What is the difference between a nebula and a galaxy?

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Another answer to a question from a subscriber. I must say that among people far from astronomy there is sometimes a misconception that a galaxy or a nebula is one and the same.

The roots of this opinion grow from the fact that one of the galaxies closest to us is called the Andromeda Nebula, which is why people think that galaxy and nebula are interchangeable concepts.

This is not true. The fact that the Andromeda galaxy is also called the Andromeda Nebula has a historical basis. Since Andromeda is the only large galaxy visible in the sky with the naked eye, it was the first to be discovered. When the first astronomers explored it, it was first classified as a nebula. Firstly, because the concept of «galaxy» then simply did not exist, and secondly, because in telescopes of the 19th century Andromeda differed little from other nebulae.

Later it turned out that Andromeda is not a nebula, but a galaxy similar to ours and one of many, but the historically established phrase «The Andromeda Nebula» has not gone out of use. In addition, the popular novel by Ivan Efremov also contributed to the popularization of this name.

The nebula is a huge cloud of ionized gas and dust. There are three types of nebula.

First, the nebula may represent

Secondly, there are so-called

In addition, nebulae can also be observed in regions where stars do not die, but are born. Stars form from clouds of interstellar gas and dust that collapse under their own gravity.

One example of such a nebula is

The galaxy is a set (billions and trillions) of gravitationally connected stars revolving around a common center of mass. The galaxy can contain many nebulae — both the remnants of exploded stars and regions of star formation.

The most famous galaxy is without a doubt our own, called the Milky Way.

Many different types of galaxies are distinguished by the way they are organized. The most common are spiral galaxies, and ours belongs to this type. In addition, there are annular, elliptical, etc.