The universe is made up of giant groups of stars which are called galaxies. One galaxy is a collection of stars, dust and gas held together by the effect of gravity. These cosmic structures form over billions of years from clouds of gas and dust that contract under their own gravity.
Types of galaxies and their formation
There are various types of galaxies and each of them has a different shape and structure. Astronomers classify galaxies into four broad categories: spirals, elliptical, lenticular e irregularThis classification was initially proposed by Edwin Hubble in the 1930s and is still used today.
The formation of a galaxy depends on several factors, such as the rotation rate of the original gas cloud, gravitational interactions with other nearby galaxies, and internal star formation processes. If a cloud has sufficient angular momentum, it can evolve into a spiral galaxy with disk and spiral arms; if it does not have one, it can become an elliptical or lenticular galaxy.
Spiral galaxies
Spiral galaxies, like our own Milky Way, are easily recognizable by their bright spiral arms that extend from a compact core. These arms are made up of young stars, dust, and interstellar gas. The arms of spiral galaxies are also home to intense star formation processes, where new stars continue to be born from compressed gas in the clouds that compose them.
Elliptical galaxies
Elliptical galaxies, on the other hand, are more rounded or oval in shape and lack defined spiral arms. They are composed primarily of old stars and contain very little gas and dust, which implies a low rate of star formation compared to spiral galaxies. The largest ones are called elliptical giants and are some of the most massive structures in the universe.
Lenticular galaxies
Lenticular galaxies are an intermediate type between spiral and elliptical galaxies. Although they have a disk, like spiral galaxies, they lack the defined structures of spiral arms. Their composition includes both old and young stars, and their star formation rate is moderate.
Irregular galaxies
Finally, irregular galaxies do not have a defined shape or structure. Many of them are the result of collisions or interactions with other galaxies. These collisions disrupt galactic structures, forming irregular galaxies with clouds of gas and dust mixed with scattered stars.
The movement and expansion of the universe
Galaxies are not static; they are all in movementThis movement is due to the gravity that affects them individually and to the expansion of the universe itself. Edwin Hubble, a famous astronomer, was the one who demonstrated in the 1920s that the universe is expanding.
Hubble observed that most galaxies are moving away from our own, implying that the universe was in constant expansion since the Big BangThis phenomenon is known as the red shift, as light waves from galaxies stretch toward the red part of the electromagnetic spectrum as they move away from us.
Using this information, astronomers have been able to calculate that the universe is about 13.800 billion years old. This expansion It not only affects the distribution of galaxies, but also their evolution. As the universe continues to expand, galaxies move further away from each other, making the space between them even vaster.
The Milky Way and our place in the universe
Our galaxy, the Milky Way, is one of the many spiral galaxies in the universe. It has a diameter of approximately 100.000 light years and is located in a group of galaxies known as the local group, which includes other notable galaxies such as Andromeda and Magellanic Clouds.
At the center of the Milky Way lies a supermassive black hole known as Sagittarius A*, around which all the stars and components of our galaxy orbit. The Milky Way also has several satellite galaxies orbiting around it, such as the aforementioned Magellanic Clouds, which are smaller and closer galaxies.
Galactic collisions and the Andromeda merger
Not only do galaxies move away from each other, many of them can also collide with each other over millions of years. Galaxy collisions can give rise to complex structures and trigger huge bursts of star formation.
A clear example of a future collision is the interaction between the andromeda galaxy and the Milky Way. Both galaxies are on a collision course and are expected to merge into a single giant elliptical galaxy in about 4.500 billion years. This merger will dramatically affect the shape and content of both galaxies.
Dark matter and galaxies
Another important part of understanding galaxies is the presence of what we call dark matterIt is a form of matter that we cannot see directly, but which exerts a strong gravitational influence on galaxies. Without the presence of dark matter, many galaxies would not be able to maintain their structure or explain their rotation rates.
When astronomers observe how galaxies spin, they find that stars at the outer edges of galaxies are moving much faster than they should, given the visible amounts of mass. To explain this discrepancy, scientists postulate the existence of dark matter, which would contribute enough mass to keep the galaxy coherent.
Vera Rubin, an American astronomer, was a pioneer in this field by studying the rotation curves of galaxies, which laid the groundwork for modern research on dark matter.
However, dark matter remains one of the greatest mysteries of modern astronomy, as it has not yet been directly observed despite many ongoing studies.
The study of galaxies has allowed astronomers to better understand the universe we live in, and although there is much we have discovered, there is still much to learn. Galaxies continue to fascinate us and challenge us to expand our horizons and knowledge.