Let's take a look at what is the largest spiral galaxy known at present.
Looking up at the sky in the dark night, you can see many stars, planets and even dim clusters and nebulae with the naked eye. But the biggest structure in the night sky is the Milky Way Dark Belt across the dome.
From our point of view, this is the inner plane of the Milky Way. For us, it is very huge, including hundreds of billions of stars with a diameter of 65438+ million light years. But the Milky Way is not the largest spiral galaxy, and Andromeda is not in this galaxy group, but it is the galaxy with the largest angle in our sky.
Andromeda galaxy can become the largest galaxy in our sky because of two factors:
First of all, it is really big! It is about 220,000 light-years in diameter and contains about one trillion stars, which is three to five times the number of stars in the Milky Way and more than twice the diameter. It is the largest galaxy in our galaxy. Secondly, it is very close to us! Only 2.54 million light years away, it is the nearest big galaxy in the universe. The galaxies in this galaxy group can be considered as a collection because they are all bound together by gravity! In a few billion years, Andromeda, the Milky Way, the Triangle galaxy and all other small galaxies in the local galaxy group will be merged to form an independent giant galaxy.
After the merger of galaxies, the final result is not a spiral galaxy, but a huge elliptical galaxy! So why did it become an ellipse instead of a spiral galaxy? Just looking at one or two isolated examples will not solve the problem. If we want to know exactly what happened in the merger of galaxies, we need to observe everything we have seen so far and draw conclusions from it.
So, if we observe the galaxies in the universe and focus on the largest galaxies, what will we find?
There are two obvious galaxies in the picture above, one of which is about the size of the Milky Way. It is the spiral structure on the right side of the image: IC 4970, which is interacting with its huge neighbor under the action of gravity.
The "giant" galaxy in the above picture has two huge, expanding and scattered spiral arms, which actually extend further than what we see in the above picture. NASA's GALEX spacecraft captured the whole picture of this giant galaxy under ultraviolet light, and this galaxy is also the largest spiral galaxy we have found in the universe so far. As shown below:
This is NGC 6872 galaxy with a diameter of about 522,000 light years. It is the largest spiral galaxy in the known universe. Of course, because the universe is so big, there is likely to be a spiral galaxy bigger than this one, but we have only determined this one at present, because it is only more than 200 million light years away from us. The mass of NGC 6872 is obviously greater than that of the Milky Way or Andromeda galaxy, and even greater than the sum of the two galaxies. In addition, its physical size is much larger. As far as we know, even if there are larger spiral galaxies in the universe, they will not be much bigger than NGC 6872.
But there are many galaxies in the universe that are much larger than NGC 6872. They are elliptical galaxies!
Giant in the Universe: Elliptic Galaxies
Even if we only look at the Virgo cluster, it is the largest cluster nearest to us. The largest galaxy is messier 87, with a diameter of about 1 100 million light years, containing trillions of stars, with a total mass of 200 times that of our galaxy!
In other words, in this ordinary galaxy type, an ordinary large elliptical galaxy dwarfs the largest spiral galaxy known. But there are bigger elliptical galaxies in the universe.
The galaxy in the above picture is IC 1 10 1, which is by far the largest single galaxy in the universe. It is/kloc-0.07 billion light years away from us, and contains about/kloc-0.00 trillion stars, which is almost/kloc-0.000 times that of the Milky Way, with a total mass of/kloc-0.0/kloc-0.5 solar masses and an estimated diameter of 5 to 6 million light years.
Compared with the known largest spiral galaxy, the known largest elliptical galaxy includes:
The number of stars is almost 100 times, and the total mass is about several hundred times. In the largest direction, it is about 10 times its physical size. Spiral galaxies are like dwarfs in front of elliptical galaxies!
How are spiral galaxies formed? Why are almost all the largest galaxies in the universe elliptical galaxies?
There are many large galaxies in the universe, usually located in the center of a huge cluster of galaxies. Their mass is usually hundreds to 1000 times that of the Milky Way, but none of them are spiral galaxies! For example, in the above picture, the bright spiral galaxy in the lower right corner of the celestial furnace cluster is not even comparable to the mass of Andromeda, and the huge elliptical galaxy is many times the total mass of Andromeda. So, why are spirals dwarfs? Let's see how spiral galaxies are formed!
In the early days of the universe, shortly after the Big Bang, tiny areas with a little more matter and energy than other areas began to attract matter by gravity first. Eventually, enough matter is attracted to the air mass, which leads to the formation of stars and clusters. The first cluster formed is a part of a larger structure (asymmetric gas cloud), which collapses first in the shortest direction and forms a disk first. As time goes by, the disk absorbs more and more substances, gathers all the nearby clusters, and becomes a spiral structure due to the density waveform. Small and medium-sized clusters merged with galaxies enter the core of galaxies and keep a spiral structure. This process takes place in the Milky Way, Andromeda, and almost all spiral galaxies we see. A huge gas collapsed, "squashed" and began to rotate because of gravity. Because of the instability of the disk, density waves appear spiral structure, and spiral galaxies have always been spiral structure, just because there is no catastrophic expansion to destroy this structure.
However, the process leading to the formation of larger galaxies in the universe (merging with one or more other spiral galaxies) is precisely destroying the spiral structure in the universe!
This process is very common in the universe, and those isolated galaxies that have never experienced merger are relatively undisturbed spiral galaxies. When the two galaxies merge gravitationally, the spiral arms of the stars will be greatly disturbed after the gravity takes effect, and they will go through a starburst stage, that is, star formation is extremely active. Once the starburst phase is over, the galaxy will enter the final elliptical state.
If you want two large galaxies to merge and keep the spiral structure, you need a very good coincidence. Statistically, this is unlikely to happen. Moreover, the more times of merging, especially the more times of merging of large galaxies, the easier it is to destroy the spiral structure and finally get an elliptical structure.
But some galaxies seem to be a mixture of spiral galaxies and elliptical galaxies, which we call a "semi-merged" state, that is, a part of an elliptical structure formed by the merger of a huge spiral galaxy and a small galaxy. Primitive spiral galaxies remain intact to some extent.
This is why the largest spiral galaxy in the universe is not so big. By merging larger galaxies, they are more likely to become giant elliptical galaxies than spiral galaxies!