GALAXY: CHARACTERISTICS, COMPONENTS, TYPES, FORMATION - ARTE - 2025

A galaxy is a conglomeration of astronomical objects and matter, such as gas and dust clouds, billions of stars, nebulae, planets, asteroids, comets, black holes, and even a lot of dark matter, all structured thanks to the force of gravity.

Our solar system is part of a large spiral galaxy called the Milky Way. This name derived from Greek can be translated as "milk path", due to its similarity to a dimly lit band that crosses the celestial sphere.

Figure 1. The beautiful lenticular galaxy known as the Sombrero Galaxy M104 in the constellation Virgo, 29.35 million light-years away, seen with the Hubble telescope. Source: Wikimedia Commons.

On clear summer nights it can be observed very well between the constellations of Scorpio and Sagittarius, since the nucleus is in that direction and where the density of stars is much higher.

History of the discovery of galaxies

The great Greek thinker and mathematician Democritus of Abdera (460-370 BC) was the first to suggest - in his day there were no telescopes - that the Milky Way was actually made up of thousands of stars so far apart that one could not be distinguished. other.

It took a while before Galileo (1564-1642) agreed with him, when when pointing his telescope he found that there were more stars in the sky than he could count.

Galileo Galilei - Source: Domenico Tintoretto

It was the German philosopher Immanuel Kant (1724-1804) who speculated that the Milky Way was made up of as many thousands of solar systems and that the whole had an elliptical shape and rotated rhythmically around a center.

Furthermore, he also suggested that other sets of stars and planets existed such as the Milky Way and called them island universes. These island universes would be visible from Earth as tiny, faint patches of light.

20 years later, in 1774, the Messier catalog appeared, a compilation of 103 deep space objects visible to date and made by the French astronomer Charles Messier (1730-1817).

Among these were some candidates for island universes, which were known simply as nebulae. The M31 nebula was one of them, being known today as the neighboring galaxy of Andromeda.

William Herschel (1738-1822) would expand the list of deep space objects to 2,500 and first described the shape of the Milky Way. However, scientists had not yet realized that certain nebulae like M31 were themselves huge conglomerates of stars similar to the Milky Way.

A telescope with sufficient resolution was needed, and it could be purchased in 1904 when the huge telescope at the Mount Wilson Observatory in California was built with a 100-inch diameter mirror. It was not until then that the size of the universe became apparent, because the already immense Milky Way is just one galaxy, among innumerable conglomerates of them.

In 1924, Edwin Hubble (1889-1953) managed to measure the distance to one of these spiral nebulae, observing the Cepheid-like stars in the object M31, the most remarkable spiral-shaped nebula called Andromeda.

Cepheids are stars that periodically change their brightness and this is proportional to the period. The brighter ones have longer periods.

By then, Harold Shapley (1885-1972) had estimated the size of the Milky Way, but it was so large that he was convinced that the Andromeda Nebula was in the interior of the Milky Way.

However, Hubble determined that the distance to the Andromeda Cepheids was much greater than the size of the Milky Way and therefore could not be found within it. Andromeda, like the Milky Way, was a galaxy in its own right, although for a long time it remained called an "extragalactic nebula."

General characteristics

Galaxies have shape and, as we will see later, they can be classified according to this criterion. They also contain mass and they are not static entities at all, since they have movement.

There are giant and very bright galaxies, such as the Milky Way and Andromeda, and also galaxies called "dwarfs", up to a thousand times less bright. To become familiar with the sizes, it is useful to know some units of measurement used in astronomy. First we have the light-year.

The light-year is a unit of distance equal to the distance that light travels in one year. Being that the speed of light is 300,000 km / s, multiplying by the number of seconds in 365 days, the result is approximately 9 and a half billion kilometers.

For comparison purposes, the distance from the Sun to the Earth is 8.5 light-minutes, about 150 million kilometers, which is roughly equivalent to one AU or astronomical unit, useful in measurements within the Solar System. The next closest star to the Sun is Proxima Centauri at 4.2 light-years.

The AU gives rise to another widely used unit: the parsec or parallax of an arc second. That a point is at the distance of a parsec, means that its parallax is equal to 1 arc second between the Earth and the Sun. The following figure clarifies it:

Figure 2. Scheme to define the parsec. Source: Wikimedia Commons. Kes47 (?).

Size, movement and chemical composition

The sizes of galaxies are extremely varied, from so small that they barely have a thousand stars, to the giant elliptical galaxies that we will talk about in detail later.

Thus, we have our Milky Way about 100,000 light-years in diameter, being a large galaxy, but not the largest. NGC 6872 is 520,000 light-years across, about 5 times the diameter of the Milky Way, and is the largest spiral galaxy known to date.

The galaxies are not static. Generally speaking, stars and clouds of gas and dust have rotational motions around the center, but not all parts of a galaxy rotate with equal speed. The stars in the center rotate faster than the outer ones, in what is called differential rotation.

Regarding chemical composition, the most common elements in the universe are hydrogen and helium. Inside the stars, like a nuclear fusion reactor, the heaviest elements that we know are formed through the periodic table.

The color and luminosity of galaxies change over time. Younger galaxies are bluer and brighter than older ones.

Ellipse-shaped galaxies tend toward red, with numerous older stars, while irregular ones are the bluest. In spiral-shaped galaxies, blue is concentrated towards the center and red towards the outskirts.

Components of galaxies

When observing a galaxy, structures such as the following can be identified, which are present in the Milky Way, which has been taken as a model because it is the best studied:

Disco and halo

The two basic structures of our galaxy are the disk and the halo. The disk is in the middle plane defined by the galaxy and contains a large amount of interstellar gas that gives rise to new stars. It also contains old stars and open clusters - a poorly structured grouping of stars.

It should be noted that not all galaxies have the same star formation rate. Elliptical galaxies are believed to have a much lower rate, unlike spirals.

The Sun is located in the galactic disk of the Milky Way, on the plane of symmetry and like all the stars in the disk, it orbits the galaxy following a path approximately circular and perpendicular to the galactic axis of rotation. It takes about 250 million years to complete one orbit.

The halo covers the galaxy with a less dense spheroidal volume, since it is a region with much less dust and gas. It contains globular clusters, stars grouped by the action of gravity and much older than the disk, individual stars and also the so-called dark matter.

Dark matter is a type of matter whose nature is unknown. It owes its name to the fact that it does not emit electromagnetic radiation and its existence has been proposed to explain the fact that stars outside move faster than expected.

The speed at which a star moves with respect to the center of the galaxy depends on how the matter is distributed, since it is the gravitational attraction due to it that a star remains in orbit. Faster speed means there is more matter that cannot be seen: dark matter.

The bulb, the galactic nucleus and the bar

Apart from the disk and the halo, in the galaxy there is the bulge, the central bulge or galactic nucleus, where there is a higher density of stars, being therefore very luminous.

Its shape is approximately spherical -although that of the Milky Way is more like a peanut- and in its center is the nucleus, made up of a black hole, a fact that seems to be common in many galaxies, especially in the spiral ones.

Objects that are in the vicinity of the nucleus rotate, as we have said, much faster than those that are further away. There the speed is proportional to the distance to the center.

Some spiral galaxies like ours have a bar, a structure that runs through the center and from which spiral arms emerge. There are more barred than unbarred spiral galaxies.

The rods are believed to allow the transport of matter from the ends to the bulb, thickening it by promoting the formation of stars in the nucleus.

Figure 3. Components of the Milky Way. The Sun is in one of the arms and has a rotational movement around the center of the galaxy, as well as a vertical movement. Source: Wikimedia Commons.

Types of galaxies

The first thing that is appreciated when observing galaxies through the telescope is their shape. The large Andromeda galaxy, for example, is spirally shaped, while its companion NGC 147 is elliptical.

The galaxy classification system is based on the shape they have and the most used today is the Hubble tuning fork or sequence, created around 1926 by Edwin Hubble, and later modified by himself and other astronomers, as new information appeared.

Hubble designed the scheme in the belief that it represented a kind of galaxy evolution, but today it is known that this is not the case. Letters are used in the sequence to designate galaxies: E for elliptical galaxies, S for spiral galaxies, and Irr for irregularly shaped ones.

Figure 4. The Hubble tuning fork. Source: Wikimedia Commons.

Elliptical galaxies

To the left, on the neck of the tuning fork, are the elliptical galaxies represented by the letter E. The stars that make them up are distributed in a more or less uniform way.

The number that accompanies the letter indicates how elliptical the galaxy is -elipticity-, starting with E0, which is the most spherical, to E7, which is the flattest. No galaxies with ellipticity greater than 7 have been observed. Denoting this parameter as є:

Є = 1 - (β / ɑ)

With α and β as the apparent major and minor semi-axes respectively of the ellipse. However, this information is relative, because we only have the view from Earth. For example, it is not possible to know if a galaxy shown on edge is elliptical, lenticular, or spiral.

Giant elliptical galaxies are among the largest objects in the universe. They are the easiest to observe, although much smaller versions, called dwarf elliptical galaxies, are much more abundant.

Figure 5. Elliptical galaxy NGC 1316, in the constellation Fornax, merging with another smaller galaxy. Source: Image credit: NASA / JPL-Caltech / CTIO.

Lenticular and spiral galaxies

Lenticular galaxies are disc-shaped, without spiral arms, but they can be barred. Their nomenclature is S0 or SB0 and they are right at the fork of the figure. Depending on the amount of dust (high absorption zones) on your disc, they are subdivided into S01, SB01 through S03 and SB03.

The S galaxies are the spiral galaxies proper, while the SB are the barred spiral galaxies, since the spirals appear to project from a bar through the central bulge. The vast majority of galaxies have this shape.

Both classes of galaxies are distinguished in turn by the degree of ease of the spiral arms and is marked with lower case. These are determined by comparing the size of the largest bulge in with the length of the disc: L bulge / L disc.

Figure 6. The beautiful spiral galaxy of Andromeda in the constellation of Cassiopea. Source: Wikimedia Commons Image from NASA).

For example, if this quotient is ≈ 0.3, the galaxies are denoted as Sa if it is simple spiral, or SBa if it is barred. In these, the spirals appear to be tighter and the concentration of stars in the arms is more tenuous.

As the sequence continues to the right, the spirals appear looser. The bulge / disk ratio for these galaxies is: L bulge / L disk ≈ 0.05.

If a galaxy has intermediate characteristics, up to two lower case letters can be added. For example the Milky Way is classified by some as SBbc.

Irregular galaxies

These are galaxies whose shape does not match any of the patterns described above.

Hubble himself divided them into two groups: Irr I and Irr II, where the former are only slightly more organized than the latter, because they have something reminiscent of the shape of the spiral arms.

Irr II galaxies are, we could say, amorphous and with no recognizable internal structure. Both Irr I and Irr II are typically smaller than elliptical galaxies or majestic spiral galaxies. Some authors prefer to refer to them as dwarf galaxies. Among the best known irregular galaxies are the neighboring Magellanic Clouds, classified as Irr I.

Figure 7. Irregular galaxy NGC 5408, discovered in the constellation Centaurus by John Herschel in 1834. At first it was believed to be a planetary nebula. Source: Wikimedia Commons.

After the publication of the Hubble sequence, the French astronomer Gerard de Vaucouleurs (1918-1995) suggested removing the Irr I and Irr II nomenclature and calling the Irr I, which have some spiral arms, as Sd - SBd galaxies, Sm - SBm or Im (The "m" is for Magellanic galaxy).

Finally, the galaxies whose shape is truly irregular and without a trace of spirals, are simply called Go. With this, the modern classification has remained like this:

How do galaxies form?

Galaxy formation is the subject of active discussion today. Cosmologists believe that the early universe was quite dark, filled with clouds of gas and dark matter. This is due to the theory that the first stars formed within a few hundred million years after the Big Bang.

Once the stellar production mechanism is put in place, it turns out to have ups and downs in the rate. And since stars are what make up galaxies, there are different mechanisms that lead to the formation of galaxies.

Gravitational attraction is the primeval force that sets in motion the formation of cosmic objects. A small accumulation of matter at some point attracts more matter and it begins to accumulate.

The Milky Way is believed to have started this way: small accumulations of matter that eventually gave rise to the globular clusters of the halo, among which are the oldest stars in the galaxy.

The rotation is inherent in the accumulation of mass that followed this initial period of star formation. And with the rotation the angular momentum is created, whose conservation produced the collapse of the spherical mass transforming it into a flat disk.

Galaxies can increase in size by merging with other smaller galaxies. This is believed to be the case today with the Milky Way and its smaller neighbors, the Magellanic clouds.

Another merger expected in the very distant future is the collision with Andromeda which, unlike most galaxies, is closing in on us. Andromeda is currently 2.2 million light-years away.

How many galaxies are there in the universe?

Although most of space is empty, there are millions of galaxies, perhaps 100 trillion of them, by some estimates. Others estimate 2 trillion galaxies. Most of the universe remains unexplored and there is no exact answer to this question.

In just 12 days, the Hubble Space Telescope found 10,000 galaxies of the most varied forms. The actual total of galaxies in the universe is unknown. When observing with a telescope it is necessary to emphasize that you are going further not only in distance, but also in time.

The sunlight that we see has taken 8.5 minutes to reach us. The view of Andromeda that we observe with binoculars is that of 2.2 million years ago. That is why what we see from Earth is in the range of the observable universe. For now there is no way to see what lies beyond.

One way to estimate how many galaxies there are in the observable universe is by taking extremely deep field shots from Hubble or XDF, which represent a small area of ​​the celestial sphere.

In one such shot, 5500 galaxies were found 13.2 billion light-years away. By multiplying this value by the amount of XDF for the entire celestial sphere, they estimated the 100,000 million galaxies mentioned.

Everything indicates that in earlier times there were more galaxies than there are now, but smaller, blue and more irregular in shape than the elegant spiral galaxies we see today.

Examples of galaxies

Despite their immense size, galaxies are not solitary, but rather are grouped into hierarchical structures.

The Milky Way belongs to the so-called Local Group, in which all the members - about 54 - are at a distance no greater than 1 Mega-parsec. Then the density of galaxies decreases until another cluster similar to the Local Group appears.

Among the immense variety of galaxies found, it is worth highlighting some surprising examples for their particularities:

Giant elliptical galaxies

The largest galaxies found so far are at the center of galaxy clusters. They are huge elliptical galaxies whose gravity pulls other galaxies, engulfing them. In these galaxies the rate of star formation is very low, so to keep growing they trap others.

Active galaxies

Active galaxies, unlike the more normal and quiet ones like the Milky Way, emit frequencies of very high energy, much higher than those emitted by the nuclei of stars, common in any galaxy.

These high-energy frequencies whose power is equivalent to billions of suns come out of the nucleus of objects such as quasars, discovered in 1963. Surprisingly, a quasar, one of the brightest objects in the universe, is capable of maintaining this rate for millions of years.

The Seyfert galaxies are another example of active galaxies. So far several hundred of them have been discovered. Its core emits highly ionized radiation, variable in time.

Figure 8. The Seyfert M 106 galaxy. Source: Wikimedia Commons. X-ray: NASA / CXC / Univ. of Maryland / AS Wilson et al.; Optical: Pal.Obs. DSS; IR: NASA / JPL-Caltech; VLA: NRAO / AUI / NSF

It is believed that near the center, a huge amount of gaseous material rushes towards the central black hole. The loss of mass releases radiant energy in the X-ray spectrum.

Radio galaxies are elliptical galaxies that emit large amounts of radio frequencies, ten thousand times more than ordinary galaxies. In these galaxies there are sources - radio lobes - linked by filaments of matter to the galactic nucleus, which emit electrons in the presence of an intense magnetic field.

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