How galaxies are classified

A galaxy is a system made up of millions of stars, gas clouds, planets, cosmic dust, dark matter, dark energy, nebulae, star clusters, multiple star systems, and other celestial bodies, which remain bound together due to gravitational interactions. .

Galaxy is a word that is derived from the Greek term ” gálaktos “ , which means milk, and which also has to do with mythology: Zeus waited for his wife Hera to fall asleep so that Hercules (his son, the product of an infidelity) could drink milk of the goddess and thus become god. But Hera woke up, and the drops of milk that spilled from her womb gave rise to our galaxy, the Milky Way.

The Milky Way and is made up of between 200 and 400 billion stars, one of which is the Sun, the center of our Solar System. Although for many centuries it was believed that only the Milky Way existed, from the technological and scientific advances in space observation, it was discovered that other galaxies existed.

See also

Milky Way .

Star .

How galaxies are classified

Galaxies are classified according to their shape. Based on this, they can be elliptical, spiral, lenticular and irregular.

Elliptical galaxies

Galaxy M87 is elliptical in shape.

They are ellipse-shaped galaxies. As the shape can vary from one galaxy to another, a system was created that allows them to be subclassed with a nomenclature that goes from E0 to E7, where E0 represents a spherical shape and E7 a very marked ellipse.

In general terms, elliptical galaxies were made up of old stars and the absence of dust and gas, essential elements for the formation of new stars.

Spiral galaxies

The Milky Way, our galaxy, is a barred spiral. The planet Earth is in one of its arms.

They are disk-shaped galaxies, in the center of which are old stars. Arms that surround the disk radiate from the central structure, generating a spiral, made up of young stars. They are the most common type of galaxy.

Spiral galaxies can have a bar that comes out from the center of their disk outward. This bar is in charge of channeling the interstellar gas from the arms of the spiral towards the center of the disk, promoting the formation of new stars.

The nomenclature of spiral galaxies includes lowercase letters from “a” to “c” to indicate the level of opening of the arms, with “a” being a level at which the arms are closely united, and “c”, in which they are more dispersed.

On the other hand, the use of the initials “SB” indicates the presence of a bar.

So “SBa”, for example, would refer to a barred spiral galaxy with arms tightly wrapped around the disk.

Lenticular galaxies

The galaxy NGC 4594, known as the sombrero galaxy, is of the lenticular type.

It is a galaxy whose shape is an intermediate between an elliptical galaxy and a spiral. They lack arms, are disk-shaped, and it is speculated that at some point they were spiral galaxies that lost much of their matter.

They are classified into three types: S0 (without central bar), SAB0 (rudimentary central bar) and SB0 (with central bar)

Irregular galaxies

The galaxy NGC 1427 has no definite shape.

To this category belong all the galaxies that do not fall into any of the previous classifications. They are classified into two types:

  • Irregular galaxy lrr-l: It appears to show some rudimentary shape, but it is not sufficiently defined to be considered elliptical, spiral, or lenticular.
  • Irregular galaxy lrr-ll: lacks any form.

They are the smallest galaxies, but since they have a large number of stars forming inside them, they are very luminous.

Formation of a galaxy

Although there are several theories to explain how galaxies formed, so far scientific evidence points to structures that appeared about 300 million years after the Big Bang .

At that time, hydrogen and helium formations were generated, which later underwent density fluctuations, giving rise to larger structures that after a billion years transforming became the first galaxies.

In that primitive stage of formation, the essential parts that make up a galaxy began to appear:

  • A galactic bulb, which is a star cluster with an ellipsoidal-type spatial distribution.
  • Globular clusters, which is a set of stars with a spherical distribution, orbiting close to the galactic nucleus.
  • A central supermassive black hole, which is speculated could be an essential structure in all galaxies by causing their spinning movements, due to its great force of gravity.

Over the next two billion years, galaxies continued to accumulate matter made up of hydrogen and helium, eventually giving rise to planets.

However, the process of galaxy formation has not stopped, and is expected to continue for the next 100 billion years. After that period, the longest and tiniest star formations will begin to disappear, while the remaining structures will be absorbed by supermassive black holes, which will be the only thing left of the galaxies.


by Abdullah Sam
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