WEBVTT FILE

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"Hubble Science"

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"Galaxy Mergers, Past and Present"

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One of the surprises that the Hubble Space
Telescope has given us over

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its decades of observation
is that galaxies have not always

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simply existed as independent entities,

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one from another, forming separately
and remaining separate, that in fact

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galaxies typically merge
with other galaxies during cosmic history.

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Now, this is a little counterintuitive
because we know the universe is expanding,

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so we would expect that galaxies

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are moving apart from each other,
which they generally are.

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But what we know
is that when galaxies are close enough

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to each other, their mutual gravitational
pull can dominate

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and draw these galaxies together as one.
We call that a merger.

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And Hubble revealed for the first time
in such detail

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many of these cases where galaxies,
particularly in the distant universe,

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were in the process of merging
with a neighbor galaxy.

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These dramatic mergers are not unusual.

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In fact, we believe now, thanks to Hubble
and subsequent observations

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that all galaxies
typically go through at least one merger

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in the process
to becoming what they are today.

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So here's some

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examples of what Hubble
has seen in terms of galaxies

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in the process of merging
or in the aftermath of merging.

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So here you can see some examples
of distorted shapes of galaxies.

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You can see here and here
none of these look like your typical

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beautiful spiral
or even spherical or elliptical shape.

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They all have some irregularities to them,
which are telltale signs

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that they are either currently merging
with another galaxy or are right

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in the aftermath of merging
with another galaxy.

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One of those telltale signs are long tails.

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So in some of these
you see these interesting long

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tail structures
coming out from the galaxy itself.

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That's often an aftereffect
or an impact of galaxies gravitationally

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interacting with each other
and creating these long tail structures.

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Another telltale sign is that you can
sometimes see two nuclei

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visible in the cores
of some of these structures.

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That means that there’s still two bright
cores of the originally separate

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galaxies that haven't quite completed
merging together.

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An irregular structure, as all of these
have, is always a sign that something

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has disrupted the original more symmetric
structure of these galaxies.

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And it's typically the interaction
with a neighboring galaxy during

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the process of merging,

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"Galaxy Merger Simulations"

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Mergers can take millions of years.

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So these are not quick processes
and in fact,

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sometimes they are very interesting
little dances where the galaxies

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can come close to each other
and then begin to rotate around one

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another before they eventually merge,
or they can actually pass through one

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another and then be pulled back again
and again before they eventually merge.

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So this can take a long time.

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So we now know that

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most galaxies have gone through mergers

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and some galaxies are still on the path
toward a future merger.

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We know that our own Milky Way
has a whole little swarm of dwarf galaxies

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orbiting around that will eventually
be swallowed into the Milky Way.

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But we have a more traditional merger
in line with the Andromeda

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Galaxy, our nearest neighbor,
a big spiral galaxy.

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It's on a head on collision course
with the Milky Way.

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Hubble has ascertained

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that this is going to happen
probably in a few billion years.

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So we don't need to think about it
too soon.

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But it is interesting
that our universe is not finished.

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We're still in the dynamics of forming
and growing galaxies, even today.

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"Follow us on social media @NASAHubble"