The famous Andromeda Galaxy, known as M31 or NGC 224, is a giant spiral galaxy, the closest to us and visible to the naked eye, even though it is 2.5 million light years from Earth. It is part of the "Local Group of Galaxies", which contains two large spiral galaxies: Andromeda and the Milky Way.
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The rest of the galaxies of the local group, about 30, are smaller; many of them are satellites of one of the largest.
Andromeda is the most studied galaxy, because it is possible to observe in it, from the outside, all the features of a galaxy: spiral structure, globular and open clusters, interstellar material, planetary nebulae, supernova remnants and the galactic nucleus.
Source Picture: Pixabay
These features are also found in the Milky Way, but it is not possible to observe them with such precision because we are immersed in it, and because most of our galaxy is hidden by interstellar dust.
It has been observed that Andromeda and the Milky Way are rapidly approaching each other at a speed of about 500,000 km/hour. This means that there would be a collision between the two in about 3,000 million years. If such a collision were to occur, it should not be understood as the collision between two solid bodies, but rather that galaxies would cross each other. As a result, part of the material of both would disperse and the rest would form a giant elliptical galaxy.
Andromeda has a calculated mass of approximately one and a half times the mass of the Milky Way, and is more than twice as bright as the Milky Way. It is visible to the naked eye and was observed by the Persian astronomer Abd-al-Rahman Al-Sufi who described and drew it in 964 A.D. in his "Book of Fixed Stars". It is known that it also appeared on a Dutch star map in the year 1500.
For a long time it was believed that the "Great Andromeda Nebula" was much closer. William Herschel believed, mistakenly of course, that its distance did not greatly exceed the distance to Sirius (17,000 light-years); he saw it as the nearest "island universe," similar to our Milky Way.
In 1917, Heber Curtis discovered twelve novae stars in Andromeda. Noting that these novas were 10 magnitudes weaker than the novas recorded in the Milky Way, he assumed that Andromeda was 500,000 light years away and that it and other similar objects, known at the time as spiral nebulae, were not nebulae but independent galaxies.
In 1920 Curtis and Harlow Shapley starred in a heated debate. Shapley argued that Andromeda was actually a nearby nebula.
However, in 1925 Edwin Hubble found cepheid stars in photographs of Andromeda. This finding made it clear that such objects are actually galaxies more or less like ours, only at great distances, so that the Andromeda Nebula (a name still found in ancient texts) became definitely known as the Andromeda Galaxy. In 1929, Hubble published his famous study of the Andromeda Nebula as an extragalactic star system, i.e. it is outside our galaxy.
In 1943 Walter Baade was the first to discern stars within the central region of the Andromeda galaxy, and he also demonstrated that there were two types of cepheids, which meant doubling their distance to a value already very close to that currently accepted.
Recent observations from the Spitzer space telescope revealed that Andromeda contains one billion stars, exceeding by ten times the number of stars in our galaxy, the Milky Way.
The Hubble Space Telescope has revealed that the Andromeda M 31 galaxy has a double core. This suggests that either it actually has two bright nuclei, probably because it has "eaten" a smaller galaxy that was once introduced to its center, or that parts of its only nucleus are obscured by dark material.
The Andromeda Galaxy can be seen with the naked eye in a dark sky from places far away from sources of light pollution.
A night similar to the one seen in the figure on the right, but a little later and when the moon is gone. As dark as possible.
To the naked eye it seems quite small, as the eye can only pick up the central part which is bright enough. But the full angular diameter of the galaxy is actually seven times that of the full moon as seen from the earth.
If you have binoculars or a telescope with a few magnifications, you can see not only its central region but also the rest of the galaxy.
With a small telescope and in good dark conditions, it is even possible to distinguish its two closest satellite galaxies (M32 and M110).
Source Picture: Pixabay