Cluster refers to a cluster of stars, that is, a relatively dense area of stars, and they originate from the same huge molecular cloud. Cluster can be roughly divided into two types, one is relatively loose open cluster, the other is relatively dense globular cluster, which spread all over the galaxy.
Open clusters are generally composed of up to several thousand stars. Due to the small number and low density of stars, they are only attracted by weak gravity. If there are other gas clouds or disturbances around them, they are easy to collapse.
Therefore, most of the open clusters have a very short life span, generally only a few hundred million years. Coupled with the radiation pressure from the clusters, the surrounding molecular clouds will slowly blow away. Therefore, although it is known as the “cradle of stars”, in fact, the materials that can eventually form stars are very limited.
There are not many open clusters in the Milky Way galaxy. At present, only more than 1000 open clusters have been found, such as the honeycomb cluster, the Pleiades cluster, the Pleiades cluster and so on.
What’s the main use of studying open clusters?
It is one of the key objects to study the evolution of stars. First of all, most stars originate from open clusters; second, because they are in the same “cradle”, the stars here are similar in age and chemical composition.
These stars are as like as two peas born just now, and they are almost identical to each other except quality.
Globular clusters usually have hundreds of thousands or even millions of stars, especially in the central region. The density is very high, and the distance between stars is usually only one light year. In addition to the difference in the number of stars, globular clusters are different from open clusters in many ways.
The shape of globular clusters is very regular. Generally speaking, globular clusters are relatively complete and oblate. However, the shape of open clusters is not very regular. After all, the mutual gravity is weak and easy to deform.
In fact, in the later stage, as the open clusters are constantly scattered, the stars have lost their gravitational ties to each other for a long time. But at this time, they can still keep the same direction of motion. At this time, the group of stars has another name, which is called stellar Association.
In addition to their different shapes, open clusters and globular clusters appear in different locations in the galaxy. Open clusters generally only appear near the galactic plane of the galaxy, while globular clusters not only appear near the disk, but also in all places above and below the disk.
This has an advantage for the observers on the earth, that is, it is not easy to be covered by gas and dust. We can see it even at a long distance, but on the whole, it is not completely evenly distributed. The closer it is to the center, the more it is distributed.
However, compared with open clusters, the number of globular clusters is much less. At present, only more than 100 globular clusters have been found in the Milky way. Even the nearby Andromeda galaxy, there may be only a few hundred globular clusters.
At present, people know little about globular clusters, such as how they form, including whether the stars in them are formed in the same batch, like open clusters, or across several generations.
At present, it is certain that no new active stars have been found in the observed globular clusters, that is to say, most of them are old stars. If the open cluster is the “Kindergarten” of stars, then the globular cluster is the “nursing home” of stars.
Is there life in the cluster?
Are there living planets like Earth in globular clusters? Although there are many stars in globular clusters, at present, it is not suitable for real life formation. In other words, there may not be traditional life in globular clusters.
Because the stars here are too dense, the orbit of the planet is too easy to be disturbed by the surrounding stars, so it is difficult to be stable. There is no stable orbit, let alone hot and cold, before long, the whole planet may directly hit the star. So the life span of planets here is often very short, and there is no time left for life to evolve.
In addition, there is little gas and dust here, because they have been washed away by these dense stars for a long time.
Blue discrete star
In globular clusters, in addition to the erratic orbits of planets, the motion of stars is also unpredictable, because the interaction between stars in globular clusters is a multi-body problem, each star interacts with n-1 stars around, and the overall complexity is increasing.
In a word, these close stars do often collide with each other, but this also makes some very special star types appear in globular clusters, such as “blue discrete star”, which has higher surface temperature than stars with the same brightness in the same group. Although it still belongs to the main sequence star, its position on the Herod diagram has deviated from the main sequence star Order belt, fell to the ground, so it is also called “blue left behind star”.
Because globular clusters exist not only in our galaxy, but also in those extragalactic galaxies. At the same time, the distribution of globular clusters in each galaxy is almost the same. Therefore, based on this, early astronomers accurately described the early galactic model by measuring globular clusters.
You know, in the early days, people always thought that the solar system was near the center of the Milky way. Later, an American astronomer named Shapley wondered why globular clusters were distributed on one side of the sky?
Accordingly, he boldly predicted that the solar system should be at the edge of the galaxy, not the center. Our solar system is not special in the Milky way, on the contrary, it is a very ordinary existence. This result also lays an important foundation for people to understand the Milky way in the future.