ANDROMEDA: DISCOVERY, ORIGIN, CHARACTERISTICS, STRUCTURE - ARTE - 2025

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

The first record of the galaxy dates from 961, when the Persian astronomer Al-Sufi described it as a small cloud in the constellation Andromeda. Most likely, other ancient peoples managed to recognize it as well.

Figure 1. The Andromeda galaxy, similar to the Milky Way, seen in ultraviolet light. Source: Wikimedia Commons.

Later, with the telescope, the astronomers who followed Galileo called it simply "nebula." In the middle of the 19th century, the most powerful telescope was 72 inches in diameter and was built by the 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 periodically on a regular basis.

The size and temperature of the Cepheids increases and decreases, relating in a very precise way the luminosity with their period. In this way, Hubble was able to establish a distance scale for 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 larger place than they had imagined.

Andromeda Features

Andromeda is a spiral galaxy whose shape is similar to that of our 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 had observed hundreds of them, classified them into elliptical (E), lenticular (L) and spiral (S), in his famous tuning fork diagram or Hubble sequence that is still used today.

Figure 2. The Hubble tuning fork. Source: Wikimedia Commons.

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 spiral galaxy Sb, although we cannot see it from the outside, but Andromeda is a simple or unbarred spiral galaxy Sb, which we see almost edge-on from here.

The most significant data of Andromeda are:

-It has a double core (see the 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 elliptical dwarf galaxies: M32 and M110 and the small spiral galaxy M33.

-Its diameter is 220 thousand light years.

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

-Close to 3% of the energy emitted by Andromeda is in the infrared region, whereas for the Milky Way this percentage is 50%. Usually this value is related to the star formation rate, therefore 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.

For its part, the NGC catalog (New General Catalog of Nebulae and Clusters of Stars) calls it NGC 224.

These designations are a good idea to remember when finding the galaxy on sky maps, as they are used in many astronomical applications for computers and phones.

To visualize Andromeda, it is convenient to first locate the constellation Cassiopea, which has a very characteristic shape in the form 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 is between it and the constellation Andromeda proper, as seen in this diagram:

Figure 3. Detail of the celestial map to locate the Andromeda galaxy. Source: F. Zapata.

Keep in mind that to see the galaxy with the naked eye, the sky must be very dark and without artificial lights in the vicinity.

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. Under these circumstances a small whitish oval is distinguished in the indicated place.

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

The most appropriate times of the year to view it are:

- Northern Hemisphere: although it is visible more less throughout the year, the optimal months are August and September.

- Southern Hemisphere: between October and December.

Finally, it is advisable to observe during the new moon, so that the sky is very dark, as well as to wear 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 groups a total of 40 galaxies. The Milky Way, Andromeda, and the Triangle galaxy are the largest members of this group.

The rest is made up of dwarf galaxies of the elliptical, spiral or irregular type that include the Magellanic Clouds.

Structure

The structure of Andromeda is basically the same as that of all spiral galaxies:

Figure 4. Structure of a typical spiral galaxy. Source: University of Manitoba.

-A nucleus, which houses a supermassive black hole.

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

-A disk of interstellar material.

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

Origin and evolution How did Andromeda originate?

The galaxies have their origin in the protogalaxies or primordial gas clouds that were organizing relatively shortly after the Big Bang, the great explosion that gave rise to the universe.

During the Big Bang the lighter elements, hydrogen and helium, were formed. In this way, the first protogalaxies were necessarily composed of these elements.

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

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

Taking into account that the estimated age of the universe is 13.7 billion years, Andromeda formed shortly after the Big Bang, just like the Milky Way.

During the course of its existence, Andromeda has absorbed other protogalaxies and galaxies, which helped give it its current shape. Also its star formation rate has varied throughout that time, since during these approaches the star formation rate increases.

Although the universe is known to expand, the Andromeda galaxy is currently rapidly approaching the Milky Way at a rate of 300 km / s, so in the distant future a "collision" between the two or at least one approach is expected. such that both are greatly 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 of equal size, they will probably lose their shape and form an elliptical galaxy or an irregular galaxy. If one is smaller, the larger one will retain its shape by absorbing it, or will experience a more or less appreciable deformation.

Cepheids and astronomical distances

Edwin Hubble used the Cepheids to determine the distance to Andromeda and to show that it was a galaxy apart from the Milky Way.

Cepheids are extremely bright stars, much brighter than the Sun, so they can be seen even from very far away. Polaris, the pole star is an example of a Cepheid.

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

The astronomer Henrietta Leavitt (1868–1921), found that any Cepheid with the same period T, has the same brightness or intrinsic magnitude Mv, according to the equation:

Mv = -1.43 - 2.81 log T

This is true for any Cepheid no matter how far away it is. Therefore, when identifying a Cepheid in a distant galaxy, examining its period will also show 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 at a distance, they may both have the same intrinsic brightness, but one of the sources may also be less bright and closer, and therefore look the same.

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

Relationship between magnitude and distance

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

m _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 from the light source (in parsecs *), in this case the star.

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

The distance between us and Andromeda that Hubble determined with this method was 285 kiloparsec, just over 929 thousand light years. The currently accepted value is 2.5 million light years, a little more than double that estimated by Hubble.

It turns out that at the time Hubble made his estimate, it was unknown that there were two classes of Cepheids and so he underestimated the distance. Despite this, he managed to prove that it was so large that Andromeda was definitely not part of the Milky Way.

* 1 parsec = 3.26 light years.

References

1. Taylor, N. The Andromeda Galaxy (M31): Location, Characteristics & Images. Recovered from: space.com.
2. Manitoba University. Research Project 1: Spiral Galaxies. Recovered from: physics.umanitoba.ca.
3. Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millenium. Third Edition. Thomson-Brooks / Cole.
4. Seeds, M. 2011. Foundations of Astronomy. Seventh Edition. Cengage Learning.
5. Wikipedia. Andromeda Galaxy. Recovered from: es.wikipedia.org.