Andromeda is a galaxy that is part of the Local Group of Galaxies.  The cover image is a photograph of the Andromeda Galaxy. Credit: NASA

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.

The rest of the galaxies of the local group, about 30, are smaller; many of them are satellites of one of the largest.

Photograph of the Andromeda Galaxy. Credit: NASA.

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.

These features are also found in the Milky Way, but they cannot be observed with such precision because we are immersed in it, and because most of our galaxy is hidden by interstellar dust.

Milky Way
Pictoric image of how the Milky Way would look from outside. Crédito: NASA

Andromeda characteristics

Andromeda has a calculated mass about 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 AD  in his “Book of the Fixed Stars“.

It is known that it also appeared on a Dutch star chart from the year 1500.

For a long time it was believed that the “Great Andromeda Nebula” was much closer.

William Herschel believed, wrongly of course, that its distance did not greatly exceed the distance to Sirius (17,000 light years).

He saw Andromeda as the closest “island-universe”, similar to our Milky Way.

Andromeda’s possible collision with the Milky Way

Andromeda and the Milky Way have been observed to rapidly approach each other with speeds of about 500,000 km / hour.

This means that there would be a collision between the two in about 3 billion years.

If it occurs, such a collision should not be understood as taking place between two solid bodies, but rather that the galaxies would pass through each other.

As a result, some of the material from both would disperse and the rest would form a giant elliptical galaxy. Click and Watch an interesting video about it.

Astronomers who observed the Andromeda galaxy

In 1864, William Huggins, the pioneer of spectroscopy, noted the difference between gaseous nebulae, with their line spectrum, and the “star-containing nebulae” that we now know to be galaxies.

He observed that Andromeda had continuous spectral lines, similar to those of stars.

In 1887, Isaac Roberts obtained the first photographs of the Andromeda Nebula, showing for the first time the basic features of its spiral structure.

Isaac Roberts ​ fue un acaudalado industrial de Gales, Reino Unido, más conocido por su actividad como astrónomo aficionado y pionero en el campo de la astrofotografía.

Isaac Roberts
Isaac Robert . Credit: Wikipedia

In 1912, Velton Melvin Slipher american astronomer of the Lowell Observatory, measured the radial velocity of the Andromeda Galaxy.

He found that Andromeda had the highest velocity ever recorded, about 300 km per second.

Heber Curtis discovered twelve nova stars in Andromeda in 1917.

When verifying that these novae were 10 magnitudes weaker than the novae recorded in the Milky Way, he assumed that Andromeda was 500,000 light years away and that both it and other similar objects, known at that time as spiral nebulae, were not nebulae but independent galaxies .

In 1920 Curtis and Harlow Shapley had 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, only at great distances.

In 1929, Hubble published his famous study of the Andromeda Nebula as an extragalactic star system, that is, it is located outside our galaxy.

So that the Andromeda Nebula became definitively known as the Galaxy of Andromeda.

In 1943, Walter Baade was the first to discern stars within the central region of the Andromeda galaxy.

He also showed that there were two types of Cepheids, which meant doubling their distance to a value already very close to that currently accepted.

Recent observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion stars, ten times more than the number of stars in our Milky Way galaxy.

Discovery of the Hubble Space Telescope

The Hubble Space Telescope has revealed that the Andromeda galaxy M 31 has a double core.

This suggests that it either actually has two bright nuclei, probably because of having “eaten” a smaller galaxy that once drove to its center, or that parts of its single nucleus are obscured by dark material.

The Andromeda Galaxy can be seen with the naked eye in a dark sky from locations far from sources of light pollution.

At first glance it seems quite small, since the eye can only capture the central part that is bright enough.

But the full angular diameter of the galaxy is actually seven times that of the full moon seen from Earth.

If, being in the indicated conditions, you have binoculars or a low magnification telescope, you can see not only its central region but 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).

moon at night
A dark night of the crescent moon  Credit: web “”