What are galaxies, nebulae, star clusters and constellations?

Galaxy: In the vast universe and ocean, there are various "islands" dotted with countless stars and various celestial bodies, which are called galaxies in astronomy. The earth where we live is in a huge galaxy-the Milky Way. In the universe outside the Milky Way, there are hundreds of millions of space giant islands similar to the Milky Way, collectively known as extragalactic galaxies.

Cluster: Among the numerous stars in the Milky Way, there are many stars gathered together, not only in the form of single star, but also in the form of double stars or multiple stars. A cluster with more than 10 stars is called a cluster. What binds these stars together is gravity. There are thousands of members in the cluster. They can be divided into open clusters and globular clusters. There are clusters everywhere in the Milky Way, but the types of clusters are different in different places.

Nebula: Nebula is a cloud-like celestial body composed of gas and dust in interstellar space. The density of matter in the nebula is very low. If measured by the standards on earth, some places are almost vacuum. However, the size of the nebula is very large, often reaching tens of light years in Fiona Fang. Therefore, nebulae are generally much heavier than the sun. Nebulae have various shapes. Some nebulae are irregular in shape, diffuse and have no clear boundaries, which are called diffuse nebulae; Some nebulae are called planetary nebulae because they are like a disk, emitting faint light, much like a big planet.

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