The remarkable "Cartwheel Galaxy"

You are familiar with the concept of galaxies. We ourselves live in a galaxy called Milky Way. There are several hundred billion stars in it. It is spiral-shaped so it's called a spiral galaxy. Likewise, there are other elliptical-shaped galaxies and we call them elliptical galaxies. And there are some, which are neither spiral nor elliptical we call them irregular galaxies such as our neighbor’ Large Magellanic clouds’. 

The above Picture is the Cartwheel galaxy. Its prominent feature is a ring. These types of galaxies are called ring galaxies. How did these galaxies form? There is a surprising story to it. You can learn about it. If you take a stone and drop it into a lake, astronomers think this Cartwheel galaxy was formed the same way as ripples are formed in the lake. This is Kainati Gup Shup (Cosmic Chit Chat) and I am Salman Hameed. The Cartwheel galaxy is 500m light-years away from us. In a sense, it is very far but compared to the width of the universe, astronomers call it a nearby galaxy. This is a picture taken by the Hubble Telescope. You can see a blue ring that dominates the picture. These are actually new stars (the blue ones) and most of them are included in this ring. The diameter of the ring is 150,000 light-years. You can fit our Milky Way galaxy easily in this. 

The Cartwheel galaxy was discovered by astronomer Fritz Zwicky in 1941. I always show his picture definitely because he was an extraordinary character as well as a remarkable astronomer. When Fritz Zwicky saw the Cartwheel galaxy he said in English ‘This is one of the most complicated structures in the universe and in reality, this is an incredible thing, this is a galaxy in which new stars are forming in the ring. In between, there is a spiral-type structure but in that, there are mostly old stars. This is inverse in a sense because usually there are new stars in the spiral arms. This is a big puzzle, how was the cartwheel galaxy formed? Now in this picture by the Hubble telescope you are seeing two other small galaxies with the cartwheel galaxy.

One is a small spiral galaxy that is yellow i.e. it mostly has old stars and alongside is another irregular galaxy that has blue stars meaning there are mostly new stars there. So how was the Cartwheel galaxy formed? Astronomers think this is a result of a collision. Actually, the astronomers think that the cartwheel galaxy, in the beginning, was a spiral galaxy but about 300M years before another small galaxy passed exactly through its center, and because of its passing through when the shock waves compressed the gases, the stars were formed in the shape of a ring. This is just like when you throw a small stone into a lake or drop it in water the ripples start moving away in a circular form. Similarly, the astronomers think that when this small Galaxy passed through the disturbance (the shock waves) compressed the gas and dust and because of that compression, new stars were formed. The ring you are seeing is the place where new stars were formed due to compression. 

In fact, there are 200 billion stars in the ring of the cartwheel galaxy. This is a computer simulation of the formation of the cartwheel galaxy. In this, we have compressed several million years into a few seconds. This is a small galaxy colliding with the cartwheel type spiral galaxy and slowly this small galaxy passed through it. it’s own shape changed but the ripples in the gas were in circular form and this is the place where the ring was formed. This was about the past of the cartwheel galaxy. The question is what is the future of the cartwheel galaxy? If you see the picture by the Hubble telescope closely you see spokes-type things. There are some lines in the rings that are connected to the central area. It means that a lot of stars have started forming in the middle also.

Astronomers think that in a little time (a little time means several billion years) this cartwheel galaxy will reform its spiral structure. Meaning after this collision, this ring formation (all a temporary phase) will become a spiral galaxy again. You can raise the question that whether our own Milky Way galaxy (which is a spiral galaxy) passed through this ring phase or did it have this type of collision in the past or not. We do not have evidence of that. The assumption is this type of Collision did not take place in the Milky Way but it happened in other galaxies because you require a specific type of orientation for this. The Galaxy must pass right through the center and we think no such collision took place in the Milky Way, but the Milky Way is not an innocent galaxy because it has devoured many small galaxies and is gobbling up more. 

In fact, these large Magellanic clouds (please see the video about the Magellanic clouds) will at some time merge into the Milky Way. But who is the prime suspect of the cartwheel galaxy? Which galaxy passed through its center? For a long time, astronomers thought that out of the two galaxies near it (the yellow one and the blue one) one passed through it but when it was studied in detail both provided some problems and some challenges. it did not look like they were responsible. Now astronomers know there is a third galaxy which is 300,000 light-years away. And you remember the interaction also took place about three lac years before so it is assumed that this Galaxy passed through this cartwheel galaxy due to which this ring structure was formed. But is there any evidence of it present or not. Yes, an astronomer got a smoking gun evidence when they saw it in hydrogen gas.

You are seeing an image here from a distance of the cartwheel galaxy and in this, you can see some contours these contours are of hydrogen gas and you see they link the cartwheel galaxy all the way to that third Galaxy. And this hydrogen gas (this type of signature) tell us if there has been an interaction in the past, These galaxies drag this gas and the signatures remain for a long time. These hydrogen bridges or tails are used by astronomers to find out this type of interaction. We now know this third Galaxy was responsible for passing through this cartwheel galaxy. This is an amazing thing. This image by the Hubble telescope is one of my favorite images. This interaction takes several hundred million years to reach this shape but the amazing thing is humans, small creatures on this earth can find out the history of these interactions. We can tell how this ring structure (which took several hundred million years to make) was formed and how did the galaxy passed through it.