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00:00Throughout our universe, galaxies slam into each other at 270,000 miles per hour.
00:05It'll completely rip everything to pieces.
00:07It will even happen to our Milky Way someday.
00:10If we want to understand galaxies, we're going to want to understand galaxy collisions.
00:15Now, the most violent collisions in our universe, when galaxies collide.
00:30Collisions. On a human scale, they're violent, leaving chaos and destruction in their wake.
00:41The bigger the collision, the greater the havoc.
00:45But once we leave Earth and head out into the universe and beyond, the scale of the collisions grows inconceivably.
00:53Galaxies smash into each other.
00:55Black holes fuse in a deadly fireworks display.
01:02Even our own galaxy marches toward a clash with its closest neighbor, Andromeda.
01:10These violent interactions astronomers believe spur on the evolution of our universe.
01:16It's not just the gradual study evolution that leads to the events that we see and the shapes and the nature of the cosmos that we live in.
01:24But it's also the occasional violent collisions between two galaxies, between two black holes, between two stars.
01:32In order for something to have gone right in our universe, things had to go very wrong.
01:38How do we know of these violent collisions?
01:40And what do they mean for us here on Earth?
01:42Technology allows us to witness destruction beyond our wildest dreams, and to learn what these impacts do to the universe.
01:52Each type of galactic collision has expanded our understanding and changed our expectations of how the universe works.
01:59The largest collision imaginable occurs between the universe's most massive objects, galaxies.
02:07A galactic collision conjures images of unparalleled destruction.
02:12Stars collide.
02:14Planets are shattered.
02:15All life forms are annihilated.
02:17But is this really what happens when galaxies collide?
02:25To begin to understand what a galactic collision means, we first have to understand the size and composition of a galaxy.
02:33A collection of gas, dust, planets, and stars.
02:42Lots of stars.
02:45A galaxy is like a city of stars.
02:48It is a collection of a couple hundred billion stars that sticks together for billions of years.
02:54It's held together by its mutual collective gravity.
02:57Our galaxy, the Milky Way galaxy, has around 100, 200 billion stars in it.
03:03And it's around 100,000 light years across from one edge of its disk to the other.
03:10So we could cross the Milky Way by traveling 100,000 light years.
03:15No problem, right?
03:17Maybe we should start by measuring a single light year.
03:20A light year is simply the distance that light can travel in a year.
03:24Light travels at an incredible speed, 300,000 kilometers a second.
03:29And if you multiply that by the number of seconds in a year, you get about 10 trillion kilometers.
03:34So the distances in astronomy are so huge that we actually have to use light years in order to measure the distances just between stars.
03:42Another way of looking at this is that a beam of light traveling 186,000 miles per second would still take 100,000 years to travel from one end of our galaxy to the other.
03:56In the sun's 4.5 billion years of life, it has orbited the center of the Milky Way a mere 20 times.
04:06Despite these great distances, objects within our universe collide all the time, as they have since the universe was young.
04:13Galaxies within the universe began forming hundreds of millions of years after the Big Bang, as clusters of stars and interstellar gas began coalescing due to gravity.
04:27These early galaxies were small, just one-tenth the size of the Milky Way.
04:32The very first galaxies to have formed in the universe were small galaxies, dwarf galaxies.
04:39And as time went on, these small building blocks combined to make bigger and bigger galaxies as a result of mergers and accretion.
04:47These dwarf galaxies found themselves, relatively speaking, caught in a traffic jam in the early universe.
04:53In the past, the early history of our universe, interactions were much more common than they are today, because galaxies were so much closer together.
05:03As small galaxies collided, they combined into larger galaxies with defined shapes.
05:09The Milky Way galaxy, home to our solar system, is a spiral galaxy.
05:14Other shapes include elliptical galaxies and lenticular galaxies, a cross between the two.
05:21Astronomers owe these galaxy classifications to Edwin Hubble, the namesake for one of NASA's great observatories, the Hubble Space Telescope.
05:33I think Hubble had a fun task, if you will, that there wasn't kind of a definitive, here's the classification system.
05:40So he got to make it up.
05:42Hubble took on the astronomical task of categorizing galaxies.
05:46What you look for is how to take care of 90%, to classify 90% of the objects there.
05:54And so you don't worry about all the strange ones.
05:58What happened to the other 10%?
06:01Everything else went into the irregular category.
06:05They're throwing off big tidal tales of debris, or there are galaxies that just look like a train wreck.
06:11For years, that's where these peculiar formations sat.
06:16But the question still stood, what could have formed these strangely shaped galaxies?
06:21And could this have something to do with cosmic collisions?
06:25It wasn't really a big topic at his time.
06:29I mean, just understanding spiral galaxies and elliptical galaxies was in its infancy.
06:35Yet, even as Hubble was more interested in sorting the majority of galaxies,
06:40he felt the ones labeled irregular warranted more attention.
06:45It was roughly a couple decades after, when he did most of his work,
06:50that people started working and thinking about the irregular and peculiar galaxies.
06:55Scientists wondered what made irregular galaxies look the way they did.
06:59One theory held that they were really snapshots of violent disruption and chaos.
07:05There was kind of a debate, say, 20, 30, 40 years ago,
07:10of whether these things were colliding and merging as we understand them now.
07:16Today, astronomers continue to use this irregular category.
07:20Irregular is kind of a catch-all term.
07:22It's referred both to little galaxies that have been so disrupted by their star formation,
07:29punching holes in their gas, that their shape is very irregular.
07:33But it can also mean galaxies that have been very highly disrupted,
07:37for example, by collisions.
07:40It incorporates a wide variety of different types of galaxies.
07:44The strange shapes of some irregular galaxies can be explained by the effects of gravity
07:50on a galactic scale.
07:53The largest objects in the universe merge in a head-on collision.
07:58Irregular galaxies are proof that these giant clusters of stars collide.
08:03They are the evidence that the universe is wrecked by destruction and pandemonium.
08:08But what happens within galaxies?
08:10What other types of collisions occur?
08:12And how can we see them?
08:15Next, colliding stars leave their fingerprints all over the universe,
08:20and yet astronomers have yet to catch them in the act.
08:32Irregularly shaped galaxies are evidence that galaxies collide.
08:36But do other celestial objects crash into each other?
08:39Our universe is both a nursery and a graveyard,
08:43host to an epic war between massive celestial bodies.
08:50Imagine this fictional scenario.
08:53Almost 3,000 light years from Earth,
08:56two sister stars have been sharing the same orbit for millions of years.
09:00Over time, they've grown closer, circling each other at nearly the speed of light.
09:08Then slam.
09:10In milliseconds, they are destroyed,
09:13sending out a shock wave that encompasses the Earth,
09:16decimating half our ozone layer,
09:18and killing everything in its path.
09:20This is what some astrophysicists theorize will happen
09:25if a type of star called a neutron star collided near Earth.
09:29For astrophysicists, 3,000 light years is near.
09:33There has been a suggestion
09:36that one or more of the mass extinctions that happened to life on Earth
09:41occurred because of such an event.
09:45Some of the most violent stellar collisions occur between neutron stars.
09:51Neutron stars are born of remnants of older giant stars
09:55that have run out of fuel.
09:57Gravity causes the whole thing to collapse.
10:03In the center, an extremely dense object forms.
10:08It's so dense that the electrons get pushed into the protons to make neutrons.
10:13The neutron star that forms at the center is the remnant,
10:17the ashes of that star.
10:20But these ashes have some life in them,
10:22and after their creation,
10:23some neutron stars orbit each other in a gravitational embrace
10:27called a binary pair.
10:30For millions of years,
10:32the stellar siblings orbit each other up to 1,000 times each second.
10:37They will be emitting gravitational waves
10:40which drain away energy from the system,
10:42getting them closer and closer and closer together
10:45until ultimately they get so close together
10:48that there's no more stable orbit for them to orbit on.
10:50When they finally collide,
10:54they release more energy than our sun would produce in its lifetime.
10:58Yet the collisions are too fast for telescopes to capture.
11:02For neutron stars,
11:04the whole collision is over in about a thousandth of a second.
11:06Then it's done.
11:07They...
11:07Once they touch, they go splat, you're done.
11:11Their collision sends a massive shockwave through space.
11:14From Earth, the collision is detected
11:18as a short gamma-ray burst, or GRB.
11:22A GRB is a short-lived yet powerful event,
11:26evidence of one of the most violent collisions in the universe.
11:30But scientists have only recently begun
11:32to fully understand what GRBs mean.
11:35There are little gamma-ray flashes
11:37that happen a couple times a day in the sky.
11:40And we pretty much monitor the whole sky for gamma rays all the time.
11:44And these have been known for about 40 years now.
11:47The U.S. Air Force had actually detected GRBs
11:51years before the scientific community even knew they existed.
11:55Gamma-ray bursts were discovered completely by accident
11:58by military satellites
12:00and came as a result of the test-ban treaty
12:05in the 1950s under Dwight Eisenhower.
12:08The nuclear test-ban treaty begun under President Eisenhower
12:13resulted in the creation of military satellites
12:15designed to search for nuclear explosions.
12:19The Air Force launched the first Vela satellites in 1963.
12:24This series of satellites was built to detect,
12:26among other things, gamma rays.
12:28Using gamma-ray detectors.
12:31The clouds of radiation that would be released
12:34in such an explosion would be releasing gamma rays.
12:37Those gamma-ray detectors were the first ones
12:41to detect cosmic gamma-ray bursts.
12:45In 1969, Ray Klebesadl, an Air Force analyst,
12:49found strange spikes in gamma rays within the Vela data
12:52that because of their intensity and duration
12:55could not be attributed to either the Earth or the Sun.
12:58So they knew they must be coming from somewhere out there.
13:02By 1973, when Klebesadl finally made his research public
13:07at an American Astronomical Society meeting,
13:10he had at least 16 confirmed gamma-ray bursts.
13:13But more sophisticated equipment would be needed to fully observe GRBs.
13:22In 1991, NASA launched the Compton Gamma-ray Observatory,
13:27a space satellite dedicated to detecting GRBs.
13:31What scientists hoped to do was catch a gamma-ray burst in action
13:35in order to determine what it was.
13:38Could GRBs be a type of cosmic collision?
13:41The Compton Gamma-ray Observatory detected a couple of thousand
13:45of these gamma-ray bursts
13:47and established that they do, in fact, happen all the time.
13:51But what was really needed is a way to quickly identify the gamma-ray burst
13:55and quickly get a telescope on there to take a picture
13:58because they fade away quick.
14:01In fact, GRBs fade completely away within just a few days.
14:06Five, four, three, two, one.
14:12We have ignition, and we have liftoff of NASA's SWIFT spacecraft
14:16on a mission to study and understand gamma-ray bursts
14:20throughout the universe.
14:22In 2004, NASA launched the aptly named SWIFT telescope
14:27to better track and react to GRBs.
14:30SWIFT is a satellite with a big gamma-ray detector
14:35that is relatively good at detecting the direction
14:39from which the gamma rays are coming.
14:42As soon as it sees a gamma-ray flash,
14:45it radios the location to the ground,
14:48and then it starts swiveling an X-ray telescope
14:51to that same location.
14:54In 2005, SWIFT detected a burst,
14:57and in less than a minute,
14:58it repositioned itself to observe its afterglow.
15:02Meanwhile, SWIFT sent the GRBs' coordinates
15:05to its network of ground telescopes.
15:08One of those telescopes is located near the summit of Palomar Mountain
15:11in San Diego, California.
15:15The Palomar Observatory combined its data with calculations
15:18from more than a dozen other institutions.
15:21Together, they were able to identify a probable cause
15:24of the short GRB,
15:26a collision between neutron stars.
15:29Since stellar collisions occur within a second to a few hours,
15:34most of what happens remains a mystery.
15:37The actual collision event is a very short-lived thing.
15:41It's for that reason that it's unlikely
15:44we'll ever in our lifetimes be able to capture
15:47a couple stars in the process of colliding.
15:51But what you can do is you can look at the aftermath.
15:56So, while astronomers have never actually witnessed
15:59a stellar collision,
16:00they know they exist
16:02by examining the ruins of these brutal star wars.
16:06Neutron stars are not the only stars
16:09that collide in violence and destruction.
16:11Even infant stars can create cosmic havoc.
16:17Starers lead a violent existence,
16:19and not just neutron-binary pairs,
16:22but virtually all stars,
16:24starting with the nature of their formation.
16:27The turbulent life of a star
16:29begins with the death of a molecular cloud.
16:32It's a dense region of interstellar gas.
16:35In these clouds, stars form, planets form,
16:40our own solar system formed.
16:42Molecular clouds grow larger
16:44as the force of gravity from them
16:46attracts more gas and particles
16:48until their density gets so large
16:51they collapse to form stars.
16:54An exterior force like those caused by a cloud collision
16:57or the shock waves of a supernova
16:59can also cause the clouds to collapse.
17:02As it collapses, it gets denser and denser,
17:05in its center.
17:06It gets hotter and hotter in its center.
17:08Recent research places the first star formation
17:11at only 100 million years after the Big Bang.
17:16Some of those stars, astronomers believe,
17:18can still be found in our own Milky Way galaxy.
17:22Today, researchers focus on compact groups of stars
17:25called globular clusters.
17:28For those researchers studying stellar collisions,
17:30globular clusters are the place to look.
17:33These dense collections of stars serve as an ideal stellar battlefield.
17:38Just like if you're walking down a street in a crowded city,
17:41you bump into people.
17:42And stars are close enough together,
17:44and they're crowded enough,
17:45they bump into each other.
17:46With about 100 billion galaxies in the universe,
17:50and with each one containing about 30 globular clusters,
17:54the number of stellar collisions can be staggering.
17:56If you count up all the galaxies in the universe,
18:01hundreds and hundreds of billions of galaxies,
18:04and ask how many regions do they have
18:07where stellar collisions could occur,
18:09it starts to be a very large number.
18:11It was a mystery within globular clusters
18:15that led astronomers to one of their biggest discoveries.
18:20Discovered in 1953,
18:23highly luminous stars in globular clusters
18:25stumped researchers for more than 40 years.
18:29They named the strange stars blue stragglers.
18:32These blue stragglers have some peculiar characteristics to them.
18:37They're called stragglers
18:39because unlike other stars of the same mass in that cluster,
18:42they haven't yet gone on to later stages of evolution.
18:45So they're stragglers and they're blue
18:47because they're hotter
18:49than the other main sequence stars in the cluster.
18:52It seemed blue stragglers
18:54had discovered the stellar fountain of youth.
18:57Scientists wondered how that could be possible.
18:59NASA's Hubble Space Telescope,
19:04launched into orbit in 1990,
19:06played a key role in answering that question.
19:10Hubble's resolution is so fantastic
19:12that if you could look with it from New York to Tokyo,
19:16you could see two fireflies.
19:18This is how good the resolution of this telescope is.
19:23Hubble's optics capture detailed images of blue stragglers,
19:27allowing scientists unprecedented access to their secrets.
19:32So by looking at these individual stars,
19:35it was able then to measure the properties of these stars,
19:40for example, their mass,
19:41how fast they rotate,
19:43their temperature, and things like that.
19:46Astronomers discovered the blue straggler's high mass,
19:49fast rate of spin,
19:51and high temperature could only mean one thing.
19:54A blue straggler, we think today,
19:58is most likely the result of a merger of two stars.
20:03And those mergers could have happened via collisions,
20:07direct collisions, two stars colliding with each other,
20:10or two stars that were in a binary system
20:13where they rotate around their center of gravity.
20:16In one example,
20:19a main-sequence star plows into another
20:21at about a half a million miles per hour.
20:24It burrows into the other star,
20:26setting off a wave of gas and debris into space.
20:30Immediately, the stars will swell
20:32and ferociously burn energy.
20:35It can take 10 million years
20:37for the stars to overcome their violent merger
20:40and settle into a blue straggler.
20:42This was the first evidence that we had
20:45that stars really could collide with each other.
20:48And this is one of the more remarkable things
20:51that stars have been found to do.
20:53It took decades for the idea of stellar collisions
20:57to move from theory to full-blown reality.
21:01But where galactic collisions are concerned,
21:03slamming stars are just the tip of the iceberg.
21:07Cosmic collisions are the rule,
21:09not the exception in the universe.
21:10And at every level,
21:13right down to the dust that wafts through space.
21:16In our own solar system,
21:18all the planets,
21:21all the objects that we see
21:22were built by collisions.
21:25Dust colliding to make sand,
21:27sand colliding to make pebbles,
21:30pebbles to rocks,
21:31boulders to planetesimals,
21:32asteroids to planets.
21:35So our own origins are from collisions.
21:39Galaxies can be violent places.
21:43The locus of death, birth, and rebirth.
21:54Galaxies collide,
21:56and so do stars within galaxies.
21:58But are there other objects
21:59that can go bump in the night?
22:01Cosmic collisions aren't limited to large bodies crashing into each other.
22:08Sometimes a collision can occur with jets of energy.
22:12Deadly energy.
22:14Lethal radiation spews out of a supermassive,
22:16mysterious object called a black hole.
22:20A quasar,
22:21the beam from what scientists have dubbed
22:23the Death Star galaxy
22:24is slamming into a smaller neighboring galaxy.
22:29What's actually happening is
22:30a jet is coming out of one black hole
22:32and it accelerates close to the speed of light
22:35and that jet strikes a companion galaxy
22:37that swung into the path of that jet.
22:41And that wreaks all sorts of havoc
22:42for any inhabitants or any Earth-like planets
22:44in that companion galaxy.
22:46The ones that are close enough would be sterilized.
22:48The ones a little bit further out
22:50would have their atmospheres damaged.
22:53These are the really extraordinarily acts of violence
22:55by a supermassive black hole.
22:57It sounds like science fiction,
23:00but it's science fact.
23:02Luckily for our galaxy,
23:04this cataclysm is taking place
23:05in galaxy 3C321,
23:08a galaxy far, far away,
23:101.4 billion light years distant.
23:13This is the first time
23:15astronomers have observed
23:17a black hole jet
23:18punching into a companion galaxy.
23:21But this event raises many questions,
23:24beginning with what is a black hole
23:26and are we Earthlings in danger
23:28from a death ray?
23:30Black holes aren't really holes at all,
23:33but instead are incredibly dense
23:35and massive objects.
23:37Black holes are so dense,
23:39their gravitational pull so strong
23:41that nothing can escape them.
23:43Not even light itself.
23:47It takes observations
23:49from multiple telescopes
23:50to make a complete picture
23:52of this unique phenomenon.
23:54It's only by combining information
23:56right across the electromagnetic spectrum
23:57that we can begin to study
23:58black holes with great precision.
24:01Part of that picture
24:02was taken at this telescope,
24:04the VLA,
24:05or Very Large Array,
24:07in New Mexico.
24:08The VLA is a radio telescope
24:11made up of 27 dish antennas,
24:14each one of them weighing 230 tons.
24:17The antennas are arranged
24:18in the shape of a Y,
24:20and each arm of that Y
24:22is 13 miles long.
24:24The VLA is especially adept
24:26at observing black holes,
24:28such as the one in our own galaxy.
24:30That thing is about 4 million times
24:33more massive than the sun,
24:35and it creates all kinds of phenomena,
24:38but gas and dust between us
24:41and that black hole
24:42obscures all of that
24:44to visible light telescopes.
24:46The VLA's ability
24:48to detect radio waves
24:50allows it to better penetrate
24:51the stellar debris.
24:53So radio telescopes
24:54and infrared telescopes
24:56are really the only way
24:57to study that.
24:58Just like a visible light telescope
25:01is collecting light waves
25:02to make an image,
25:03we're collecting radio waves
25:04to make an image.
25:06Optical and short wavelength radio
25:08cannot see into the inner parts
25:10of these regions.
25:11Radio can.
25:13And radio astronomers
25:14of long wavelengths,
25:15such as this facility,
25:17can peer into the inner parts
25:18of these regions
25:19that are obscured
25:21to other wave bands.
25:23Most galaxies have
25:24supermassive black holes
25:26at their centers.
25:27They can be 1 million
25:28to 1 billion times
25:30the mass of our sun.
25:33Despite their mass,
25:34we initially couldn't see black holes.
25:37So how did we eventually know
25:38they were there?
25:40As material flows in
25:41towards a black hole,
25:42it's much like water
25:43going down a bath plug.
25:44It goes down
25:46and it spins around.
25:48And essentially,
25:49through frictional forces,
25:50that friction generates heat
25:51and it generates light.
25:52And that light is so intense
25:54and so energetic
25:54that it forms X-rays.
25:57It was the disappearance
25:59of objects in this drain
26:01that led 18th century astronomers
26:03to suggest the existence
26:05of an invisible star.
26:07In 1783,
26:11the Reverend John Mitchell
26:13laid out beautifully
26:15the idea of an event horizon.
26:17Mitchell showed
26:18that a body that's very massive
26:20with a small enough radius
26:22would have an escape velocity
26:24that is the speed of light.
26:27It would trap the light
26:28and be invisible,
26:30be dark.
26:32That is the idea
26:33of the event horizon.
26:36John Wheeler,
26:37a close collaborator
26:38of Albert Einstein's,
26:40coined the phrase
26:41black hole in 1967.
26:44It was then still a theory.
26:46Scientists hadn't even realized
26:48that they had already found
26:49their first black hole.
26:51The evidence was X-rays,
26:53invisible energy produced
26:55by black holes.
26:56In 1971,
26:59the X-ray source
27:00was identified
27:01with a very massive,
27:03giant blue star.
27:05Later in 1971,
27:07we were all startled
27:08to find
27:09that this optical star
27:13that was so massive
27:14was orbiting the X-ray source
27:16at a very high velocity.
27:19This high velocity
27:20means that the X-ray source
27:23is more massive
27:24than three times
27:25the mass of our sun.
27:26This essentially established
27:28it to be a black hole.
27:30Probably the biggest boon
27:32to the study of black holes
27:33came in the form
27:34of another of NASA's
27:35great observatories,
27:37the Chandra X-ray Observatory.
27:41We've often been asked,
27:42why don't we do X-ray astronomy
27:43from the ground?
27:44Because, of course,
27:44it's riskier,
27:45more expensive,
27:46and challenging
27:47to put your telescope
27:47up into space.
27:49The Earth's atmosphere
27:50is actually
27:50very opaque to X-rays.
27:53It blots out
27:54all the X-rays
27:54that come from stars
27:55and galaxies
27:56that are far off.
27:58If we want to do
27:58X-ray astronomy,
27:59we have to go into space.
28:01And Chandra is the most capable
28:02X-ray telescope
28:03that we've had.
28:05Chandra is particularly
28:06suited for studying
28:08places and objects
28:10in the universe
28:10where violent conditions,
28:12extreme conditions,
28:14generate X-rays
28:15as a primary signature.
28:16We're talking about,
28:17for example,
28:18matter falling
28:19into a black hole,
28:20stars exploding
28:21as supernovae
28:22or galaxies colliding.
28:24And it's this kind
28:25of cosmic violence
28:26that Chandra's
28:27particularly adept
28:28at studying.
28:29X-rays are present
28:30whenever a celestial object
28:32produces large amounts
28:33of heat.
28:34Neutron stars
28:35and black holes
28:36can create X-rays
28:37100,000 times
28:39the strength
28:40of the X-rays
28:41the Sun produces.
28:42Chandra was pivotal
28:43in helping astronomers
28:45understand
28:45what's occurring
28:47in the Death Star galaxy.
28:48So Chandra allowed us
28:50to see the central
28:51black hole sources.
28:53It allowed us
28:53to determine
28:54the presence
28:55of this X-ray jet.
28:57And it's this jet
28:58that gave rise
29:00to the name
29:00of the Death Star galaxy
29:02because the jet
29:02points directly
29:03in the direction
29:04of the companion galaxy
29:06and you can actually
29:07see the effects
29:07of that radiated beam
29:09of energy and particles
29:10as it hits
29:10the neighboring
29:11or companion galaxy.
29:13Like most cosmic collisions,
29:15there's a silver lining.
29:16Once the Death Star jet
29:18has devastated
29:19the nearby galaxy,
29:21it will likely unleash
29:22a new round
29:23of star and planet formations.
29:25It's all part
29:26of the universe's
29:26cycle of life
29:28and death.
29:29Black holes
29:29aren't necessarily
29:30these cosmic cannonballs.
29:32They don't necessarily
29:33run through the universe
29:34sucking up everything
29:35in their path.
29:36In fact,
29:36one of the ultimate
29:37legacies of black holes
29:39could actually lead
29:39to the creation
29:40as well as destruction
29:41of life.
29:43Our solar system
29:44has managed
29:45to avoid death rays
29:46only to be thrust
29:47into a galactic calamity
29:49of staggering proportions.
29:51In the distant future,
29:53our galaxy,
29:53the Milky Way,
29:54and its neighbor,
29:55Andromeda,
29:56will collide.
29:58But will this collision
29:59be a source of creation
30:00or destruction?
30:02But what could a galactic
30:10collision like this mean?
30:13It turns out that
30:14the greatest danger
30:15comes from collisions
30:16with those thermonuclear reactors,
30:19also known as stars.
30:20If a star comes sufficiently close
30:23to the solar system,
30:24it may send comets
30:26in our way.
30:27If it comes even closer,
30:29it may disturb the orbit
30:30of the Earth
30:31around the sun.
30:32And so life,
30:33as we know it,
30:34will cease to exist
30:35because either
30:36all the water
30:37will get evaporated
30:38or else it would freeze
30:40if we are sent out
30:42away from the sun.
30:44The collision
30:44between the Milky Way
30:45and Andromeda
30:46was destined
30:47from the time
30:48in the early universe
30:49when these two galaxies
30:51were born side by side.
30:53Things that merge
30:54are born bound
30:55and eventually
30:56come together.
30:59Things that are born unbound
31:00generally don't merge.
31:01They just separate.
31:03Most galaxies
31:04are diverging
31:05in the expanding universe.
31:07And all galaxies,
31:08it turns out,
31:09that are far away from us
31:11will recede away from us
31:13and disappear from view
31:14within a finite amount of time.
31:16The only exception
31:17is the Andromeda galaxy,
31:19the nearest neighbor,
31:20our sister galaxy.
31:21Andromeda and Milky Way,
31:24their gravity
31:25has actually
31:26at some point
31:27in the past
31:28reversed any
31:29receding velocity
31:30between the two of them
31:31and they're now
31:32approaching one another
31:34because of that
31:34mutual attraction.
31:36We're actually
31:36on a collision course.
31:38Seem to be coming
31:39straight at each other
31:40and it's looking pretty,
31:41looking pretty bad for us.
31:44The Andromeda galaxy,
31:45within about
31:45five billion years,
31:47will collide
31:47with the Milky Way galaxy.
31:49But what's the evidence
31:50of this coming collision?
31:52It turns out
31:53that the light signatures
31:54that Andromeda gives off
31:55are the strongest evidence
31:57of its approach.
31:59It's the same stuff
32:00that makes the fireworks colorful.
32:02You look for a particular color
32:04and you know
32:04it's coming from iron
32:06or oxygen or sulfur
32:08or something
32:08and you know
32:08what color it should be
32:10and then you just see
32:10what color it seems to be.
32:12Is it bluer or redder?
32:13If it's bluer,
32:14it's most probably
32:15because it's coming
32:15straight at you
32:16and if it's redder
32:17it's because it's
32:18going further away.
32:19You look at Andromeda
32:20and it's blue shifted
32:21and the speed
32:23at which it has to be moving
32:24is a few hundred kilometers
32:25per second.
32:26It's coming at us
32:27and coming at us fast.
32:28But wait,
32:29a collision that destroys
32:30life on Earth
32:31is only one possible scenario
32:33and not at all
32:35the most likely one.
32:36So what will actually happen
32:38in two billion years
32:40when these two galaxies
32:41begin to approach each other?
32:42Because galaxies
32:46are incredibly massive
32:47and the distances
32:48between them so great,
32:50a collision takes
32:51billions of years
32:52from start to finish.
32:54As a result,
32:55astronomers can only observe
32:57snapshots in the sky.
32:59To understand
32:59the whole process,
33:01they rely on simulations.
33:04Every snapshot we get
33:05of a galaxy collision
33:07in the universe
33:07is really just one piece
33:09of a very long sequence.
33:11And so these simulations
33:13allow you to piece together
33:15the various situations
33:16and see how it develops
33:18from approach
33:19to passing by
33:20to the title tales
33:21to smashing together
33:22to mixing all up.
33:24In the 1970s,
33:26astrophysicists
33:27and brothers
33:28Alar and Yuri Tumre
33:30created the first simulations
33:31of colliding spiral galaxies.
33:35Their simulations
33:36duplicated features
33:37that have since been observed
33:39in colliding galaxies.
33:40their sequence
33:44detailing the steps
33:45from galactic approach
33:46through merger
33:47has become known
33:48as the Tumre sequence.
33:51Today, Frank Summers,
33:53an astrophysicist
33:54at the Space Telescope
33:55Science Institute
33:56in Baltimore, Maryland,
33:58uses state-of-the-art
33:59visual effects programs
34:01and banks of computers
34:02to compress galactic collisions
34:04into simulations
34:06depicting 10 million years
34:07of galactic action per second.
34:11It all starts
34:12with the approach.
34:13Two spiral galaxies
34:15draw near to each other.
34:17You can see here
34:17that just before
34:18they start to crash,
34:19there really isn't
34:20any tidal distortion,
34:21which I thought was really strange
34:22when I looked at the visualization.
34:23Right.
34:24They're still pretty symmetric there.
34:25It's only when you kind of
34:26start sending out
34:26these tidal tales
34:27that they would be put
34:29into the Tumre sequence.
34:30Okay.
34:30And then you can kind of watch
34:31as it comes along here.
34:33They start kind of first pass
34:35and then they come back together.
34:37The simulation shows
34:39the tremendous gravitational attraction
34:41between colliding spiral galaxies.
34:44Bass strings of stars and gases
34:47known as tidal tails
34:48are thrown out
34:49into distant space.
34:51This dramatic stage
34:53can be verified
34:54by images of colliding galaxies
34:56like the antennae galaxies
34:58and the mice.
35:09Astronomers predict
35:10that the Andromeda-Milkyway collision
35:12will progress
35:13with the same shearing stage.
35:16The tidal tails
35:18are basically the outskirts
35:19of the Milky Way
35:20being ripped apart.
35:21The Milky Way is a thin,
35:24fairly fragile,
35:25delicate galaxy.
35:27And the Andromeda
35:28is actually bigger than we are
35:29and more massive.
35:30And when it zooms past us originally,
35:33its gravitational field
35:34is going to tear out the Milky Way.
35:37But as the two galaxies
35:38come together
35:39in two billion years,
35:41their approach
35:41will look less like a collision
35:43and more like
35:44the swing of a pendulum,
35:46momentum carrying the galaxies
35:48past each other
35:49and gravity drawing them back.
35:52They'll keep on moving
35:53because of all this momentum
35:55they've built up.
35:56And as they move apart
35:57from one another now,
35:59the Andromeda galaxy
36:00and the Milky Way galaxy
36:01will start to slow down
36:02because of the gravitational
36:04attraction between them.
36:05The recessional velocity
36:07will decrease, decrease
36:08until ultimately
36:09they turn around
36:10and come back
36:10for another passage
36:12past one another.
36:14Four and a half billion years
36:16from now.
36:18Once again,
36:18the galaxies swing
36:19toward each other.
36:21This time,
36:22it will be a direct hit.
36:24These beautiful disk structures
36:26of these two spiral galaxies
36:27are probably going to get
36:28totally destroyed.
36:30They'll be pulled out
36:30into these long tidal tails
36:32and then when they come
36:32smashing together,
36:34well, the orbits
36:34of all the stars
36:35will randomize
36:36and what you're going to end up
36:37with is more of an elliptical
36:39shape in the center
36:40and these big long tidal tails
36:42that'll come raining back in
36:43over the course
36:44of billions of years.
36:46The collision has become
36:48a merger.
36:50And then the central parts here
36:52are now merging together
36:53and now you have just a central
36:55kind of one single galaxy
36:57at the center
36:57and so the outer tidal tails
36:59would kind of dilute
37:00and you'd come back
37:01in another, you know,
37:02half a billion years or so
37:03and you'd see one galaxy.
37:05You can see how they merge
37:06together here
37:07and so now you really have
37:08essentially one core.
37:10You still have some tidal tails.
37:12Those will tend to fade
37:12with time
37:13and so if we take this
37:14a little bit farther out there
37:16we would think we have
37:17just one object
37:18and it would look
37:19pretty much like
37:20an elliptical galaxy
37:20at that point.
37:22Astronomers speculate
37:23that the elliptical galaxies
37:25in the universe
37:26are the product
37:27of galactic collisions
37:28like these.
37:30But what actually happens
37:31inside colliding galaxies
37:33as they go through
37:34this violent process
37:35and what does it mean
37:37for our planet
37:38as we endure
37:39the cataclysm?
37:41The images
37:42of colliding galaxies
37:44are ethereal.
37:45Tidal tails
37:46stretch out
37:47like spider webs.
37:48Galactic cores
37:49orbit each other
37:50locked in attraction.
37:52But do these
37:53beautiful images
37:54mask wholesale destruction?
37:56Stars slamming
37:57into each other,
37:58planets crashing,
38:00moons atomized
38:01into space dust?
38:03One of the coolest things
38:04about galaxy collisions
38:05is that when these
38:06huge galaxies
38:07smash together,
38:09the stars don't.
38:10If you take a star
38:11and another star,
38:13the distances between
38:13them are millions
38:14of times larger
38:15than the diameters
38:16of the stars.
38:17And during this
38:18galactic collision
38:19and ultimate merger,
38:21what are the chances
38:22that Earth
38:23will be smashed
38:24to bits?
38:25According to astronomers,
38:27quite low.
38:28Each star has
38:30something like
38:31the solar system
38:32around it.
38:33Even the space
38:34between those
38:35stars is very,
38:36very large,
38:37such that the
38:38probability of even
38:39one planet
38:41colliding with another
38:43is negligible.
38:45But that doesn't mean
38:46that Earth
38:47is entirely off the hook.
38:50There's somewhere
38:50around a 10% chance
38:52or so that the Earth
38:54would get ejected
38:56with the Sun
38:56and the solar system
38:57in one of these
38:59tidal tails.
39:01And then we would
39:01be far away,
39:02and we'd sort of have,
39:03you know, a bird's-eye view
39:05of our galaxy.
39:06So really,
39:07the main implication
39:08for what we will see
39:10on our sky
39:11has to do with
39:12how the orbit
39:13of our Sun
39:13is changed
39:14during the collision.
39:16Stars and planets
39:17can pass by each other
39:19without incident,
39:20but not gas.
39:22A galactic collision
39:23is really about
39:24smacking.
39:24We'll immediately
39:33make a lot of stars.
39:34We'll probably get,
39:35I don't know,
39:3710 billion stars
39:38made fairly quickly.
39:39This is what we call
39:40a starburst,
39:41and this rampant star formation
39:43seems to be an indicator
39:45of galaxy interactions.
39:47So if we were on a planet
39:48within one of these galaxies,
39:50the night sky
39:51would be very dramatic,
39:52especially when the starburst
39:53is going on.
39:54It would just be lit
39:55with the fireworks
39:55of all these massive stars
39:57that are being born.
39:59In general,
40:00it wouldn't necessarily
40:00be dangerous
40:01unless you were nearby
40:02one of those regions
40:04where all the stars
40:05are being born.
40:06But once the galaxies
40:07combine into
40:08an elliptical galaxy,
40:10star formation slows down.
40:13You go from
40:14a normal galaxy
40:16minding its own business,
40:17it crashes into another one,
40:18you have this huge amount
40:19of star formation,
40:20this dramatic starburst,
40:22but then in a relatively
40:23short period of time,
40:24all the gas is used up,
40:25all the fireworks are over,
40:27and the galaxy
40:28just basically goes
40:29on its merry way,
40:31not doing much of anything.
40:33The merger
40:33of the Milky Way
40:34and Andromeda,
40:36dubbed Milcomeda
40:37by some astronomers,
40:38will also undergo
40:39the merger
40:40of their dense cores,
40:42each harboring
40:42a black hole.
40:44The supermassive black hole
40:45from one galaxy
40:46finds the supermassive
40:47black hole
40:47from the other galaxy,
40:48they come together,
40:49they become a bound pair,
40:50and then the orbit shrinks,
40:52and they merge.
40:52And then you get
40:53an even bigger
40:53supermassive black hole.
40:55This process is violent,
40:57and during that process,
40:59gas may feed
41:00the black hole
41:01and produce the appearance
41:03of a very bright quasar.
41:08But the greatest danger
41:09to Earth
41:10during the collision
41:11of the Milky Way
41:11and Andromeda galaxies
41:13is something completely
41:15unrelated
41:15to the galactic collision.
41:17As the new galaxy
41:19is being born,
41:20our sun
41:21will be going
41:22through its death throes.
41:24The analogy of life
41:25works really well
41:26for stars.
41:27They're definitely born.
41:28There's a point
41:28at which they start
41:29and they turn on
41:30where they're converting
41:31hydrogen to helium
41:33and extracting energy.
41:36They go through
41:37middle age as well.
41:38They tend to swell up.
41:40They get a little bit brighter.
41:41And then towards the end
41:42of their lives,
41:43they really change dramatically.
41:44As the sun dies,
41:47it runs out
41:48of its fuel source,
41:49hydrogen.
41:51It now becomes
41:52a red giant star.
41:54Its size will extend
41:56all the way out
41:57to where the Earth
41:58is currently orbiting
42:00away from the sun.
42:01So it'll be around
42:0390 or 100 million miles
42:05in radius.
42:06That's not good
42:07for life on Earth.
42:08The Earth will lose
42:09its oceans,
42:11it will lose
42:11its atmosphere,
42:12and it really won't
42:13be hospitable
42:14for life.
42:16Provided we can make
42:17it this long,
42:18billions of years
42:19in the future,
42:20we'll have to have
42:21either colonized
42:22nearby star systems
42:24or built some kind
42:25of artificial planet
42:26further away
42:26from our star
42:27so that we could be
42:28at a nice,
42:29safe distance.
42:31Though our neighborhood
42:32may have become
42:33a lifeless hot zone,
42:35astronomers predict
42:36conditions in the new
42:37Milcometa galaxy
42:38will be conducive
42:40to life.
42:40There's going to be
42:42close to somewhere
42:43around a trillion stars
42:45in this merged galaxy.
42:48And these stars
42:49are still going
42:50to have planets,
42:51even if only
42:52a small fraction
42:53of the planets
42:54are habitable.
42:55The numbers are
42:56so overwhelming
42:57that with conservative
42:59estimates,
43:00it's expected
43:00that you would have
43:01to have life
43:02somewhere in Milcometa,
43:04if not many places
43:06within this merged galaxy.
43:10And in our universe,
43:12the most fundamental
43:13merger is that
43:14between death
43:15and life.
43:16The host of collisions
43:18between stars,
43:20black holes,
43:21and even galaxies
43:22represent the most
43:24violent of events
43:25that bring on rebirth.