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Andromeda: discovery, origin, characteristics, structure

Andromeda is a galaxy made up of a conglomeration of star systems, dust and gas, all subject to the force of gravity. It is 2.5 million light years away from Earth and is the only object visible to the naked eye that does not belong to the Milky Way.

The galaxy’s first record dates back to 961, when the Persian astronomer Al-Sufi described it as a small cloud in the constellation Andromeda. Certainly, other peoples of antiquity managed to recognize it as well.

Later, using the telescope, the astronomers who followed Galileo called it simply “the nebula.” In the mid-19th century, the most powerful telescope was 72 inches in diameter and was built by Irish astronomer William Parsons, who directly observed the curious spiral structure of some nebulae.

It was in 1924 that astronomer Edwin Hubble realized that the Andromeda spiral nebula was not part of the Milky Way. For this, he used the properties of the Cepheids, a class of stars whose brightness varies regularly. 

The size and temperature of the cepheids increase and decrease, relating the luminosity very precisely to the period. In this way, Hubble was able to establish a distance scale to the universe and estimate the distance between Andromeda and the Milky Way. This confirmed that the nebula was, in fact, an independent galaxy and the universe a much bigger place than they imagined.

Andromeda Characteristics

Andromeda is a spiral galaxy whose shape is similar to the Milky Way. It is shaped like a flat disc, with a bulge in the center and several spiral arms. Not all galaxies have this design.

Hubble, who observed hundreds of them, classified them into elliptical (E), lenticular (L), and spiral (S) in his famous tuning fork or sequence diagram that is still used today.

In turn, spiral galaxies are distinguished into two groups, those with a central bar and those without. 

The current consensus is that our Milky Way is a barred Sbb spiral galaxy, although we can’t see it from the outside, but Andromeda is a simple or unbarred Sb spiral galaxy, which we see almost singing from here.

The most significant data from Andromeda are:

-Has a dual core (see Structure section below)

-Its dimensions are comparable to the Milky Way. Andromeda is only slightly larger in size, but the Milky Way is more massive, with more dark matter.

-Andromeda has several satellite galaxies, with which it interacts gravitationally: the dwarf elliptical galaxies: M32 and M110 and the small spiral galaxy M33.

– Its diameter is 220 thousand light years.

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-It is approximately twice as bright as the Milky Way, with 1 billion stars.

– About 3% of the energy emitted by Andromeda is in the infrared region, while in the Milky Way this percentage is 50%. Typically, this value is related to the rate of star formation, so in the Milky Way it is high and in Andromeda it is lower.

How to see Andromeda?

The Messier catalog, a list of 110 astronomical objects dating back to 1774, names the Andromeda galaxy, visible in the constellation of the same name, as object M31. 

In turn, the NGC catalog (New General Catalog of Nebulae and Star Clusters) calls it NGC 224. 

It is convenient to remember these designations for finding the galaxy on celestial maps, as they are used in numerous astronomical applications for computer and telephone.

To visualize Andromeda, it is convenient to first locate the constellation Cassiopea, which has a very characteristic shape in the shape of the letter W or M, depending on how you see it.

Cassiopea is very easy to visualize in the sky and the Andromeda galaxy lies between it and the constellation Andromeda itself, as seen in this diagram:

Keep in mind that to see the galaxy with the naked eye, the sky must be very dark and no artificial lights nearby.

However, it is possible to see the galaxy even from a populated city on a clear night, but always with the help of binoculars at least. In these circumstances, a small whitish oval is distinguished at the indicated location.

With a telescope, many more details of the galaxy are distinguished and two of its small complementary galaxies can also be located.

The most appropriate times of year to view it are:

– Northern Hemisphere : although it is more visible throughout the year, the ideal months are August and September.

– Southern Hemisphere : between October and December.

Finally, it is advisable to do the observation during the new moon so that the sky is very dark, as well as wearing appropriate clothing for the season.

The local group of galaxies

Both the Andromeda Galaxy and the Milky Way itself belong to the Local Group of Galaxies, which comprises a total of 40 galaxies. The Milky Way, Andromeda and the Triangle Galaxy are the largest members of this group.

The rest are dwarf galaxies of the elliptical, spiral or irregular type that include the Magellanic clouds.


Andromeda’s structure is basically the same as all spiral galaxies:

-A core, which houses a supermassive black hole.

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-The bulb, surrounding the core and full of stars advanced in its evolution.

-A disc of interstellar material.

-The halo, a huge diffuse sphere that surrounds the structures already named and which is confused with the halo of the neighboring Milky Way.

Origin and Evolution How did Andromeda originate?

Galaxies have their origin in the protogalaxies or primordial gas clouds that were organized relatively soon after the Big Bang, the great explosion that gave rise to the universe.

During the Big Bang, the lightest elements, hydrogen and helium, were formed. Thus, the first protogalaxies were necessarily composed of these elements.

In the beginning, the subject was distributed evenly, but in some points it accumulated a little more than in others. In places where the density was higher, the force of gravity came in and caused more matter to accumulate. Over time, gravitational contraction gave rise to protogalaxies.

Andromeda may be the result of the fusion of several protogalaxies that occurred about 10 billion years ago.

Taking into account that the universe is estimated to be 13.7 billion years old, Andromeda was formed just after the Big Bang, just like the Milky Way.

During the course of its existence, Andromeda absorbed other protogalaxies and galaxies, which helped to give it its current shape. Likewise, their rate of star formation has varied throughout this time, as during these approaches the rate of star formation increases.

Despite the fact that the universe is known to expand, the Andromeda Galaxy is currently rapidly approaching the Milky Way at a rate of 300 km/s; therefore, a “collision” between the two is expected, or at least an approach in the distant future. so that both are very deformed.

Such events are not uncommon and are not necessarily violent or destructive, given the great distance between the stars. 

If the colliding galaxies are the same size, they will likely lose their shape and an elliptical galaxy or an irregular galaxy will originate. If one is smaller, the larger one will maintain its shape by absorbing it or will suffer a more or less appreciable deformation.

Cepheids and astronomical distances

Edwin Hubble used the Cepheids to determine the distance of Andromeda and demonstrate that it was a separate galaxy from the Milky Way.

Cepheids are extremely bright stars, much brighter than the Sun, so they can be seen even far away. Polaris, the North Star is an example of Cepheid.

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They are characterized by the fact that they undergo periodic expansions and contractions, during which their brightness rises and falls at regular intervals. This is why they are known as pulsating stars .

Astronomer Henrietta Leavitt (1868-1921) found that any cepheid with the same period T has the same intrinsic brightness or magnitude Mv, according to the equation:

M v = -1.43 – 2.81 log T

This is true of any Cepheid, no matter how far away it is. Therefore, when identifying a cepheid in a distant galaxy, the examination of its period will also have its magnitude, since there are previously calibrated magnitude versus period curves .

Now, any light source has intrinsic magnitude and apparent magnitude.

When two equally bright lights are seen at night from a distance, they may both have the same intrinsic brightness, but one of the sources may also be dim and closer and therefore look the same.

The intrinsic magnitude of a star is related to its luminosity: of course, the greater the magnitude, the greater the luminosity. In turn, the difference between the apparent and the intrinsic magnitude is related to the distance from the source.

Relationship between magnitude and distance

Astronomers use the following equation that relates the three variables mentioned; intrinsic magnitude, apparent magnitude and distance:

v – M v  = -5 + 5 log d

Where m v  is the apparent magnitude, M v  is the absolute magnitude and d is the distance at which the light source is (in parsecs *), in this case the star. 

In this way, Hubble found Cepheids in the Andromeda nebula with very small magnitudes, meaning they were very far away.

The distance between us and Andromeda that Hubble determined with this method was 285 kiloparsec, just over 929,000 light years. The currently accepted value is 2.5 million light years, almost double the figure estimated by Hubble.

It turns out that, at the date Hubble made its estimate, there were no two known classes of Cepheids and therefore it underestimated the distance. Despite this, he managed to prove that it was so big that Andromeda was definitely not part of the Milky Way.

 * 1 parsec = 3.26 light years.

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