What is a galaxy?
Galaxies have been observed since time immemorial. A person with acute vision can discern light spots in the night sky, similar to drops of milk. In the 10th century, the Persian astronomer Abd al-Raman al-Sufi mentioned two similar spots in his Book of Fixed Stars, now known as the Large Magellanic Cloud and the Galaxy M31, also known as Andromeda. With the advent of telescopes, astronomers have observed more and more such objects, called nebulae. If the English astronomer Edmund Halley in 1716 listed only six nebulae, then the catalog published in 1784 by the French Navy astronomer Charles Messier already contained 110 - and among them four dozen real galaxies (including M31). In 1802, William Herschel published a list of 2, 500 nebulae, and his son John in 1864 published a catalog containing more than 5, 000 nebulae.
The nature of these objects eluded understanding for a long time. In the middle of the XVIII century, some penetrating minds saw in them stellar systems similar to the Milky Way, but telescopes at that time did not provide an opportunity to test this hypothesis. A century later, the belief that every nebula is a gas cloud, illuminated from the inside by a young star, triumphed. Later, astronomers were convinced that some nebulae, including Andromeda, contain many stars, but for a long time it was not clear whether they are located in our Galaxy or beyond. It was only in 1923-1924 that Edwin Hubble determined that the distance from Earth to Andromeda is at least three times the diameter of the Milky Way (actually about 20 times) and that M33, another nebula from the Messier catalog, is no less than the distance. These results laid the foundation for a new scientific discipline - galactic astronomy.
Dwarfs and giants
The universe is filled with galaxies of different sizes and different masses. Their number is known very approximately. Seven years ago, the Hubble orbital telescope discovered about 10, 000 galaxies in three and a half months, scanning a portion of the sky in the southern constellation of the Furnace, a hundred times smaller than the area of the lunar disk. If we assume that the galaxies are distributed over the celestial sphere with the same density, it turns out that there are 200 billion of them in the observed space. However, this estimate is greatly underestimated, because the telescope could not notice a great many very dim galaxies.