The fastest star in the Milky Way
We all know planets orbit around their host star. Some of them go faster than others,
and all of this depends on how distant they are from the star.
For example, Jupiter travels at an average velocity of 13.7 kilometres per hour in
its orbit around the Sun, while the Earth at about 29.78 km per hour.
But have you ever wondered what is the average velocity of a star?
Even if lesser than planets, stars could be very different from each other.
A lot of them are giants, some others are dwarfs. Some of them are very dense, while
others have a big radius, but it's just because the gas they're made of has a low density.
Said so, how do we measure the velocity of a star?
Follow me in this video to get to know the answer to these questions,
and to hear more about the fastest star in the Milky Way. Spoiler: it goes faster than you can
imagine.
Not a long time ago,
we spotted the fastest star in the Milky Way. Shortly, it travels at 8% of the speed of light.
This means its average velocity is about...24.000 km per second, or 15.000 miles per second.
The star is dubbed S4714, but,
from now on we will call it “Bolt”, just to make this video less boring!
Bolt survives in the extreme environment at the centre of our galaxy.
He's orbiting an unusual object, a very peculiar one, its majesty Sagittarius A*,
the supermassive black hole of the Milky Way.
As we said, on its journey it reaches a speed of 8 per cent of the speed of light,
but what if I told you that is not even the most amazing thing about it?
In fact, Bolt is a weird type of star, it's a squeezar.
Squeezars-type stars were proposed 20 years ago by astrophysicists Tal Alexander and Mark Morris,
They published an article on the astrophysical journey in which they proposed the existence of
a class of transient sources, called squeezars, which are stars caught in highly eccentric orbits
around a massive black hole, whose atypically high luminosity is powered by tidal interactions
with the black hole. Their existence follows from the presence of a mass sink,
the supermassive black hole, in the galactic centre, which
drives a flow of stars into nearly radial orbits to replace those it has destroyed.
So, our star, Bolt, has given us the first candidates for a type of star
originally proposed nearly 20 years ago! In other words, squeezars are stars that get
so close to a black hole, that they are squeezed by its tidal forces.
Bolt is just one of a group of stars that have now been discovered skimming Sgr A* on closer orbits
than any other stars discovered previously. Even though the Milky Way is a quiet galaxy
compared to more active galaxies with quasars in their centre,
the environment around Sagittarius A* can get pretty wild.
It's actually a bulge, around there. Stars are constantly orbiting the
black hole along unusual paths. Astronomers, for example, have
identified a number of stars on looping, long, and sharply elliptical orbits in that region.
You just have to imagine an oval, the path, with the black hole at the centre.
These are insane trajectories. Ad if you add the fact that they move along their
paths with an absurd velocity, at some per cent of the speed of light, you get
the recipe for the most peculiar kinematics discoveries we can make in our Universe.
Isn't it amazing? However, these crazy going stars are called S
stars, and we can use them to probe the properties of the gargantuan invisible object they orbit.
Before Bolt's detection, for years, a star called S2 was regarded as the nearest star to
the black hole. At is periapse, that is the point in which the star reach is the maximum velocity,
the gravitational kick from the black hole approach can accelerate this star
to 3 per cent of the speed of light.
But then, a team from the University of Cologne, Germany, found another star,
S62, that was harder to spot because it was much fainter. It took 9.9 years to
orbit Sagittarius A*. That's closer than the average distance between Uranus and the Sun.
It was also much closer to the black hole, and this implied it travelled much faster than S2.
In fact, astronomers estimated its maximum velocity to reach 6.7% of the speed of light.
But Peissker, this was the lead team author's name, wasn't done. After years of work,
the team now discovered five new S stars even closer than S2 - S4711, S4712,
S4713, S4714, our Bolt, and S4715. "I am happy that I had the opportunity
to work with and observe (with the Very Large Telescope) the Galactic centre for
the last 7 years," Peissker declared. Of the new stars, S4711 and S4714
are the most important ones. Let's see what we know about S4711.
We know it is a blue B-type star around 150 million years old, and it goes around
Sagittarius A* once every 7.6 years, with a periapse distance of 21.5 billion kilometres.
The period of 7.6 years is shorter than S62 one, and this tells us it has the shortest
mean distance to the black hole throughout its entire orbit that we've yet discovered.
S4714, Bolt instead has a longer orbital period than S4711 - 12
years - but its orbit is extremely eccentric. Eccentricity is a measure of the “roundness” of
an orbit, and it takes values from 0 to 1. A null eccentricity corresponds
to a perfect circumference. A big eccentricity means the ellipse
shape is elongated, 1 being an escape orbit. S4714 has an orbital eccentricity of 0.985.
These extreme stars, Peissker said, are the first real candidates for squeezars,
first theorised back in 2003. Astrophysicists Tal Alexander and
Mark Morris proposed a class of stars on highly eccentric orbits around massive black holes.
At each pass, the tidal forces convert a fraction of the star's close orbital energy into heat.
This, firstly, makes the star shine more brightly than it normally would; and,
secondly, contributes to the star's orbital decay. In other words, squeezars are dead stars orbiting.
According to Peissker, "At least S4711 and S4714 are squeezar candidates,"
"I would say, I am sure about S4711 since the orbital elements
are consistent with the predictions of Tal Alexander in 2003. In this sense,
S4711 is the very first squeezar ever detected."
If confirmed, these stars could help to revolutionize our understanding
of the interactions between black holes and the stars they devour.
All of these studies involved a monumental use of stellar kinematics.
But what is it? And why is important to study stellar kinematics?
Before answering this question, please be sure to like or dislike the video so
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In astronomy, as one can imagine, stellar kinematics is the observational study
or measurement of the kinematics or motions of stars through space.
Stellar kinematics encompasses the measurement of stellar velocities in the Milky Way
and its satellites as well as the internal kinematics of more distant galaxies.
Measurement of the kinematics of stars in different subcomponents of the Milky Way
including the thin disk, the thick disk, the bulge, and the stellar halo provides
important information about the formation and evolutionary history of our Galaxy.
Kinematic measurements can also identify exotic phenomena such as
hypervelocity stars escaping from the Milky Way, which are interpreted as the result of
gravitational encounters of binary stars with the supermassive black hole at the Galactic Center.
Stellar kinematics is related to but distinct from the subject of stellar dynamics, which
involves the theoretical study or modelling of the motions of stars under the influence of gravity.
Stellar-dynamical models of systems such as galaxies or star clusters are often compared with
or tested against stellar-kinematic data to study their evolutionary history and mass distributions,
and to detect the presence of dark matter or supermassive black holes through their
gravitational influence on stellar orbits.
Now, as for what we said above we can use kinematics to test our theories.
For example, since these stars reach relativistic velocities,
we can use them to test general relativity. Both the way the star's light stretches when
it approaches the black hole, and the way its orbit shifts around like a spirograph,
confirmed Einstein's theory in some of the most strenuous tests yet.
I know what you're thinking: is it possible to find stars
travelling at even more extreme velocities around Sagittarius A*?
Of course, this is possible. With more powerful telescopes launching
in the coming years, including the Extremely Large Telescope, we should be able to find them.
We just have to be patient and, as always, to keep it curious!
"This video ends here! Thanks for watching everyone!
What do you think about this video? Do you think we will spot faster stars in the upcoming years?
Let us know in the comment below! See you next time on the channel!