To explain what are Galaxies, it is necessary to emphasize that they are a set of stars. These also have gas clouds, planets, cosmic dust, dark matter and energy. These elements come together gravitationally with a more or less defined structure. The origin of the word galaxy is Greek, since they were the ones who attributed the birth of a galaxy to the drops of milk spilled in the Universe.
The belief of the Greeks, with respect to the Galaxies, is that by which they considered a goddess: Hera. They thought that while she fed her son Hercules drops of milk were spilled in the Universe. It is then, when for them the Galaxies were given birth. Today, the amount of stars that make up a galaxy it is uncountable. However, they are the ones that are part of 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 done according to their apparent shape. Also called visual morphology. This is a common shape, like the elliptical galaxy, which has the luminous profile of an ellipse, hence its name. Spiral galaxies, on the other hand, are circular in shape but with a structure of curved arms wrapped in dust. If you would like more detailed information on other characteristics of galaxies, I recommend visiting this link. the characteristics of galaxies.
There was also another way of classifying them, with the unusual galaxies. They are also named after irregular galaxies. This classification of Galaxies, are the result of disturbances caused by the gravitational attraction of neighboring galaxies. Then, existing interactions between neighboring galaxies, is when the fusion of galaxies can be caused, being able to induce the intense birth of stars.
Galaxy Studies
The final result obtained, as a result of the interaction between neighboring galaxies, are small galaxies. This type of Galaxies, are the ones that lack a coherent structure. For this reason, they are also called irregular galaxies. The estimate that was published, according to studies from 2016, is that there are at least 2 billion galaxies. It would be a total of two million million. Of course, we are talking about the Observable Universe.
Previously it was estimated that there were ten times less than the estimate mentioned in the previous paragraph. In addition to this, according to research, it is estimated that the most galaxies they have a diameter between one hundred and one hundred thousand parsecs. As for their separation, Galaxies are usually separated by distances of the order of a million parsecs. Intergalactic space is made up of tenuous gas.
The average density of intergalactic space does not exceed one atom per cubic meter. In addition to this, many of the galaxies are arranged in a hierarchy of aggregates, which are called clusters. In turn, the hierarchy of aggregates can form larger aggregates, these are called superclusters. On the other hand, the larger structures are arranged in sheets or filaments surrounded by huge voids in the universe.
Some astronomers speculate that the dark matter makes up 90% of mass in most galaxies. But the nature of this component is not proven and for now, it appears only as a theoretical resource to support the observed stability in galaxies. Dark matter was initially proposed in 1933 by the Swiss astronomer Fritz Zwicky.
Dark matter and galaxy shapes
The discovery in dark matter in galaxies It happened, because the observed rotation in the 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 have not yet 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 as many as one hundred billion galaxies in the 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 ours. They have curved arms that make them look like pinwheels. To learn more about this, you can read the article on types of galaxies.
There are also galaxies that are smooth and oval in shape. These are names elliptical galaxies. On the other hand, there are also galaxies that are neither spiral nor oval in shape. These types of galaxies are irregular in shape and look like spots in universal space. The light that we see from each of these galaxies comes from the stars within them.
There is enough distance between them to walk around the space that corresponds to them. However, sometimes galaxies get so close that they collide with each other. According to what some scholars explain on this subject, the Milky Way will one day collide with Andromeda, which is its closest galactic neighbor. It is estimated that this event will take place within five billion years.
The good news about this event is that galaxies are too big. So much so that they are so expanded at the ends that, even if they collide with each other, the planets between them and the solar systems often do not collide.
galactic stars
Thanks to a telescope used by Galileo Galilei in the year 1610, what was then called the milky ribbon in the night sky can be seen. which was called Milky Way for this reason. In the discovery that he made, he indicates that it is composed of an immense number of small stars. By the year 1755, Immanuel Kant theorized about the structure and groupings of stars.
In Kant's treatise General History of Nature and Theory of Heaven, he builds on earlier work by Thomas Wright. Kant claimed that the Milky Way was a system made up of thousands of solar systems like ours. These systems mentioned, would be grouped in a higher order structure and with characteristics similar to those of the planetary systems, substantially flat, elliptical in shape, rotating around a center and governed by the same celestial mechanics.
discovered galaxies
Andromeda: A neighboring galaxy
In Andromeda Constellation, the galaxy that bears the same name was observed in 1917 by Hebert Curtis. This great scholar managed to locate it in the "nebula" of Messier M31. However, searching the photographic records, he found another 11 novae. In this way, he was able to observe that on average, these novae were 10 orders of magnitude fainter than those that occurred in our galaxy.
The investigation resulted in this observation being able to predict that such novae should be found at a distance of 150.000 parsecs. This is how Hebert became a famous defender of the «island universe» hypothesis, arguing that «spiral nebulae» were really independent galaxies. Three years later, a debate erupts between Harlow Shapley and Heber Curtis about the nature of our galaxy.
What could be discussed, apart from our galaxy, was the subject of "spiral nebulae" and the dimension of the universe. However, to defend the claim that M31 was an external galaxy, Curtis argued that the dark lines seen in this "nebula" were similar to the dust clouds seen in our own. The key to the novelty was the use of a new telescope. In this way, Edwin Hubble was allowed to resolve the outer parts of some "spiral nebulae" as collections of individual stars.
In addition to this, Hubble managed to identify these Cepheid variable stars and these allowed him to estimate the distance to these "nebulae": they were too far away to be part of the Milky Way. During the year 1936, Hubble organized a system of galaxy classification which is still used today, bearing the name: the Hubble sequence.
Our galaxy: The Milky Way
The shape of the Milky Way was first described by William Herschel. This first attempt at description was recorded in the year 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 the year 1920, Kapteyn used a refinement of the technique used by Herschel and suggested the image of a small ellipsoidal galaxy. This small galaxy was 15 kiloparsecs across, with the Sun near the center.
On the other hand and with a different method, based on the distribution of globular clusters, by Harlow Shapley, a radically different image emerged: a flat disk with a diameter of approximately 70 kiloparsecs and with a Sun far from its center. However, neither analysis took into account the absorption of interstellar light and dust present in the galactic plane.
Finally it was Robert Julius Trumpler who took these effects into account in 1930. That year open clusters were studied by this researcher, thus producing the currently accepted image of our galaxy: the Milky Way. It is a spiral galaxy with a diameter of approximately 30 kiloparsecs.
As such, the Milky Way belongs to a Local Group of some forty-six galaxies dominated by the Milky Way and the andromeda galaxy, on the whole. This cluster lies at the edge of a "super cluster" comprising nearly five thousand galaxies. The super cluster, at the same time, is also part of another enormous concentration of galaxies gathered in compact and smooth masses.
Types of galaxies according to Hubble
Galaxies can be observed from their different configurations, among them are: elliptical, spiral, lenticular and irregular. There is a somewhat more detailed description. This description is the one based especially on the appearance of galaxies and was provided by the hubble sequence, proposed in 1936. It is a scheme that only rests on visual appearance, so it does not take into account other aspects, such as the rate of star formation or the activity of the galactic nucleus.
Elliptical galaxies
This type of galaxies, as its name indicates, are those that have shape of an ellipse. They can be named E0 through E7, where the number means how oval the ellipse is; thus, E0 would be a sphere shape and E7 a plate or disk. In addition to this, it can be said that the number indicates its eccentricity multiplied by 10.
La appearance of these galaxiesshow few structures. And even typically, they have relatively little interstellar matter. This means that these galaxies also have a low number of open clusters, and the rate of star formation is low. On the other hand, contrary to this, these galaxies are dominated by old, long-evolving stars.
These old stars are the ones that orbit around the nucleus in random directions. In this sense, they bear some resemblance to globular clusters. As a curious fact, the largest galaxies are elliptical giants. In fact, most elliptical galaxies are thought to be the result of colliding and merging galaxies. These can grow to enormous sizes and are often 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 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 band 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
They are those that constitute a transition group between elliptical and spiral galaxies, and are divided into three subgroups: SO1, SO2 and SO3. They have a disk, a very important central condensation and an extensive envelope. They include the lenticular bars (SBO), which comprise three groups: in the first (SBO-1), the bar is wide and diffuse; in the second (SBO-2) it is more luminous at the extremities than in the center; and in the third (SBO-3) it is already very bright and well defined.
Irregular galaxies
They are irregular galaxies, since they are galaxies that do not yet fit into any of the galaxy classifications of the Hubble sequence. They are galaxies without spiral or elliptical form. These types of galaxies have their own typology. There are two kinds of irregular galaxies. An Irr-I galaxy is an irregular galaxy that shows some structure but not enough to fit neatly into the Hubble Sequence classification.
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 show any structure that would fit it into the Hubble sequence. In addition to that, irregular dwarf galaxies they are usually labeled as dI. Some irregular galaxies are small spiral galaxies distorted by the gravity of a much larger neighbor.
It is curious to know that of the total number of galaxies observed to date, only one five percent of the bright galaxies are called irregular galaxies.
active galaxies
This class of galaxies release large amounts of energy and matter into the interstellar medium. They do it through processes that are not related to the ordinary stellar processes. Few galaxies can be classified as active galaxies, in fact they are approximately ten percent.
Regarding the energy emitted by the active galaxies, most come from a small, bright region in the galaxy's core. In many cases, broad and narrow emission spectral lines are observed. These lines are the ones that show the existence of large masses of gas rotating around the center of the galaxy.
Types of active galaxies
Seyfert Galaxy
This class of galaxies are actually spiral galaxies that are characterized by having a very bright point nucleus. According to their spectrum they are distinguished in this way:
- 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
This class of active galaxies are those in which huge numbers of stars are forming. Many of these stars, after dying, explode producing supernova. Even after this phenomenon has occurred, it is part of the stellar evolution and formally this group would not be in our classification. This abnormally high formation of stars could be linked to internal mechanisms of the nucleus of the galaxy.
radio galaxies
The so-called radio galaxies are those that are usually associated with type E galaxies with an active nucleus. These emit at radio wavelengths and some can be relatively weak. They are usually galaxies that extend over large areas of space. They have a bright core and are usually surrounded by two jets of large particles. In addition, in many of them it has been detected synchrotron radiation.
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.
The quasars that have been located in the closest places of the observable Universe, have had as an investigative result, that a diffuse cloudiness. What reveals that this type of objects are nothing more than nuclei of active galaxies that are very far away from which we are only able to detect their nucleus. However, what is known is that the mass of these objects is very high and they generally have a structured shape.
Formation and evolution of galaxies
These are one of the areas of research that have the most activity in recent years. astrophysical studies. Among the ideas that are already widely accepted are computer simulations that have predicted the current structures and distribution seen in galaxies.
galaxy formation
According to the investigations carried out by scientists and astronomers, the galaxies are forming new stars and interacting with each other. 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 check out this article on the process of galaxy and star formation.
The most renewed models that are shown with reference to the cosmos of the beginnings of the universe, are based on the possible Big Bang theory. Some 300 years after this supposed event, hydrogen and helium atoms began to form in a new event called recombination. Almost all hydrogen was neutral (not ionized) and readily absorbed light.
The period in which stars have not yet formed is called Dark Age. It was from the density fluctuations (or anisotropic irregularities) in this primordial matter that the larger structures began to appear. Because of this activity, masses of baryonic matter condensed into halos of cold dark matter. These primordial structures would eventually become the galaxies we see today.
Early Galaxies In 2006, evidence was found that referred to the early appearance of galaxies. This occurred when it was discovered that the galaxy IOK-1 had a red shift abnormally high (6,96), it was corresponding to only 750 million years after the supposed Big Bang. This was what made it the oldest and most distant galaxy ever seen.
On the other hand, some scientists argue that other objects like Abell 1835 IR1916 have higher redshifts and are therefore at an earlier stage of the evolution of the universe, age and composition of IOK-1 has been established with greater reliability. The last month of the year 2012, was when several astronomers reported that UDFj-39546284 was the most distant known astronomical object.
This object had a redshift value of 11. It is estimated that the object came into existence about 380 million years after the possible Big Bang. This means that the light that reaches us has traveled about 13.420 million light years. The existence of these early protogalaxies suggests that they must have formed in the so-called Dark Ages.
An open question in astrophysics is the detailed process by which the stars were formed. first galaxies. Theories that come from there can be divided into two categories: top-down and bottom-up. In top-down theories, such as the ELS model (from Eggen, Lynden-Bell, and Sandage), protogalaxies form in a large-scale, simultaneous collapse over about a hundred million years.
On the other hand, in bottom-up theories, such as the SZ model (by Searle and Zinn), small structures similar to globular clusters and then several of these objects come together to form a larger galaxy.
evolution of galaxies
By spending a billion years in the formation, it is that the key structures begin to appear to start with the evolution of a galaxy: the globular clusters, the central supermassive black hole and a bulge formed by population II stars poor in metal. Although it may not seem like it, it is the creation of the supermassive black hole that seems to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added. During this early period, galaxies undergo a major burst of star formation.
After this, two billion years later, the accumulated matter is the one that sits on a disk. On the other hand, the galaxy will continue to absorb material falling from high-speed clouds and dwarf galaxies throughout its lifetime. This matter is mainly hydrogen and helium. The stellar cycle of birth and death slowly increases the abundance of heavy elements, eventually allowing planets to form.
What may be affecting the evolution of galaxies significantly are concurrent interactions and collisions. Galaxy mergers were common in early times. In fact, most of the galaxies looked very peculiar. Taking into account the distance between the stars, the vast majority of galaxy star systems in collision are not affected.
In addition to this, it is important to note that the action of gravity on the interstellar gas and dust of the spiral arms produces long rows of stars known as tidal tails. Some examples that can be mentioned, regarding these formations, can be seen in NGC 4676 27 and the Antennae galaxies.
We have already mentioned that the Milky Way and the nearby Andromeda galaxy are moving towards each other. They do this at an approximate speed of about 130 km/s. In addition to this, research also leads us to think that depending on the lateral movements, 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 increasing evidence of past collisions of the Milky Way with small dwarf galaxies.