To explain what galaxies are , it's important to understand that they are collections of stars. They also contain gas clouds, planets, cosmic dust, dark matter, and energy. These elements are held together by gravity, forming a more or less defined structure. The word "galaxy" comes from Greek, as the Greeks attributed the birth of galaxies to drops of milk spilled in the universe.
The Greeks believed that galaxies originated from their goddess, Hera. They thought that as she nursed her son Hercules , drops of milk spilled into the universe. This, they believed, was the moment when galaxies were born. Today, the number of stars that make up a galaxy is countless. However, these are the stars that comprise a galaxy.
On the other hand, there are also substructures such as nebulae, star clusters, and multiple star systems. The historical classification of galaxies was based on their apparent shape, also called visual morphology. One common shape is the elliptical galaxy, which has the luminous profile of an ellipse, hence its name. Spiral galaxies, on the other hand, have a circular shape but with a structure of curved arms enveloped in dust. If you would like more detailed information on other characteristics of galaxies, I recommend visiting this link about galaxy characteristics.
There was also another way to classify them: unusual galaxies . They are also called irregular galaxies. This classification of galaxies is the result of disturbances caused by the gravitational pull of neighboring galaxies. Therefore, interactions between neighboring galaxies can lead to galaxy mergers, potentially inducing intense star formation.
Galaxy Studies
The end result of interactions between neighboring galaxies is the formation of small galaxies. These galaxies lack a coherent structure , which is why they are also called irregular galaxies. Studies published in 2016 estimate that there are at least two trillion galaxies—two million million. Of course, this refers to the observable universe.
Previously, it was estimated that there were ten times fewer galaxies than the estimate mentioned in the previous paragraph. Furthermore, according to research, most galaxies are estimated to have a diameter between one hundred and one hundred thousand parsecs. As for their separation, galaxies are usually separated by distances on the order of one million parsecs. Intergalactic space is composed of a tenuous gas.
The average density of intergalactic space does not exceed one atom per cubic meter. Furthermore, many galaxies are arranged in a hierarchy of clusters. These clusters, in turn, can form even larger clusters called superclusters. Finally , the largest structures are arranged in sheets or filaments surrounded by vast regions of emptiness in the universe.
Some astronomers speculate that dark matter constitutes 90% of the mass in most galaxies. However, the nature of this component is not yet proven and, for now, appears only as a theoretical resource to support the observed stability of galaxies. Dark matter was initially proposed in 1933 by the Swiss astronomer Fritz Zwicky.
Dark matter and galaxy shapes
The discovery of dark matter in galaxies occurred because the observed rotation of galaxies indicated the presence of a large amount of matter that did not emit light. However, there are many other galaxies besides our own—the Milky Way. There are so many that we haven't even been able to count them all! The Hubble Space Telescope observed a small portion of space for 12 days.
Hubble managed to discover ten thousand galaxies of all sizes, shapes, and colors. Some scientists believe there could be around one hundred billion galaxies in the entire universe. But for now, we can only rely on studies of the observable universe. In visible space, there are several types of galaxies. Some of them are spiral-shaped, like our own. They have curved arms that make them look like pinwheels. To learn more about this, you can consult the article on types of galaxies.
There are also galaxies that are smooth and oval-shaped. These are called elliptical galaxies . On the other hand, there are also galaxies that are neither spiral nor oval-shaped. These types of galaxies are irregularly shaped and appear as smudges in space. The light we see from each of these galaxies comes from the stars within them.
There is enough distance between them for them to wander through their respective spheres of space. However, sometimes galaxies get so close that they collide. According to some researchers, the Milky Way will one day collide with Andromeda , its closest galactic neighbor. This event is estimated to occur in about five billion years.
The good news regarding this event is that galaxies are so vast . So vast, in fact, that they are so expanded at their edges that even if they collide with each other, the planets between them and solar systems often do not actually collide.
galactic stars
Thanks to a telescope used by Galileo Galilei in 1610, we were able to observe what was then called the milky band in the night sky. It was named the Milky Way for this reason. In his discovery, he indicated that it is composed of an immense number of small stars. By 1755, Immanuel Kant had theorized about the structure and groupings of stars.
In his treatise *General History of Nature and Theory of the Heavens*, Kant draws upon the earlier work of Thomas Wright. Kant asserted that the Milky Way was a system composed of thousands of solar systems like our own. These systems, he claimed, were grouped into a higher-order structure with characteristics similar to those of planetary systems : essentially flat, elliptical in shape, rotating around a center, and governed by the same celestial mechanics.
discovered galaxies
Andromeda: A neighboring galaxy
In the Andromeda constellation , the galaxy of the same name was observed in 1917 by Herbert Curtis. This great scholar managed to locate it within the Messier M31 "nebula." However, upon searching through photographic records, he found 11 other novae. He observed that, on average, these novae were 10 orders of magnitude fainter than those occurring in our own galaxy.
The research, based on this observation, allowed scientists to predict that these novae should be located at a distance of 150.000 parsecs. This led Hebert to become a renowned proponent of the "island universe" hypothesis, arguing that " spiral nebulae " were actually independent galaxies. Three years later, a debate arose between Harlow Shapley and Heber Curtis regarding the nature of our galaxy.
What could be debated, besides our own galaxy, was the topic of spiral nebulae and the size of the universe . However, to defend the claim that M31 was an external galaxy, Curtis argued that the dark lines observed in this nebula were similar to the dust clouds seen in our own. The key to this novelty was the use of a new telescope. This allowed Edwin Hubble to resolve the outer parts of some spiral nebulae as collections of individual stars.
In addition, Hubble was able to identify Cepheid variable stars within these nebulae, which allowed him to estimate their distance: they were too far away to be part of the Milky Way. During 1936, Hubble developed a galaxy classification system that is still used today and is known as the Hubble sequence.
Our galaxy: The Milky Way
The shape of the Milky Way was first described by William Herschel. This initial attempt at description was recorded in 1785. During that year, Herschel carefully counted the number of stars in different regions of the sky. A century and a half later, specifically in 1920, Kapteyn used a refinement of Herschel's technique and suggested the image of a small ellipsoidal galaxy . This small galaxy was 15 kiloparsecs in diameter, with the Sun near its center.
On the other hand, using a different method based on the distribution of globular clusters , developed by Harlow Shapley, a radically different image emerged: a flat disk with an approximate diameter of 70 kiloparsecs and a Sun located far from its center. However, neither analysis took into account the absorption of light and the interstellar dust present in the galactic plane.
It was Robert Julius Trumpler who finally took these effects into account in 1930. That year, he studied open clusters, producing the image of our galaxy, the Milky Way , that is currently accepted . It is a spiral galaxy with an approximate diameter of 30 kiloparsecs.
As such, the Milky Way belongs to a Local Group of about forty-six galaxies, dominated by the Milky Way and the Andromeda Galaxy . This cluster lies on the edge of a "supercluster" comprising nearly five thousand galaxies. The supercluster, in turn, is also part of another enormous concentration of galaxies gathered in compact, smooth masses.
Types of galaxies according to Hubble
Galaxies can be observed in their various configurations, including elliptical, spiral, lenticular, and irregular. A more detailed description exists, based primarily on the appearance of galaxies, and was provided by the Hubble sequence , proposed in 1936. This sequence relies solely on visual appearance and therefore does not consider other aspects, such as the rate of star formation or the activity of the galactic nucleus.
Elliptical galaxies
These types of galaxies, as their name suggests, are those shaped like an ellipse . They can be designated from E0 to E7, where the number indicates how elongated the ellipse is; thus, E0 would be a sphere and E7 a plate or disk. Additionally, the number indicates their eccentricity multiplied by 10.
These galaxies appear to have few structures. They also typically contain relatively little interstellar matter. Consequently, they have few open clusters and a low rate of star formation. On the other hand, these galaxies are dominated by old, long-evolved stars.
These ancient stars orbit the nucleus in random directions. In this respect, they bear some resemblance to globular clusters. Interestingly, the largest galaxies are giant ellipticals . In fact, it is believed that most elliptical galaxies are the result of galaxy collisions and mergers. These can reach enormous sizes and are frequently found in larger clusters of galaxies, near the nucleus.
Spiral galaxies
These types of galaxies are rotating disks of stars and interstellar matter, with a central bulge composed mainly of stars even older than those in other galaxies. Spiral arms of varying brightness extend from this bulge. For this reason, they are called spiral galaxies . Barred spiral galaxies, in particular, are those with a central bar of stars.
Intermediate spiral galaxies are those that, according to their shape, are classified between a barred spiral galaxy and a spiral galaxy without a bar.
Lenticular galaxies
These galaxies constitute a transitional group between elliptical and spiral galaxies and are divided into three subgroups: SO1, SO2, and SO3. They possess a disk, a prominent central condensation, and an extensive halo. They include barred lenticular galaxies (SBO), which comprise three groups: in the first (SBO-1), the bar is broad and diffuse; in the second (SBO-2), it is brighter at the ends than in the center; and in the third (SBO-3), it is very bright and well-defined.
Irregular galaxies
These are irregular galaxies, as they are galaxies that do not yet fit into any of the classifications of galaxies in the Hubble sequence. They are galaxies without a spiral or elliptical shape. This type of galaxy has its own typology. There are two types of irregular galaxies . An Irr-I galaxy is an irregular galaxy that shows some structure but not enough to clearly place it within the classification of the Hubble sequence.
On the other hand, the second type of irregular galaxy is an Irr-II (Irr II) galaxy. This is an irregular galaxy that does not exhibit any structure that would place it within the Hubble sequence. Additionally, dwarf irregular galaxies are often labeled dI. Some irregular galaxies are small spiral galaxies distorted by the gravity of a much larger neighbor.
It is interesting to know that of all the galaxies observed to date, only five percent of the bright galaxies are called irregular galaxies.
active galaxies
These types of galaxies release large amounts of energy and matter into the interstellar medium. They do so through processes unrelated to ordinary stellar processes . Few galaxies can be classified as active galaxies; in fact, they represent approximately ten percent.
Regarding the energy emitted by active galaxies , most of it comes from a small, bright region in the galaxy's nucleus. In many cases, broad and narrow emission spectral lines are observed. These lines provide evidence of large masses of gas rotating around the galaxy's center.
Types of active galaxies
Seyfert Galaxy
These types of galaxies are actually spiral galaxies characterized by a very bright, point-like nucleus . Based on their spectrum, they are distinguished as follows:
- Seyfert Galaxy Type I: They are those with wide emission lines.
- Seyfert Galaxy Type II: On the contrary, they have narrow emission lines.
It is also observed that these galaxies emit weakly in radio.
starburst galaxies
These types of active galaxies are those in which enormous numbers of stars are forming. Many of these stars, after dying, explode as supernovae. Even after this phenomenon occurs, they remain part of stellar evolution , and formally, this group would not be included in our classification. This abnormally high rate of star formation could be linked to internal mechanisms within the galaxy's core.
radio galaxies
Radio galaxies are typically associated with E-type galaxies with active nuclei. They emit radio waves, and some can be relatively faint. These galaxies usually extend across large areas of space. They have a bright nucleus and are typically surrounded by two large jets of particles. Synchrotron radiation has also been detected in many of them.
quasars
A quasar cannot be differentiated from another celestial body. This happens because quasars apparently have the same appearance as a star. In fact, that is the reason for its name, since it comes from the English contraction quasi-stellar, due to its resemblance. But essentially, quasars consist of a very luminous, unresolved nucleus with strong broad and narrow emission lines.
Quasars located in the closest regions of the observable Universe have been found to exhibit diffuse cloudiness . This reveals that these objects are actually the nuclei of very distant active galaxies, of which we are only able to detect their cores. However, it is known that these objects have a very high mass and generally exhibit a structured shape.
Formation and evolution of galaxies
These are among the most active areas of research in astrophysical studies . Among the widely accepted ideas are computer simulations that have predicted the current structures and distribution observed in galaxies.
galaxy formation
According to research conducted by scientists and astronomers, galaxies are forming new stars and interacting with one another. This scene closely resembles the Cobweb Galaxy (MRC 1138-262) and its surroundings, one of the best-studied protoclusters. If you'd like to learn more about galaxy and star formation, I suggest you consult this article on the process of galaxy and star formation.
The most recent models presented regarding the early universe are based on the Big Bang theory. About 300,000 years after this supposed event, hydrogen and helium atoms began to form in a process called recombination. Almost all of the hydrogen was neutral (not ionized) and readily absorbed light.
The period before stars had formed is called the Dark Ages . It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. This activity caused masses of baryonic matter to condense within halos of cold dark matter. These primordial structures would eventually become the galaxies we see today.
Early Galaxies: In 2006, evidence was found pointing to an early appearance of galaxies. This occurred when the galaxy IOK-1 was discovered to have an abnormally high redshift (6,96), corresponding to a period only 750 million years after the supposed Big Bang. This made it the most distant and oldest galaxy ever observed.
On the other hand, some scientists argue that other objects, such as Abell 1835 IR1916, have higher redshifts and are therefore at an earlier stage in the evolution of the universe. The age and composition of IOK-1 have been established with greater reliability. In the last month of 2012, several astronomers reported that UDFj-39546284 was the most distant known astronomical object.
This object had a redshift of 11. It is estimated that the object began to exist about 380 million years after the possible Big Bang. This means that the light reaching us has traveled about 13.420 billion light-years. The existence of these early protogalaxies suggests that they must have formed during the so-called Dark Ages.
An open question in astrophysics is the detailed process by which the first galaxies formed . The theories that arise from this can be divided into two categories: top-down and bottom-up. In top-down theories, such as the ELS model (Eggen, Lynden-Bell, and Sandage), protogalaxies form in a simultaneous, large-scale collapse over approximately one hundred million years.
On the other hand, in bottom-up theories, such as the SZ model (of Searle and Zinn), small globular cluster -like structures form first , and then several of these objects come together to form a larger galaxy.
evolution of galaxies
After a billion years of formation, the key structures for the evolution of a galaxy begin to appear : globular clusters, the central supermassive black hole, and a bulge of metal-poor Population II stars. Although it may not seem obvious, the creation of the supermassive black hole appears to play a key role in actively regulating galaxy growth by limiting the total amount of additional matter added. During this early period, galaxies experience a major burst of star formation.
After this, two billion years later, the accumulated matter settles into a disk. Furthermore, the galaxy will continue to absorb material falling from high-speed clouds and dwarf galaxies throughout its lifetime. This material is primarily hydrogen and helium. The stellar cycle of birth and death slowly increases the abundance of heavy elements, eventually allowing for the formation of planets.
What may be significantly affecting galaxy evolution are concurrent interactions and collisions. Galaxy mergers were common in early epochs. In fact, most galaxies had a very peculiar appearance. Considering the distance between stars, the vast majority of star systems in colliding galaxies are unaffected.
In addition, it's important to note that the gravitational pull on the interstellar gas and dust in spiral arms produces long strings of stars known as tidal tails . Examples of these formations can be seen in NGC 4676 and the Antennae Galaxies.
We've already mentioned that the Milky Way and the nearby Andromeda Galaxy are moving towards each other at a speed of approximately 130 km/s. Furthermore, research suggests that, depending on their lateral motions , the two could collide in about five or six million years. Although the Milky Way has never collided with a galaxy as large as Andromeda, there is growing evidence of past collisions between the Milky Way and smaller dwarf galaxies.