Galaxy (Meaning and Explanation)

We explain what galaxies are, how they were formed, what types and how many exist. Also, what are galaxy clusters.

What is a galaxy?

A galaxy is an astronomical structure that groups groups 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 that orbit 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 Ancient Greek Culture, since observers of the night sky at that time assumed that that enormous white spot that crosses the sky were remains of breast milk spilled by the goddess Hera when she was breastfeeding 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 moving matter in space.

Galileo Galilei discovered in 1610 that the Milky Way is made of thousands of small stars. That was a very important step in human understanding of celestial structures, especially those larger than our Solar System.

How are galaxies formed?

Galaxies were formed in the same way as the rest of the stars and astronomical objects, and traces of galaxies have been found so old that they would have emerged just 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 not clear, but there are two possible approaches from multiple proposed theories:

Those that go from bottom to top, that is, that assume that cumulus clouds arose first and small agglomerations of stars that little by little were organized as a system.
Those from top to bottom, which on the contrary assume that protogalaxies were initially formed as a result of a large-scale collapse over a hundred billion years.

The key and now recognizable structures of 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 These are rotating disks of stars and interstellar gases that orbit a bright core of older stars, forming “arms” around them in a spiral shape, of less intense brightness. 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 have a central bar or band of stars in the core.
- Intermediate spiral galaxies Those that are between barred galaxies and those without 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 disc). They tend to show little structure to the observer, and are dominated by old stars, which orbit the center in random directions.
Lenticular galaxies This is a transitional group between spiral and elliptical galaxies, although they also have a disk and an extensive envelope. They can be barred or not.
Irregular galaxies Finally there are galaxies whose shape does not fit into any of the previous categories. They may have a certain degree of structure or be more dispersed, and this may be because they are still forming, or they are the product of some collision between galaxies that occurred long ago.

How many galaxies exist?

It is estimated, according to 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 are usually part of larger structures known as clusters which in turn can join together 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

galaxy milky way sky — Our galaxy contains between 200,000 and 400,000 million stars.

Some of the best known galaxies are:

The Milky Way Our spiral galaxy has a diameter of about 100,000 light years and contains around 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 approximately five billion years. It is the most distant object visible to the naked eye from Earth, located 2.5 million light years away, above the constellation of Andromeda and is a spiral galaxy, like ours.
The Triangle Galaxy Also known as M33 or NGC 598, it is located in the triangle constellation (triangle) about 2.8 million light years from Earth. It is gravitationally attracted to the Andromeda Galaxy, with a separation of just 720,000 light years from it, although it is much smaller in size (“barely” between 30,000 and 40,000 million stars).