We explain what galaxies are, how they formed, what types and how many exist. Also, what are galaxy clusters.
What is a galaxy?
A galaxy is an astronomical structure that groups together sets of stars (in their respective solar systems) and interstellar matter such as gases, asteroid fields, etc., in the same more or less defined astronomical system. That is, the galaxy is a set of stars and planetary systems orbiting around a defined center or axis.
Our planetary system is part of a galaxy we call the “Milky Way”. It is located in one of its outer regions and far from the center.
Its name comes from the Ancient Greek Culture, since the observers of the night sky at that time assumed that that huge white spot that furrows the sky was the remains of the mother’s milk spilled by the goddess Hera when she was nursing the mythical Heracles (Hercules).
Galaxies are immense structures, as will be understood, that vary greatly in shape, size, and composition, but they are among the brightest objects observable with the help of specialized telescopes.
It is estimated that galaxies are composed of 90% dark matter, although the existence of the latter has not been proven. Although they have different forms of organization, the vast majority of galaxies are flat disks of matter in motion in the space.
Galileo Galilei discovered in 1610 that the Milky Way is made of thousands of tiny stars. That was a very important step in the human understanding of celestial structures, especially those larger than our Solar System.
However, the formal understanding of the existence of a galaxy was not recognized until the late 18th century. Newly at the end of the 19th century William Parsons built a telescope that allowed the first observation of galaxies. Until then they were simply called “nebulae”.
How do galaxies form?
The galaxies were formed in the same way as the rest of the stars and astronomical objects, and traces of galaxies so old have been found that they would have arisen barely 750 million years after the Big Bang (we are talking about the IOK-1 galaxy).
The exact mechanism of formation of these galactic systems is unclear, but they do exist two possible approximations from multiple proposed theories:
- Those that go from bottom to top, that is, that they assume that clusters arose first and small agglomerations of stars that little by little were organized as a system.
- Those from top to bottom, that on the contrary presume that protogalaxies were initially formed, as the result of a large-scale collapse over a hundred billion years.
The key and now recognizable structures of the galaxies appeared after billions of years of evolution and formation. They were affected by mutual attractions and eventual collisions, as a result of which many galaxies merged or were absorbed by larger ones.
Types of galaxies
There are, according to the model proposed by Edward Hubble (the “Hubble sequence” of 1936) and still in force, four types of galaxies according to their apparent shape:
- Spiral galaxies. They are rotating disks of stars and interstellar gases that orbit a bright core of older stars, forming less intense spiral “arms” around them. These galaxies in turn can be classified into:
- Spiral galaxies with star-forming arms. Those that present “arms” with greater or lesser proximity to the nucleus.
- Barred spiral galaxies. Those that present a central bar or band of stars in the nucleus.
- Intermediate spiral galaxies. Those that are between the barred galaxies and those lacking a “bar” in the center.
- Elliptical galaxies. Those that have an ellipse shape, and that are usually named from E0 to E7, indicating with the number how oval their shape is (E0 a sphere and E7 a disk). They tend to show little structure to the observer, and are dominated by old stars, which orbit the center in random directions.
- Lenticular galaxies. It is a transition group between spiral and elliptical galaxies, although they also have a disk and an extensive envelope. They can be barred or not.
- Irregular galaxies. Lastly, there are the galaxies whose shape does not fit into any of the previous categories. They can have a certain degree of structure or be more dispersed, and this may be because they are still in formation, or that they are the product of some collision between galaxies that occurred long ago.
How many galaxies are there?
It is estimated, based on observations from the Hubble telescope in 2016, that there are at least 2 billion (2,000,000,000) galaxies in the observable universe, almost ten times more than previously thought.
Galaxy clusters
Galaxies are not simply scattered throughout the universe, but They are usually part of larger structures known as clusters, which in turn can coalesce and form superclusters.
Galaxy clusters consist of a hierarchy of aggregates. Between them there are gigantic extensions of dead (or empty) space in the universe.
Examples of galaxies
Some of the best known galaxies are:
- The Milky Way. Our spiral galaxy has a diameter of about 100,000 light years and contains about 200,000 to 400,000 million different stars, of which the Sun is just one of the smallest, located at a distance of 25,756 light years from the galactic center.
- The Andromeda Galaxy. Also known as M31 or NGC 224, this is our neighboring galaxy, with which the Milky Way will collide and merge in about five billion years. It is the farthest object visible to the naked eye from Earth, located 2.5 million light years above the constellation Andromeda and is a spiral galaxy, like ours.
- The Triangle Galaxy. Also known as M33 or NGC 598, it is located in the constellation of the triangle (triangulum) about 2.8 million light years from Earth. It is gravitationally attracted to the Andromeda Galaxy, barely 720,000 light years apart from it, although it is much smaller in size (“just” between 30,000 and 40,000 million stars).