00:00At first glance, it looks like any other star.
00:04Until you look away for just a second.
00:07You look again.
00:09And it is unobtainable.
00:10The reason is that it is probably the fastest of the stars.
00:15Listen a little.
00:16Astronomers have recently discovered a new amazing star class.
00:20They have received the name of hypervelocity stars.
00:23They have spotted about a dozen.
00:26And it turns out that they are so fast
00:28that one day they may leave our galaxy, the Milky Way,
00:32to embark on an intergalactic journey.
00:35You see, the stars are constantly moving through the arms
00:38in a spiral of our Milky Way
00:40like pressed cars in the middle of high-speed traffic.
00:45But when you look at the night sky,
00:47it seems to you that these luminous points
00:50are as immobile as the monumental pyramids of Giza.
00:54Now, among these 200 to 400 billion stars
00:57that consider the Milky Way as their home,
01:00a tiny fraction stands out from the others.
01:03Gravitational interactions have accelerated these massive stars
01:06to the point that they move two or three times faster than our sun.
01:11Yes, these are the famous hypervelocity stars
01:14whose destiny is to free themselves
01:16one day from the gravitational constriction of our galaxy.
01:19It looks like they are trying to reach unprecedented speeds.
01:23They move at more than 126,000 km per hour.
01:28A hypervelocity star could cross the distance
01:31that separates the Moon from our planet in just 20 minutes.
01:36These stars are solitary
01:38and very different from the hypervelocities we have known so far.
01:41The original hypervelocities were large and blue
01:44and seemed to be born in the heart of the galaxy.
01:47But these new stars, they are quite small,
01:50not bigger than our sun,
01:52and do not seem to come from the center of the Milky Way,
01:55which is surprising.
01:57They are most often discovered escaping from relatively young stars.
02:01Most of the hypervelocity stars
02:03are in the outer halo of the galaxy,
02:06about 150,000 light years from the center of it.
02:10It is interesting to note that an astronomer
02:12from the National Laboratory of Los Alamos in New Mexico
02:16predicted the existence of hypervelocity stars in 1988,
02:20but scientists only began to discover them in 2005.
02:25The first star spotted was 350,000 light years from the heart of the Milky Way
02:30and was 2.4 million km away from its center per hour.
02:36Astronomers say it is extremely difficult
02:39to chase a star out of the galaxy.
02:42When it happens,
02:43it is most of the time because of a supermassive black hole.
02:47And according to experts,
02:48it is probably Sagittarius A-star
02:52that accelerates hypervelocity stars,
02:54or at least some of them.
02:57To reach the speed of evasion,
02:59a star must receive an impulse of an unimaginable force for us.
03:03And as the central black hole of the Milky Way
03:06has a mass of 4 million suns,
03:08it probably has enough power
03:10to produce something of this kind.
03:12In the most common scenario,
03:14we have a binary star,
03:16a system made up of two gravitationally linked stars
03:20orbiting each other.
03:22Suppose these stars are caught in the core of our black hole.
03:26As one of the stars goes in a spiral towards the black hole,
03:30its sister is projected outward at a phenomenal speed.
03:34This is what is called a three-body exchange.
03:38Astronomers have already discovered 18 giant stars
03:41that could have been made hypervelocity by this process.
03:44There is also a theory
03:46according to which hypervelocity stars
03:48could have been ejected from their stellar masses
03:51by supernova explosions.
03:53Hypervelocity stars,
03:55recently discovered,
03:56seem to have the same composition as normal stars.
04:00This is why astronomers do not think
04:02they come from an exotic place
04:04located outside the galaxy
04:06or in the outer halo of the Milky Way.
04:09This halo is actually a lifeless region
04:12filled with old poor metal stars
04:15and very weak masses.
04:17It is unlikely that a hypervelocity star is found there,
04:20unless it has been ejected in this direction.
04:23Hypervelocity stars remain a mystery,
04:26but they are not the only ones to behave strangely.
04:30For example, in 2017,
04:32astronomers spotted a bright star
04:35rushing out of the Milky Way.
04:37It was moving incredibly fast,
04:39at a speed of 3.2 million km per hour,
04:42almost four times faster than the sun in orbit
04:45around the center of the galaxy.
04:47The main problem with this wandering star
04:50was that it was moving in the opposite direction
04:52compared to most of the other stars
04:54orbiting the center of our galaxy.
04:57But this is not the only strangeness about it.
05:00Its appearance also raised many questions.
05:04Was it a hypervelocity star?
05:06Was it even a star?
05:08This wanderer seemed to be made up of
05:11a totally different stellar matter from its congeners.
05:14Astronomers managed to identify its composition.
05:17It was made up of heavy and metallic atoms.
05:20Most of the other stars are made up of much lighter elements.
05:24This star was called LP40-365.
05:28It was moving so fast
05:30that it literally left our galaxy.
05:33Scientists therefore thought
05:35that it had been ejected by a cosmic disaster.
05:38For example, a supernova.
05:40Today, astronomers claim that the star,
05:43which was previously considered a white dwarf,
05:46was actually a stellar fragment,
05:48a kind of residue caused by a supernova explosion.
05:52The explosion must have been so powerful
05:54that it pushed LP40-365 very far out of the galaxy.
06:00This star fragment continues to turn slowly around its axis.
06:04Thanks to this observation,
06:06astronomers can confirm the fact that LP40-365
06:11is actually only a space debris,
06:13and not an entire star.
06:15But such a conclusion means
06:17that it would have been a witness to a supernova.
06:20Even if this event occurred at the speed of light,
06:23it is the entire composition of this space object that has been modified.
06:27Most stars are mainly composed of helium and hydrogen,
06:31but LP40-365 is different.
06:35It contains heavy elements,
06:37magnesium, oxygen and neon.
06:39It is undoubtedly the supernova
06:41that added its atoms to the composition of this star.
06:44Astronomers also consider
06:46that all the heavier elements than helium are metals.
06:49This means that after being a witness to the supernova,
06:53LP40-365 has become metallic.
06:57Our space traveler lights up
06:59and then darkens every 8 or 9 hours.
07:02This could mean that it emits pulsations.
07:04But in general,
07:06stellar pulsations are much less regular.
07:09A more plausible explanation
07:11is that the surface of the star is irregular.
07:13It is the rotation of the star
07:15that would cause these differences in brightness.
07:17Now let's move on to another curious space corse.
07:21This is the 18th brightest star in the night sky,
07:24called Fomalhaut.
07:26If you observe it with a telescope,
07:29you will be confronted with a terrifying spectacle.
07:32It is not surprising that it is nicknamed
07:34the Eye of Sauron.
07:35A ring of dust and debris surrounds it,
07:38which gives it the appearance of a giant eye
07:40that looks straight into your soul.
07:42This intimidating star
07:44has a mass more than twice that of our sun
07:47and is 25 light-years from Earth,
07:50which is not so far
07:52if we consider the distances in space.
07:54There is also a star that tears itself apart.
07:57It has a beautiful name,
07:59Vega.
08:00At first glance, it seems normal.
08:02Big, bright and quite young.
08:05But the more we advance in time,
08:07the more it becomes bizarre.
08:09For example, it is not really round.
08:11It looks like an egg.
08:13The reason may be that it rotates so fast
08:15that it ends up deforming.
08:17This has led to temperature variations on its surface.
08:21In addition, it reaches 93% of the maximum speed
08:25that a star of this size can tolerate,
08:27i.e. 273 km per second.
08:31Astronomers do not know exactly
08:33why Vega rotates so fast
08:35that it almost tears itself apart.
08:37Mira is sometimes one of the weakest stars
08:40and sometimes one of the brightest in the night sky.
08:43This system is actually made up of two stars.
08:46One of them is a red giant
08:48that once looked like our sun.
08:51The other is a white dwarf.
08:53The red giant constantly gets rid of its external layers,
08:57creating a cloud of debris around the system.
09:00This phenomenon could be the cause
09:02of the regular darkening of the stars.
09:04It could also explain why the intervals
09:06between these periods of darkening are not regular.
09:09But the most interesting thing is
09:11that this binary star has a tail
09:13similar to that of comets.
09:15Transcribed by ESO. Translated by —
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