Have you ever wondered at what speed our Sun moves in the Milky Way? Or, what speed would be required by a star to escape the gravitational clutches of this massive galaxy? And the biggest question - can a star even escape the galaxy permanently? The answer to the last question is - yes! It happens in a few ways but the one way we will focus on will lead us to the most interesting stars in any galaxy (or out of *wink* *wink*) - the hypervelocity stars.
We will understand these stars by taking a look at their story behind their discovery and everything that lead to it. Of course, we will never leave out any important science in its midst and you are more than welcome to read more from the links to research papers provided in the text!
Discovery
First, to get the facts straight - the Sun moves with a velocity of roughly ~ 200 km/s and the escape velocity of our galaxy is 550 km/s. Obviously, our Sun is wayyyyy below the escape limit. And anyway, how would a regular star gain so much velocity as to leave the galaxy itself, right? It’s just a star!
But in 2005, Warren Brown, Margaret Geller, Scott Kenyon, and Michael Kurtz discovered a star moving with a velocity of 850 km/s which is ~ 2 million miles per hour! That means the velocity of the discovered star was not just above the escape velocity but wayyy above the escape velocity by 300 km/s!
Safe to say that the star was certainly unbound or free from the gravitational pull of our magnanimous galaxy. As Brown himself said, it was quite ironical because this group was actually trying to observe stars bound to the galaxy to get the measurement of the total mass of our Milky Way. And by a twist of fate, they discovered a star which was unbound.
So, what could have pushed a star so strongly to be able to escape the clutches of such massive gravitational force of the galaxy?
The mechanism behind the escape
For this, we go way back in time in 1988. In this year, theorist Jack Hills published his paper theorizing the existence of such stars. He called them the Hyper-velocity stars. According to him, these stars gained these massive speeds due to a three-body exchange mechanism.
It starts with two bodies initially. These two bodies are gravitationally bound and orbiting one another in a stable system. This system is the binary stars system which is very common in galaxies. Speaking in Physics, these systems have a negative binding energy. And more negative the binding energy, the more stable the system is. So stable that the binary stars can stay bound to each other forever!
But then comes the third body. According to Hills, this third body that leads to a hypervelocity star has to be a supermassive blackhole (SMBH)! During this time there were speculations of the existence of a supermassive blackhole at the center of our galaxy but Jack Hills took this to a whole new level. He imagined what it would be like for other stars because of its existence.
Back to the mechanism! Now we have a third body (SMBH) introduced in a binary system. This is when things start getting bad. The previously highly stable system becomes highly unstable or in Physics terms – chaotic.
The SMBH starts forming a tighter orbit with one of the two binaries. There is an exchange of energy and since we have such a massive entity in the system (SMBH) the energy is a LOT. This excessive energy of the system manifests in the form of the other binary being pushed out. It is enough to subsequently unbind the star from the gravitational pull of the system. But that’s not all. It is a supermassive blackhole! The energy should be enough to not only free the star from the gravitational clutches of the binary system but also the gravitational pull of the galaxy too! We see this force in the form of the excessively high velocity gained by the star.
But of course some of these stars end up just getting ripped apart in what we call the tidal disruption events! But according to Jack Hill’s calculations, there should be between 1000 to 10000 hypervelocity stars just in the 33000 lightyears of the galactic center itself!
Phew! Imagine being an astrophysicist during the time when Jack Hills published this paper. People would have easily overlooked it because of how crazy it sounded regardless how scientifically sane it actually was! As you might expect, his paper didn’t get a lot of attention then. That is, until 2005 when a hypervelocity star was actually discovered.
Many theories were then put forth to explain the mechanism. And all had to agree that only a really massive object such as SMBH would be able to make it possible. In the end, it was Jack Hills’ theory that made the most sense after conducting simulations to test each theory. These simulations also suggested that the mass of the black hole must be around 4 x 10^6 solar masses for it to have ejected out any star with that force.
But what happens to the stars bound to the SMBH? And what about the unbound stars? Can we know more about the SMBH just from studying these hyper-velocity stars?
Let us take a glance at the observations made over the years that might help us answer these questions.
Observations
Since the accidental discovery of the first hyper velocity star, the group (Warren Brown, Margaret Geller, Scott Kenyon, and Michael Kurtz) decided to dedicate their time to finding more of these and understand them better. But how were they to find them?
One way was to look at the stars in the halo. Specifically look for the stars which might look younger than the others in that region. Why? Because the halo is dominated by older stars so any younger stars present there might mean they might not have originated there. And what method was used? They actually used the age-old method of understanding the doppler shift in the spectra of the stars. And using this method, they actually found 21 unbound stars!
Well of course, there were other astronomers too who were trying to find and study them. They used different methods with larger uncertainties but nonetheless caused a lot of conversations. Let us understand what picture these observations painted about the blackhole.
Through their calculations, the astronomers knew that if it were a case of two SMBHs there would be a single burst of hyper-velocity stars before merging of the SMBH. But the observations didn’t show a single such outburst of hypervelocity stars. That meant there weren’t two but only one SMBH at the centre of the galaxy.
Later observations threw light on the situation with the left-behind binary counter-parts which were stuck with the SMBH. The images obtained showed that there were multiple stars orbiting the SMBH in tight, chaotic orbits. You can learn more about the SMBH and the stars orbiting it in 'Sagittarius A: Milky Way's galactic center'.
As for the unbound stars, before the southern-hemisphere surveys, they were found clumped in the galactic longitude at the northern hemisphere. The idea behind their further study is that they might show us the galaxy's exact limits which can’t quite be mapped because of the invisible dark matter surrounding the galaxies.
But how strange is it that a star born in the galaxy, bound to its gravitational field for eternity is flung out by the very heart of the galaxy, to sail out of its limits? Stranger still is the fact that this concept of hypervelocity stars was an idea for a sci-fi movie (literally used in a movie once) before it was even discovered and science made sense of it all!
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