WEBVTT

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[MUSIC PLAYING]

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What is a galaxy?

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Until the 1920s, we only knew of
our own galaxy, the Milky Way.

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Even with telescopes,
we could barely

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see little fuzzy spiral
shapes in the sky,

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and we wondered what
exactly they were.

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Dusty nebulas or other
galaxies entirely?

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There was a great
debate about the issue.

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Ultimately, Henrietta
Swan Leavitt's work

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on distances to stars
enabled Edwin Hubble

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to prove that the largest
fuzzy spiral shape in the sky

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was actually a galaxy entirely
separate from our own,

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the Andromeda Galaxy.

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So cool.

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It was the beginning of our
discovering and grasping

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the vastness of space.

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Since Andromeda,
we've learned there

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are billions more galaxies
outside of the Milky Way.

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Galaxies are concentrations
of stars, gas, dust,

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and dark matter, all
held together by gravity.

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Many galaxies are spiral
galaxies, like the Milky Way.

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But there are also elliptical
and irregular galaxies.

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As far as we know,
all large galaxies

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have a supermassive black
hole at their centers.

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We also know that galaxies
interact with each other,

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creating beautiful new forms
over millions of years.

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This is true for Andromeda
and the Milky Way too.

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Our galaxy is on
a collision course

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with our nearest neighbor.

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While that may seem
violent to you and me,

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galaxies are so huge that
most stars will pass right

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by each other.

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Phew.

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Eventually, the Milky
Way and Andromeda

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will merge into one giant
new elliptical galaxy.

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It's not for another
several billion years,

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but it's sure to be
a spectacular show

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for the stargazers of
the distant future.

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