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One of the big mysteries
in astronomy

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is how galaxies grow
and evolve over time.

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Collisions between galaxies
are thought to be

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key events that shape
the development.

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A stunning collection of
59 new images of colliding galaxies

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has been released to mark
the 18th anniversary

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of the NASA / ESA
Hubble Space Telescope.

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They give us a unique insight

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into how galaxies merge
to form larger galaxies.

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This is the Hubblecast.

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News and images from the
NASA / ESA Hubble Space Telescope.

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Travelling through time and space
with our host, Dr. J

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EPISODE 16: Galaxies gone wild!
a.k.a. Dr. Joe Liske.

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Welcome to this special edition
of the Hubblecast.

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Today we have a real treat
in store for you,

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but before we start, the whole team
here at the Hubblecast

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would like to say thank you
to all of you

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for watching and for making
the Hubblecast

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such a big success
over the past year or so.

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We've received many e-mails
with questions and comments

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and it's great to see that there are
so many people out there

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who share our excitement
about astronomy.

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On 24th of April 2008,

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the NASA / ESA
Hubble Space Telescope

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would have been in orbit
around Earth for 18 years.

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Hubble is one of the most
successful scientific projects ever.

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Orbiting the Earth
at an altitude of 600 km.

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it has peered to the very edges
of the known Universe

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and it has delivered some of
the most breathtaking images

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of the cosmos ever taken.

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Among its achievements are that
it has helped scientists to discover

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what the atmospheres
of extrasolar planets are made of,

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that there are black holes at
the centres of nearly old galaxies

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and that the expansion of the Universe
is now accelerating.

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To celebrate the 18th anniversary
of Hubble's launch

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we are releasing a collection
of 59 brand new images

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of colliding galaxies.

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This is the larges collection
of Hubble images

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ever released
to the public simultaneously

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and we are proud to share them
with you today.

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Astronomers think
that the first galaxies formed

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out of clumps of gas
in the early Universe.

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These protogalactic clumps
then went on to coalesce

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and to merge together to form
bigger and bigger galaxies

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as time goes on and as
their gravity attracts each other.

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So, what exactly happens
when two big galaxies collide?

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Well, that is precisely what Hubble
is showing us in this new images,

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where it has captured some of
these "merger" events in the act.

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These cosmic collisions beautifully
portrayed in the new Hubble images

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are not as swift and ferocious
as we might think.

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They are not like collisions
between everyday objects.

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They are slow stately affairs

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that may take hundreds of millions
of years to complete.

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In this series of images,

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snapshots of different pairs
of interacting galaxies are seen

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representing the different stages
in this process.

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A gradual waltz of stars and gas
choreographed by gravity.

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Despite taking hundreds of millions
of years to eventually merge,

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the two galaxies of a pair
actually approach each other

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at very high velocities.

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Up to a million kilometres
per hour or so.

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Also, because the distances
between individual stars are so large

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is actually very rare
for two stars to hit each other.

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The hundreds of millions of stars
in each of the galaxies

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all move individually,
following the pull of gravity

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from all the other stars and
the dark matter in the galaxies.

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This interwoven tidal forces can produce
the most intricate and varied effects

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as the two galaxies pass each
other and eventually merge.

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The new collection of Hubble images
illustrates how galaxy collisions

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produce a remarkable variety of intricate
and delicately interwoven galactic shapes.

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Typically, the first
signs of an interaction

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appear during the first
encounter of the galaxies.

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The tug of gravity teases out stars
and gas from the passing galaxies

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often building bridges between them.

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During subsequent encounters,
long streamers of gas and dust,

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known as tidal tails, stretch out and
sweep back to wrap around the cores.

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These long, often spectacular tidal tails
are the clear signature of an interaction

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and can persist long after
the main action is over.

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As the galaxy cores
finally approach each other

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their gas and dust clouds are
buffeted and accelerated dramatically

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by the push and pull of
matter from various directions.

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These forces can result in shock-waves
rippling through the interstellar clouds.

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Gas and dust are siphoned into
the active central regions,

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fuelling bursts of star formation

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that appear as characteristic
blue knots of young stars.

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As the clouds of dust build they are
heated so that they irradiate strongly

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becoming some of the brightest
infrared objects in the sky.

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Some of the galaxies show
striking highly distorted features,

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dust lanes crossing between galaxies
and long filaments of stars and gas

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extending far beyond the central regions.

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We see beautifully interwoven galaxies
twisted by these gargantuan encounters.

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Triggered by colossal and violent
interaction between the galaxies

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stars form from large clouds of
gas in dramatic and brilliant bursts

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of stellar fireworks, creating
new blue star clusters.

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Here is an image of NGC 6670, 
a pair of overlapping galaxies,

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seen edge-on, that kind of
resemble a leaping dolphin.

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Scientists believe that it has already
experienced at leas one close encounter

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and that it is now in an
early stage of a second.

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The next image shows an
astonishing pair of galaxies,

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composed of two spiral galaxies,
of similar mass and size:

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NGC 5257 and NGC 5258.

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The galaxies are clearly
interacting with each other

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creating a bridge of dim stars
connecting the two galaxies.

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Almost like two dancer holding
hands while performing a pirouette.

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Both galaxies harbour super-massive
black holes in their centers

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and are actively forming
new stars in their discs.

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This galaxy pair dubbed
IC 694 and 3690

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is thought to have already experienced
one close pass some 700 million years ago.

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As a result of this interaction the system
underwent a fierce burst of star formation.

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In the last 15 years or so
no less than six supernovae

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have gone off in the outer
reaches of the galaxy,

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making the system a
notable supernova factory.

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The image of this
beautiful interacting pair

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shows long tails sweeping out
from the centre of the galaxies.

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These are tidal tails of stars and gas

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that have been pulled from the
distorted discs of the merging galaxies.

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So we can see that whilst
each of these images

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represent the demise of
two beautiful galaxies,

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the distorted destruction they experience
also results in a flood of newborn stars

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and some incredibly
striking galactic landscapes.

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If you would like to see all
of the 59 new Hubble images,

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please visit our web
site at spacetelescope.org

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This is Dr. J
signing off for the Hubblecast.

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Once again, nature has surprised us
beyond our wildest imagination...

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Hubblecast is produced
by ESA / Hubble

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at the European
Southern Observatory in Germany.

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The Hubble mission is a project
of international cooperation

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between NASA and
the European Space Agency.