00:00An event so violent was produced in the past of the Volacty
00:03that it would have been returning our entire galaxy on the side.
00:07It was obviously not affected the life on Earth.
00:09The Earth didn't even exist at the time.
00:11But if it was really happened,
00:14it was completely written the story of our Soleil.
00:19Our little tranquillity of the galaxy was perhaps not as calm as that.
00:23And most of the stars of the Volacty
00:25might follow perhaps the trajectories completely different from today.
00:29Today, y compris, our own Soleil.
00:31We could literally be seated on the debris of an ancient carambolage galactique.
00:37And until now, no one can doubt it.
00:40There is a big cosmic mystery
00:42that has broken up the astronomers for years.
00:45Our galaxy has a large flat disk in rotation,
00:49the part with all the spiral arms that we all saw in photo.
00:53This disk turns quickly, very quickly even,
00:55at about 220 km per second.
00:59But around this disk,
01:01there is another region,
01:02called the halo stellar.
01:04It's a nuage ténu
01:06and clairsemé of ancient stars
01:07which float well beyond the disk principal.
01:11We would say a diffuse
01:12around the plate plate in rotation.
01:14And the most strange,
01:16it's that this halo
01:17doesn't turn almost no longer.
01:19In 2017,
01:21a team of scientists
01:22measured precisely
01:23this length rotation
01:24of the halo for the first time.
01:26In combining the data
01:27of the satellite Gaia
01:28and the Sloan Digital Sky Survey,
01:31they discovered that the halo
01:32turns at only 10
01:34to 20 km per second
01:35around.
01:37For us,
01:38terriens,
01:38it seems fast,
01:40but compared to the rotation
01:41of the disk,
01:42at about 800 000 km per second,
01:44it's almost immobile.
01:46So why a part of the galaxy
01:48would turn so fast
01:49while the other
01:50moves a little?
01:52It's the halo stellar
01:54that can help us find the answer.
01:57Even if it contains
01:58just a low percentage
01:59of the stars
02:00of the galaxy,
02:01it's in some sort
02:02the memory
02:02of our galaxy.
02:05The fact is that
02:06many of these stars
02:07of the halo
02:08are not born here.
02:09They belong to
02:10small galaxies
02:11distincts
02:12that are too close,
02:14they have been
02:14dislocated by
02:15our gravity
02:16and absorbed
02:17by the galaxy
02:17during the time.
02:21Imagine a big fish
02:22that has plenty
02:23of small fish.
02:25Even after having
02:25eaten,
02:26there are still traces
02:27of what he ate
02:28inside.
02:31When these small galaxies
02:32had been
02:33déchiquetées,
02:34their stars continued
02:35to travel
02:35in the same direction
02:37and at the same speed
02:38as before.
02:40So, today,
02:42in measuring
02:42the movement
02:43of these absorbed
02:43these stars,
02:44we can learn a lot
02:46on all the ancient galaxies
02:47englouties
02:48by the Voie lactate.
02:51It's like
02:52a fossil record
02:53but,
02:53at least
02:54these stars
02:55are still
02:56in space.
02:57And,
02:58thanks to this
02:58fossil fossil
02:59space,
03:00we know
03:01that,
03:01the Voie lactate
03:04has suffered
03:04a massive collision
03:05with another galaxy
03:07called Gaia
03:08Sausage
03:09Enceladus.
03:11We will talk about it
03:12a little later.
03:15Our solar system
03:16has about 4,6
03:18billion years
03:19years
03:19So,
03:20it happened
03:20before the existence
03:21of the Earth.
03:23scientists
03:23discovered this collision
03:25in studying the stars
03:26who float in our galaxy
03:27today,
03:28and more precisely,
03:30their
03:30movements
03:30The most
03:32stars
03:33follow
03:33quite regularly
03:36like
03:36cars
03:37on a circuit
03:37But,
03:38this group of stars
03:39in particular
03:39of
03:41to the center
03:43on a long way
03:46very elongated
03:47When scientists
03:49have mapped out
03:49these movements
03:50all the points
03:51of the data
03:51form a
03:52a thin shape
03:53When we look,
03:54it looks like
03:55a sausage
03:55So,
03:56that's why
03:56the astronomer
03:57called the sausage
04:00autre thing
04:01the word collision
04:03can give
04:03a bad image
04:04of what happened
04:06You can imagine probably two solid objects that are going through each other, like two cars
04:12that are going through, explosions, lights, debris and a total destruction.
04:17But this is not at all what happens with the galaxy.
04:21The galaxy contains a lot of vertigineous stars, it is true.
04:25But these stars are incredibly far away from each other.
04:29When two galaxies are going through, it is in fact very little probable
04:32that the stars are going through directly.
04:34What really causes the damage, it is gravity.
04:39When the small galaxy comes across, the gravity much more powerful of our galaxy
04:43is pulling, pulling these stars one by one and project them on new trajectories.
04:49The gas clouds of the two galaxies are going through and comprising
04:53and all these gravitational changes can curve or incline the disk of the largest galaxy.
05:00It's a bit like to put cream in a coffee.
05:02It doesn't remain in a small bowl.
05:05It's a little bit different.
05:06It's a completely different collision.
05:09This ancient collision is a very well documented event in the past of our galaxy.
05:14And the dominant theory today,
05:16is that this cosmic cataclysmic was so violent
05:19that it literally returned the entire disk of our galaxy.
05:23But there is a problem.
05:24No one can point a telescope towards the sky
05:27and just look at it all happen.
05:30This event was produced it hundreds of years ago,
05:33before humans and before the telescopes.
05:35There is no image of the collision
05:37and no way to re-bobine it to see it in direct.
05:40So, in the place,
05:41scientists have built
05:42the mathematical simulations of galaxies.
05:44These are, in general,
05:46the virtual voies,
05:46which obey the real laws of physics.
05:49How the real galaxies grow,
05:51grow,
05:52attract gas
05:52and engloutisent
05:53the smaller galaxies
05:54over billions of years.
05:56The researchers have used
05:57a specific set of these simulations,
06:00called the simulations Auriga.
06:03They created 25 galaxies
06:04fictives
06:05similar to the voies lactate
06:06and let them evolve over billions of years.
06:09The scientists
06:10looked for a recurring scheme.
06:13Every time
06:13the halo stellar
06:14of a simulated galaxy
06:15ended up turning
06:16like the ours,
06:18what caused that?
06:20What caused that?
06:21The answer came
06:21systematically
06:22to the same events
06:23of events.
06:24Every time.
06:26The galaxy was formed
06:28relatively early.
06:29She had suffered a major fusion
06:30frontale
06:31similar to our real collision
06:33Gaia-Sausage.
06:35And its disk
06:36was turned on,
06:37the three together,
06:38again and again,
06:40in dozens of galaxies
06:41simulated.
06:42It could not be a coincidence.
06:45We can still be sure
06:47that it is exactly
06:48what happened
06:49there are hundreds of years
06:50but according to the simulations,
06:52it is far the most probable explanation.
06:55So how do we turn
06:57a whole galaxy?
06:59The space
07:00is not high
07:00nor low
07:01so scientists
07:02have followed
07:03the magnetic moment
07:04of the disk.
07:05It is basically
07:06a imaginary flash
07:07pointing along
07:08its axis of rotation
07:09like the
07:10wheel of a wheel
07:11that turns.
07:13If this flash
07:14ends up moving
07:15to more than 90 degrees
07:16compared to its initial position
07:18it counts
07:19as an official return.
07:21It was not instantaneous
07:22of course
07:23because the disk
07:25has not turned
07:26like a plate
07:27launched through a table.
07:28The stars
07:29have continued
07:30to orbit
07:30all this time
07:31while the gravity
07:32reorienting
07:34slowly
07:34slowly
07:34until it
07:37stabilizes
07:38in a new orientation.
07:39The
07:40returns
07:40the most rapid
07:41observed by the researchers
07:42have taken
07:44millions
07:44even billions
07:45of years.
07:46Nothing happens
07:47in the space.
07:48The Earth
07:49did not exist yet
07:50when it was produced.
07:52But even
07:53if it existed,
07:54we would probably
07:55have not noticed.
07:56Our sun
07:57would have been
07:58carried out with the rest
07:59of the disk.
08:00And this is not the only
08:01proof that
08:01suggests an ancient inclination.
08:04In 2026,
08:06another team
08:07of scientists
08:07used the data
08:08of over 600,000
08:10giant stars
08:11to cartograph
08:12the shape of
08:13the black halo
08:13of our galaxy.
08:16It is
08:16invisible
08:16that we can't see
08:18but whose gravity
08:19shapes everything
08:20around us.
08:21They discovered
08:22that the exterior
08:22of this black halo
08:24is practically vertical
08:26compared to
08:27our rotation
08:27and the disk
08:27could mean
08:30that the halo
08:31and the disk
08:32are inclined
08:33together
08:34at a moment
08:35while the halo
08:36more ancient
08:38is simply
08:39remained in place.
08:40Scientists want
08:41to continue
08:42looking for
08:42other cicatrices
08:43by such an event.
08:45It is different
08:46distances,
08:47different ages
08:48different
08:49angles discordant
08:50between the disk,
08:51the halo
08:51and the black material
08:53This is part
08:54of the work
08:55to understand
08:55exactly
08:56how our galaxy
08:57is becoming
08:58what it is today.
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