In summer if you go to a dark place at night and see the sky you realize that you see Galactic Center. You will see a beautiful band of stars. This is actually our on galaxy (Milky Way). Our sun and earth are in a way it’s citizens and there are approximately 200 billion other stars included in the Galaxy. But are there any stars in our universe which are not a part of any Galaxy.
Location of Stars
For thousands of years human had thought that we are at center of the universe and everything in the universe is orbiting the Earth. And in a way one result of this was that we are the most important thing of the universe. Few hundred year before we learned that our earth is a planet which is orbiting our sun. And there are other planets also present in our solar system. Not only this our sun is actually a part of the Milky Way galaxy and there are approximately 200 billion stars present in this galaxy.
And now our estimate is that the number of planets is in trillions. Leave the center of the universe we came to know that even our sun is not at the center of our Milky Way galaxy. In fact, the carefully done mapping of our Milky Way, this is a difficult job mapping the Milky Way because we live inside this galaxy.
But this mapping showed us that our sun is at a distance of 30,000 light-year from the center of our galaxy meaning if somebody sent us a message from the center of our galaxy at the speed of light that will take 30,000 years to reach us.
And we are so far that it takes our sun 225 million years to complete one orbit of our galaxy. So our sun and our solar system has completed about 20 orbits around the Galaxy in its life. Because it takes 225 million years, so the last time our Sun was at this place in its galaxy the Ali dinosaurs were leaving on this earth at that time.
Now I do not know whether the dinosaur saw beautiful thing on the sky at night or not. I think there was no light pollution at that time so that spectacle would be wonderful. It is not known whether the dinosaurs saw that thing or not but today specially in summer if you see the sky from a dark place you will see the Milky Way galaxy, which is the disc galaxy on the sky. And we are looking toward the center of our galaxy from the place where our sun is located in our galaxy at that time. In a way the Milky Way is our home and a very beautiful home.
Discovery of Rough Stars
Astronomers had a suspicion that this type of intergalactic stars (starts between the galaxies) must be present but a big problem is how to detect this type of stars. When we are seeing the picture of the galaxies, specially Andromeda or other galaxies
The light we see is the light of several billion stars at the time.
But to detect that individual stars between the galaxies would be very difficult. In the late 1990 century for the first time this type of intergalactic stars were discovered. These were actually discovered in a Virgo cluster.
We know that there are several hundred billion stars in a galaxy but there are galaxy clusters also in which many galaxies are present. This Virgo Cluster is at a distance of 60 million light years from us. This is in a nearby cluster.
In 1997 Astra numbers, for the first time, discovered some stars in that (cluster) which were not a part of any galaxy. How did they find out this? Actually, astronomers used the Hubble space telescope that took a deep image of a blank part of the Virgo Cluster. Do you know about the Hubble deep field where different galaxies were visible. This was the same type of deep field and that you’re such an area where there were no bright galaxies.
Astronomers in this blank piece of the sky discovered about 600 stars.
Now what type of stars were these?
It was found at that this is a stage of the stars “Red Giant”.
Many stars (our sun also pass through this stage after some billion years) when there will be near their end stage, its size overall will become big. Because of its bigger size its overall brightness (called luminosity in Astronomy) will also increase. This is called by astronomers “Red Giant Phase” and every start passes through this stage and because their size and brightness increase it is very easy to detect them.
So, in the Virgo cluster Snr must have discovered 600 red giant stars in this blank piece of the sky. Made this mean that if there were 600 red giant over there then there must be several thousand other stars also in that part whose light we could not detect. In this way, astronomers calculate, what is the overall population.
But the important of this was first time, astronomers saw space between the galaxies. The nearest galaxy was 300,000 light years away from these stars. This means these stars are not a part of any galaxy.
Now, these stars are not small in numbers, in fact the estimate of astronomers is that 1 trillion stars in the Virgo Cluster or 10% of the total starred in this Virgo Cluster are these intergalactic stars that is between the galaxies. Now the question arises how did this type of stars reach the space between the galaxies?
Now the question arises how did this type of stars (in virgo cluster) reach the space between the galaxies?
Astronomers, think there are two way of getting the stars into intergalactic space. I did think that all the stars no doubt were formed in the galaxies but one way of getting them into the intergalactic space is through galaxy collisions. Galaxies collide with one another. We have a lot of evidence of that at the present. Rather the Milky Way and Andromeda Galaxy are also going to collide with one another at that time.
You are seeing in front of you the picture of a Galaxy pair called “Mice Galaxy”. These galaxies are colliding with one another nowadays and this merger (eventually, they will form one galaxy) will take about 1 billion years. But then galaxies collide let me remind you once again, stars, do not collide with one another.
The reason is the space between the stars is much more compared to the size of the stars. When galaxies collide the stars easily passed through but because of the impact of gravity their orbits may be disturbed. And the idea some stars may inject from these galaxies as a result of those mergers.
Rather in the small galaxy is called DWARF galaxies, the Assumption is because their gravity is less the ejection of stars from them (galaxies) is relatively easy. In these big galaxies, as in the collision of mice is galaxies The idea is the stars may also eject to this collision.
And the tail you are seeing in the picture, you can imagine if the stars at the end of the tail get a little more kick they may go further away from the gravity the gravity of these galaxies
There is another wonderful way of ejecting these stars and its connection with supermassive black holes which are found at the center of the galaxies. Astronomers have up till now discovered 16 hypervelocity stars in the Milky Way.
Hypervelocity means their speed is much more compared to the speed of other stars. The speed at which our sun for example is moving around our Galaxy, the speed of these stars is almost 3 times more than that. This means that these stars will definitely eject from our Milky Way. But when astronomer traced their trajectories, the trajectory of all of them was related to the supermassive Black hole at the center of our galaxy.
What example the tracing star revealed that about hundred million years before
It was a triple star system meaning there were three stars in the system. It past near the supermassive Black hole of the milky Way. One star was pulled by the black hole toward itself and because of that due to the conservation of angular momentum, the other stars received a kick and now they are going with more speed toward the outer side.
Mapping Dark Matter
The idea is that all these hypervelocity stars almost all of them, had an interaction of their orbits with the supermassive Black hole and the speed increased. Now astronomers think this is a very important ways by which these intergalactic stars are formed.
Now some among you will say this was very cool. Can you tell something which is more cool? Yes of course! Actually, this intergalactic or intra cluster stars can tell you the distribution of dark matter. How? These stars are not impacted by the gravity of the galaxies. Most of the matter or material in these galaxy clusters such as Virgo Cluster is dark matter.
If you want to trace dark matter you cannot see it, but these stars which are between the galaxies their distribution can tell you the distribution of the dark matter (in a way) because the moment is due to gravity of the whole cluster and the gravity of the cluster is because of dark matter.
Read the first image of the James Webb space telescope remember the SMACS image
which had clusters and gravitational lensing etc.
This image has been further analyzed by astronomers.
And now they have detected there are faint stars and the light is visible in the background. This images is actually a reverse inverted image. In this, you can see the intergalactic light (very faint) which is due to these stars. This is only the beginning. The James Webb space telescope is going to image many more clusters and you will start getting more of these intergalactic lights and these will tell you how is the dark matter distributed in the clusters.
But not from the big galaxy scale let’s come again to small galaxy scale. Just imagine these stars which are between the galaxies. There must be planets around many of them waiting. There are planets around every star. In some there will be life and in some there will be intelligent life also. So you can imagine there will be intelligent beings on some planets
Who must be thinking that probably they are also at the center of the universe or they will imagine that the faraway galaxies looking small to them maybe they’re inhabitants around those stars. In a way, whether you are inside of the galaxy or outside the galaxy we are all citizens of this universe.