49,481
which makes the gas
around the stars glow.
475
00:26:50,567 --> 00:26:53,319
It's literally fluorescing
in response to the light...
476
00:26:53,361 --> 00:26:57,197
coming out of the massive stars
that are near the nebula...
477
00:26:57,240 --> 00:27:00,743
and this nebula literally
glows and can be seen.
478
00:27:00,785 --> 00:27:02,536
You can see it
with your naked eye.
479
00:27:04,080 --> 00:27:05,623
When you see
the Orion Nebula...
480
00:27:05,665 --> 00:27:09,001
in a real way, those are
baby pictures for us.
481
00:27:09,044 --> 00:27:12,046
Five billion years ago,
we were in a glowing hot nebula...
482
00:27:12,088 --> 00:27:14,465
and the Sun and the planets
were forming together...
483
00:27:14,507 --> 00:27:15,758
under the influence of gravity.
484
00:27:18,094 --> 00:27:20,387
Orion,
which contains hot stars...
485
00:27:20,430 --> 00:27:23,682
ionizing its gases
with ultraviolet light...
486
00:27:23,725 --> 00:27:26,518
is called a diffuse
or emission nebula.
487
00:27:28,271 --> 00:27:32,775
Astronomers classify
two other categories of nebulae.
488
00:27:32,817 --> 00:27:35,819
Some nebulae are what are
called reflection nebulae.
489
00:27:35,862 --> 00:27:38,238
They're simply-
the dust in these nebulae...
490
00:27:38,281 --> 00:27:41,659
is simply reflecting the starlight
from the bright star nearby.
491
00:27:44,287 --> 00:27:48,832
The Witch Head Nebula
is an example of a reflection nebula...
492
00:27:48,875 --> 00:27:51,210
borrowing light
from the star Rigel.
493
00:27:52,212 --> 00:27:57,132
Reflection nebulae appear blue
for the same reason our sky does.
494
00:27:57,175 --> 00:27:59,843
Blue light is
more easily reflected than red.
495
00:28:01,054 --> 00:28:04,890
And last there are dark nebulae
like the Horsehead.
496
00:28:08,770 --> 00:28:11,647
When low-mass stars
like our Sun die...
497
00:28:11,690 --> 00:28:16,193
they form another kind of nebula
called a planetary nebula.
498
00:28:18,571 --> 00:28:22,491
These dim, short-lived nebulae,
like the Cat's Eye Nebula...
499
00:28:22,534 --> 00:28:25,744
spew elements
back into the galaxy.
500
00:28:25,787 --> 00:28:30,708
These may become raw materials
for new suns and new planets.
501
00:28:36,256 --> 00:28:39,925
Just as dying stars spew out
clouds of dust and gas...
502
00:28:40,760 --> 00:28:44,263
dust and gas can come together
to form stars.
503
00:28:47,267 --> 00:28:51,645
At a construction site, you have
old buildings being torn down...
504
00:28:51,688 --> 00:28:53,564
and new buildings going up.
505
00:28:53,606 --> 00:28:57,151
And it's very much the same
in the Milky Way galaxy.
506
00:28:57,193 --> 00:28:59,278
You have old stars explode...
507
00:28:59,320 --> 00:29:02,030
and they cast out new material,
raw material...
508
00:29:02,073 --> 00:29:05,284
gas and dust that can be used
to form new stars.
509
00:29:09,038 --> 00:29:12,750
Nebulae are the galaxy's
recycling centers...
510
00:29:12,792 --> 00:29:15,252
where old becomes new again.
511
00:29:16,296 --> 00:29:18,756
Recycling is not just
a good idea here on Earth.
512
00:29:18,798 --> 00:29:22,676
It's a natural cosmic law.
513
00:29:22,719 --> 00:29:26,096
In fact, our own bodies
are made out of recycled material...
514
00:29:26,139 --> 00:29:28,223
from earlier generations of stars...
515
00:29:28,266 --> 00:29:31,018
that had dispersed material
into the interstellar medium...
516
00:29:31,060 --> 00:29:33,020
before our own
solar system formed.
517
00:29:34,773 --> 00:29:38,525
Each generation of stars
creates heavier elements...
518
00:29:38,568 --> 00:29:41,570
which become the ingredients
for everything in the universe.
519
00:29:42,655 --> 00:29:45,157
Most of the galaxy's
hot, young stars...
520
00:29:45,200 --> 00:29:47,868
get built in the Milky Way's
spiral arms.
521
00:29:49,913 --> 00:29:53,332
As gas clouds orbit the center
of the galaxy like the stars do...
522
00:29:54,334 --> 00:29:57,503
they get squeezed as they go
through a spiral arm.
523
00:29:58,421 --> 00:30:03,509
Remember, a spiral arm is simply
a wave in the pattern of stars.
524
00:30:03,551 --> 00:30:05,969
And because stars
are denser there...
525
00:30:06,012 --> 00:30:09,473
the gas clouds that orbit through it
tend to get compressed.
526
00:30:09,516 --> 00:30:14,186
That compression allows gravity
to get a hold of that gas...
527
00:30:14,229 --> 00:30:16,438
and cause it to collapse
to form stars...
528
00:30:16,481 --> 00:30:18,482
more readily there
than anywhere else.
529
00:30:21,152 --> 00:30:23,654
Stars often die
in the spiral arms...
530
00:30:23,696 --> 00:30:26,156
because they are formed here
more frequently...
531
00:30:26,199 --> 00:30:29,034
victims of
their own enormous mass.
532
00:30:30,203 --> 00:30:32,871
The more massive stars
are extremely powerful...
533
00:30:32,914 --> 00:30:34,289
extremely luminous.
534
00:30:34,332 --> 00:30:35,499
And to be that luminous...
535
00:30:35,542 --> 00:30:40,003
they have to use up
their energy source very quickly.
536
00:30:40,046 --> 00:30:43,757
So massive stars
live only short lives...
537
00:30:43,800 --> 00:30:50,264
and they are thus found in or near
their birthplaces, the spiral arms...
538
00:30:50,306 --> 00:30:52,683
because they simply
don't have enough time...
539
00:30:52,725 --> 00:30:55,894
to wander away from the places
where they were born.
540
00:30:58,565 --> 00:31:01,525
Although we can anticipate
the future of some stars...
541
00:31:01,568 --> 00:31:04,027
it's often difficult
to learn their history.
542
00:31:04,988 --> 00:31:07,239
But new techniques
are revealing new secrets...
543
00:31:07,282 --> 00:31:11,702
about a well-studied star
in the Milky Way called Mira A.
544
00:31:12,579 --> 00:31:16,039
Mira has actually been a favorite star
of astronomers for 400 years.
545
00:31:16,082 --> 00:31:18,500
It's a very easily visible star
in the night sky.
546
00:31:19,544 --> 00:31:24,172
Recently, the GALEX spacecraft,
the Galaxy Evolution Explorer...
547
00:31:24,215 --> 00:31:28,343
photographed Mira
in invisible ultraviolet light...
548
00:31:28,386 --> 00:31:34,266
and revealed that it's leaving a trail
thirteen light-years long behind it.
549
00:31:34,309 --> 00:31:35,851
We think that's actually
caused by the fact...
550
00:31:35,894 --> 00:31:38,645
that as the star plows
through the gas...
551
00:31:38,688 --> 00:31:40,898
the gas heats up
in a bow shock...
552
00:31:40,940 --> 00:31:43,817
very much like waves
breaking up against a boat.
553
00:31:43,860 --> 00:31:47,571
And then that streams out
into a wake of hot material.
554
00:31:47,614 --> 00:31:49,948
You're actually looking at Mira
acting very much like a boat...
555
00:31:49,991 --> 00:31:51,783
plowing through the water.
556
00:31:51,826 --> 00:31:53,827
When you look at how fast
Mira's going right now...
557
00:31:53,870 --> 00:31:56,246
about 291,000 miles an hour...
558
00:31:56,289 --> 00:31:57,706
and you do the calculations...
559
00:31:57,749 --> 00:32:01,335
that long tail is its path
the last 30,000 years.
560
00:32:01,377 --> 00:32:04,004
We can't predict exactly
where the Sun will go...
561
00:32:04,047 --> 00:32:05,505
in its orbit
around the Milky Way.
562
00:32:05,548 --> 00:32:07,925
There's all kinds of things
it could interact with.
563
00:32:07,967 --> 00:32:09,593
But here we have
the history of one star.
564
00:32:09,636 --> 00:32:11,929
We know this is the path it took.
565
00:32:11,971 --> 00:32:14,598
And that'll help us model
how the galaxy works...
566
00:32:14,641 --> 00:32:17,142
and how all the stars move
around the middle of the galaxy.
567
00:32:19,479 --> 00:32:22,856
Within the Milky Way's
suburban spiral arms...
568
00:32:22,899 --> 00:32:26,026
young stars enjoy plenty
of space to move around.
569
00:32:26,986 --> 00:32:29,279
As we move
into the galactic bulge...
570
00:32:29,322 --> 00:32:32,616
conditions get much more
crowded and urban.
571
00:32:33,493 --> 00:32:34,952
The closest star to the Sun...
572
00:32:34,994 --> 00:32:37,329
is a little more
than four light-years away.
573
00:32:37,372 --> 00:32:39,331
And when we look up
into the night sky...
574
00:32:39,374 --> 00:32:42,709
even on a perfectly clear night
with no lights around...
575
00:32:42,752 --> 00:32:46,129
you can't see more than
about 2,000 or 3,000 stars.
576
00:32:48,091 --> 00:32:52,636
But if our planet was down
in the middle of the galaxy...
577
00:32:52,679 --> 00:32:56,056
there would be a million stars
in the night sky...
578
00:32:56,099 --> 00:32:59,643
as bright as the brightest star
that we've ever seen in our sky.
579
00:32:59,686 --> 00:33:02,229
And it would be so bright that, in fact,
it wouldn't be nighttime.
580
00:33:02,271 --> 00:33:04,064
It would be daytime
all the time.
581
00:33:05,858 --> 00:33:09,236
Life, as we know it,
would be completely different.
582
00:33:09,278 --> 00:33:11,655
So what do we owe
our position to?
583
00:33:12,615 --> 00:33:16,410
Scientists believe that gravity
has a lot to do with it.
584
00:33:20,957 --> 00:33:25,210
Gravity is the power
that drives the galaxy...
585
00:33:25,253 --> 00:33:29,339
and at the galaxy's center,
churns the engine it feeds.
586
00:33:30,383 --> 00:33:35,012
Galaxies are like a city
in that they are ever-changing...
587
00:33:35,054 --> 00:33:38,390
and you're constantly being
rebuilt and reinvigorated.
588
00:33:40,184 --> 00:33:42,477
There's no question
that the Milky Way...
589
00:33:42,520 --> 00:33:43,562
a few billion years ago...
590
00:33:43,604 --> 00:33:45,939
looked a lot different
than it does today.
591
00:33:46,983 --> 00:33:49,026
It probably was smaller.
592
00:33:49,068 --> 00:33:51,194
It probably didn't have
the beautiful spiral shape...
593
00:33:51,237 --> 00:33:52,571
that we see today.
594
00:33:53,489 --> 00:33:57,492
As things collapse under gravity,
you tend to naturally form a disk.
595
00:33:57,535 --> 00:33:59,369
You'll notice
there are disks everywhere.
596
00:33:59,412 --> 00:34:02,581
Our solar system is a disk,
our galaxy is a disk.
597
00:34:02,623 --> 00:34:05,375
So the stars all start rotating
in the same direction.
598
00:34:08,755 --> 00:34:13,633
Just as in a city, not all of
the stars in the galaxy are natives.
599
00:34:13,676 --> 00:34:16,428
Some stars,
born beyond the Milky Way...
600
00:34:16,471 --> 00:34:19,848
settle here
and begin to make their mark.
601
00:34:20,475 --> 00:34:22,809
But then,
if another galaxy comes by...
602
00:34:22,852 --> 00:34:25,395
the gravity affects the way
the stars move...
603
00:34:25,438 --> 00:34:27,773
and this may initiate
the spiral arms.
604
00:34:28,691 --> 00:34:32,611
So, in fact, our spiral shape
may be some evidence...
605
00:34:32,653 --> 00:34:35,405
that the Milky Way is composed
of more than one small galaxy...
606
00:34:35,448 --> 00:34:37,824
that came together
a long time ago.
607
00:34:39,952 --> 00:34:42,120
Historically,
the center of the galaxy...
608
00:34:42,163 --> 00:34:44,956
has been
an impenetrable mystery...
609
00:34:44,999 --> 00:34:48,460
until we developed
X-ray vision.
610
00:34:49,295 --> 00:34:51,129
One of the first ways
we really identified...
611
00:34:51,172 --> 00:34:53,131
where the exact center
of the galaxy was...
612
00:34:53,174 --> 00:34:55,050
was with an X-ray telescope.
613
00:34:55,093 --> 00:34:57,260
Well, the X-rays were
able to pass through...
614
00:34:57,303 --> 00:35:00,263
all of the dust and gas
in the disk of our galaxy.
615
00:35:00,306 --> 00:35:04,267
And so, even though we can't really
see this bright center to the galaxy...
616
00:35:04,310 --> 00:35:07,437
in X-rays, there's this giant,
glowing hot source...
617
00:35:07,480 --> 00:35:09,106
right in the middle.
618
00:35:09,148 --> 00:35:12,234
The X-ray emitter,
Sagittarius A star...
619
00:35:12,276 --> 00:35:15,320
is associated with
the supermassive black hole...
620
00:35:15,363 --> 00:35:17,155
in the Milky Way's center.
621
00:35:18,491 --> 00:35:21,118
By definition, a black hole
doesn't allow light...
622
00:35:21,160 --> 00:35:23,787
or even X-rays to escape.
623
00:35:23,830 --> 00:35:27,332
The radiation comes from gas
caught in its gravity...
624
00:35:27,375 --> 00:35:31,002
spun and heated
to millions of degrees.
625
00:35:32,463 --> 00:35:35,924
They're moving
at extremely high velocities.
626
00:35:35,967 --> 00:35:40,053
For example, the more extreme cases
that we've been able to observe...
627
00:35:40,096 --> 00:35:43,932
the stars are moving
as they pass by the black hole...
628
00:35:43,975 --> 00:35:47,769
at a speed of 10,000
kilometers per second.
629
00:35:48,938 --> 00:35:51,940
That's like going around
the world in four seconds.
630
00:35:53,234 --> 00:35:56,903
And we're talking about a whole star
moving at that speed.
631
00:35:58,197 --> 00:36:01,366
When we watch stars orbiting
the very center of our galaxy...
632
00:36:01,409 --> 00:36:04,161
it's obvious that there's
some sort of invisible monster there.
633
00:36:06,205 --> 00:36:08,373
They're orbiting
around a giant mass.
634
00:36:08,416 --> 00:36:10,667
And the orbits of the stars
imply that there's about...
635
00:36:10,710 --> 00:36:13,712
three to four million times
the mass of the Sun...
636
00:36:13,754 --> 00:36:15,839
in the very center of our galaxy.
637
00:36:15,882 --> 00:36:19,176
We're not sure which came first,
the galaxy or the black hole...
638
00:36:19,218 --> 00:36:22,637
but we know that it's there,
and it's tremendous.
639
00:36:24,390 --> 00:36:28,268
For all its power and weirdness,
the supermassive black hole...
640
00:36:28,311 --> 00:36:32,314
is pretty typical for a galaxy
the size of the Milky Way.
641
00:36:33,774 --> 00:36:36,985
Other spiral galaxies
and big elliptical galaxies...
642
00:36:37,028 --> 00:36:40,989
also seem to have supermassive
black holes in their middle...
643
00:36:41,032 --> 00:36:44,034
ranging from a million times
the mass of our Sun...
644
00:36:44,076 --> 00:36:47,412
up to several billion times
the mass of our Sun.
645
00:36:49,749 --> 00:36:53,043
The stars surrounding
the black hole are ancient.
646
00:36:53,085 --> 00:36:57,088
Many are red giants, hundreds
of times bigger than our Sun.
647
00:36:58,382 --> 00:37:01,051
The galactic center
is crowded with them...
648
00:37:01,093 --> 00:37:04,387
like an urban downtown
crowded with people.
649
00:37:05,264 --> 00:37:08,475
And stars, like people,
can be pushy.
650
00:37:09,435 --> 00:37:10,810
Most stars
in the galactic center...
651
00:37:10,853 --> 00:37:13,688
simply just keep orbiting
the central black hole.
652
00:37:13,731 --> 00:37:14,856
But in the galactic center...
653
00:37:14,899 --> 00:37:16,983
there are so many stars
packed so close together...
654
00:37:17,026 --> 00:37:19,402
that stars are constantly
nudging each other...
655
00:37:19,445 --> 00:37:20,820
a little bit gravitationally.
656
00:37:20,863 --> 00:37:23,657
And their orbits
are being perturbed...
657
00:37:23,699 --> 00:37:24,741
changed a little bit.
658
00:37:26,661 --> 00:37:29,746
A bumped star can get
stripped of its atmosphere...
659
00:37:29,789 --> 00:37:32,207
leaving just its orbiting core.
660
00:37:32,250 --> 00:37:36,795
Or, rarely, it can tumble
into the black hole and vanish.
661
00:37:38,756 --> 00:37:41,758
But something else is happening
around the black hole.
662
00:37:42,635 --> 00:37:44,761
This turbulent,
dangerous neighborhood...
663
00:37:44,804 --> 00:37:47,430
is also a stellar nursery.
664
00:37:48,641 --> 00:37:50,433
The stars that we're observing...
665
00:37:50,476 --> 00:37:52,477
moving fastest
around the black hole...
666
00:37:52,520 --> 00:37:56,856
are the young stars
that have very recently formed.
667
00:37:56,899 --> 00:37:59,693
And it's something we call
the paradox of youth...
668
00:37:59,735 --> 00:38:01,945
because it's hard to imagine
how to form...
669
00:38:01,988 --> 00:38:05,240
these massive young stars
in the presence of a black hole...
670
00:38:05,283 --> 00:38:06,574
and yet there they are.
671
00:38:08,828 --> 00:38:10,787
Recently,
astronomers discovered...
672
00:38:10,830 --> 00:38:13,957
that not all stars caught
in the black hole's grip...
673
00:38:14,000 --> 00:38:15,667
are doomed to stay there.
674
00:38:17,003 --> 00:38:21,172
A few manage to break away
and see the universe.
675
00:38:24,635 --> 00:38:25,969
Barreling through the universe...
676
00:38:26,012 --> 00:38:29,431
at one and a half million
miles per hour...
677
00:38:29,473 --> 00:38:33,435
hyper-velocity stars
are the escapees of the galaxy.
678
00:38:34,854 --> 00:38:37,188
And what's interesting
about high-velocity stars...
679
00:38:37,231 --> 00:38:39,649
is the only way to explain
their extreme velocities...
680
00:38:39,692 --> 00:38:42,485
is that they were ejected
by a supermassive black hole.
681
00:38:44,155 --> 00:38:45,655
For a star to go ballistic...
682
00:38:45,698 --> 00:38:49,492
takes a very specific
set of circumstances...
683
00:38:49,535 --> 00:38:52,162
and, in fact,
it requires two stars.
684
00:38:53,497 --> 00:38:55,457
Most of the stars
you see in the sky...
685
00:38:55,499 --> 00:39:00,337
are not single stars,
but pairs or binary stars.
686
00:39:03,049 --> 00:39:07,010
They orbit around each other,
linked by gravity's embrace.
687
00:39:07,887 --> 00:39:10,305
But a star pair
in the galactic center...
688
00:39:10,348 --> 00:39:13,016
might get jostled
by surrounding stars...
689
00:39:13,059 --> 00:39:15,560
and stray too close
to the black hole.
690
00:39:17,104 --> 00:39:18,355
When that happens...
691
00:39:18,397 --> 00:39:21,691
the moment that the gravitational
pull of the black hole...
692
00:39:21,734 --> 00:39:25,320
exceeds the gravity that's
binding the two stars together...
693
00:39:25,363 --> 00:39:28,156
the pair of stars is broken apart.
694
00:39:28,199 --> 00:39:30,950
One of the stars will be
captured by the black hole...
695
00:39:30,993 --> 00:39:33,578
usually into a very tight orbit
around the black hole...
696
00:39:33,621 --> 00:39:34,704
and the other star
will then gain...
697
00:39:34,747 --> 00:39:36,206
all the energy of that system
and it'll be ejected...
698
00:39:36,248 --> 00:39:37,916
with this incredible velocity.
699
00:39:39,043 --> 00:39:41,252
If the galaxy were a city...
700
00:39:41,295 --> 00:39:44,881
where most of the stars
would be cars or pedestrians...
701
00:39:44,924 --> 00:39:48,051
a high-velocity star
would be more like an airplane...
702
00:39:48,094 --> 00:39:51,096
or a high-speed train
rushing out of the country.
703
00:39:52,598 --> 00:39:57,435
If you were on a high-velocity star,
the ride would be quite amazing.
704
00:39:57,478 --> 00:39:59,396
The sky would be covered
with stars...
705
00:39:59,438 --> 00:40:01,731
as bright as the full Moon
in every direction.
706
00:40:01,774 --> 00:40:03,400
But that view
would quickly change...
707
00:40:03,442 --> 00:40:06,069
'cause the high-velocity star
moves so quickly out of the galaxy...
708
00:40:06,112 --> 00:40:08,696
the stars would appear
fewer and fewer in the night sky.
709
00:40:10,491 --> 00:40:13,159
The galaxy is
constantly in motion...
710
00:40:13,202 --> 00:40:16,621
like a giant wheel
or a sprawling metropolis.
711
00:40:18,582 --> 00:40:19,916
In the heart of town...
712
00:40:19,959 --> 00:40:23,086
the supermassive black hole's gravity
whips stars around...
713
00:40:23,129 --> 00:40:25,713
in an orbit
of around eleven minutes.
714
00:40:27,633 --> 00:40:31,678
Where the Earth sits, two-thirds
of the way out on a spiral arm...
715
00:40:31,720 --> 00:40:36,724
we traverse the Milky Way
once every 250 million years.
716
00:40:38,394 --> 00:40:40,520
Our solar system
has been around the block...
717
00:40:40,563 --> 00:40:43,690
only eighteen times
since it formed.
718
00:40:44,567 --> 00:40:47,527
The Milky Way's
incomprehensible size...
719
00:40:47,570 --> 00:40:51,239
makes it easy to forget
it's just one small part...
720
00:40:51,282 --> 00:40:53,283
of an expanding universe.
721
00:40:54,160 --> 00:40:55,910
When people hear
about the expanding universe...
722
00:40:55,953 --> 00:40:58,788
a common misconception is
that everything is expanding.
723
00:40:58,831 --> 00:41:01,416
And in fact,
I'm not expanding right now.
724
00:41:01,459 --> 00:41:03,126
My atoms are the same size.
725
00:41:03,169 --> 00:41:04,669
My cells are the same size.
726
00:41:05,671 --> 00:41:08,840
The Earth is not getting
farther away from the Sun.
727
00:41:08,883 --> 00:41:12,302
The expansion of the universe
only applies to celestial objects...
728
00:41:12,344 --> 00:41:14,345
that aren't bound together
by gravity.
729
00:41:15,681 --> 00:41:17,724
Since the planets
within the Milky Way...
730
00:41:17,766 --> 00:41:19,684
have stronger gravitational pulls...
731
00:41:19,727 --> 00:41:23,104
than the expanding forces
outside our galaxy...
732
00:41:23,147 --> 00:41:27,108
the expansion of the universe
doesn't affect our solar system.
733
00:41:28,903 --> 00:41:31,529
Our own Milky Way,
a spiral galaxy...
734
00:41:31,572 --> 00:41:34,866
is on a collision course
with another spiral galaxy...
735
00:41:34,909 --> 00:41:36,367
the largest spiral near us...
736
00:41:36,410 --> 00:41:38,286
and that is
the Andromeda galaxy.
737
00:41:38,329 --> 00:41:42,123
We think that in maybe three
or four billion years...
738
00:41:42,166 --> 00:41:44,834
our two galaxies
may merge together.
739
00:41:44,877 --> 00:41:46,628
It will be very interesting
to see what happens.
740
00:41:48,255 --> 00:41:51,257
What probably won't happen
is a collision of stars.
741
00:41:52,301 --> 00:41:55,887
Even though both galaxies
contain billions of stars...
742
00:41:55,930 --> 00:41:57,931
the space between them
is enormous.
743
00:42:00,059 --> 00:42:02,227
They will
gravitationally interact...
744
00:42:02,269 --> 00:42:05,021
changing
their direction and motion.
745
00:42:05,064 --> 00:42:08,483
Eventually, the merged spirals
will settle down...
746
00:42:08,526 --> 00:42:10,818
to become an elliptical galaxy.
747
00:42:11,904 --> 00:42:16,074
Essentially, all of the several
dozen galaxies in our local group...
748
00:42:16,116 --> 00:42:19,202
will be part of one supergalaxy.
749
00:42:20,246 --> 00:42:23,831
And then, gradually, that supergalaxy
will start losing stars...
750
00:42:23,874 --> 00:42:28,461
because of gravitational interactions
among the stars within that galaxy.
751
00:42:28,504 --> 00:42:32,757
Some will get flung away
into intergalactic space.
752
00:42:34,843 --> 00:42:39,514
When it first formed, the Milky Way
built stars at a rapid pace...
753
00:42:39,557 --> 00:42:44,018
using raw materials that were
ejected in space from the Big Bang.
754
00:42:44,061 --> 00:42:45,728
As the galaxy aged...
755
00:42:45,771 --> 00:42:49,315
the star production slowed down
from a few hundred a year...
756
00:42:49,358 --> 00:42:53,361
to about four to six new stars
each year today.
757
00:42:53,404 --> 00:42:56,155
Over time, the Milky Way galaxy
has changed dramatically...
758
00:42:56,198 --> 00:42:58,741
and we don't know exactly
what it looked like long ago.
759
00:42:58,784 --> 00:43:02,120
But probably early on,
there was a lot more gas and dust...
760
00:43:02,162 --> 00:43:03,496
and probably fewer stars.
761
00:43:03,539 --> 00:43:06,583
And you had a lot of very large,
very massive stars...
762
00:43:06,625 --> 00:43:08,042
that would've formed early on.
763
00:43:08,085 --> 00:43:12,130
And these early stars
exploded fantastically...
764
00:43:12,172 --> 00:43:15,758
and spat out new material,
heavier metals...
765
00:43:15,801 --> 00:43:19,220
that could be used to form
smaller second-generation...
766
00:43:19,263 --> 00:43:21,014
and then third-generation stars.
767
00:43:23,475 --> 00:43:27,812
Some younger galaxies are still
enjoying that kind of building boom.
768
00:43:29,690 --> 00:43:30,815
We can see some galaxies...
769
00:43:30,858 --> 00:43:33,943
where the rate of star
formation is very high...
770
00:43:33,986 --> 00:43:35,111
compared to our galaxy.
771
00:43:35,154 --> 00:43:37,614
Those are called
starburst galaxies.
772
00:43:37,656 --> 00:43:39,532
The rate of star formation
there can be anywhere...
773
00:43:39,575 --> 00:43:42,577
from ten to a hundred times
what it is now in our galaxy.
774
00:43:43,787 --> 00:43:48,124
With every generation,
star production slows down...
775
00:43:48,167 --> 00:43:52,003
and the Milky Way has been
in business for thirteen billion years.
776
00:43:52,921 --> 00:43:56,174
One of the reasons that the rate
of star formation in our galaxy...
777
00:43:56,216 --> 00:43:59,135
has changed over time,
going from a very high rate...
778
00:43:59,178 --> 00:44:01,638
to the current modest rate
of star formation...
779
00:44:01,680 --> 00:44:04,599
is because the gas
is being used up.
780
00:44:04,642 --> 00:44:07,226
Gas is used up to form stars.
781
00:44:07,269 --> 00:44:09,562
And we're running out of gas,
literally.
782
00:44:10,439 --> 00:44:16,277
Eventually, over trillions of years,
star formation will stop completely.
783
00:44:17,529 --> 00:44:21,199
The great galactic construction
project will shut down...
784
00:44:22,368 --> 00:44:28,164
and one by one,
the twinkling stars will fade away.
67361
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