Galaxies

We explain what galaxies are, how they are composed and the types that exist. Also, what are their general characteristics and how they are formed.

What are galaxies?

Galaxies are collections of stars, planets, gases, cosmic dust and dark matter bound together by gravitational forces.. These sets are so large that the number of stars that make them up can exceed billions.

Our galaxy, the Milky Way, has about 100 billion stars and a diameter of approximately 100,000 light years. However, there are larger and smaller galaxies. There are believed to be at least two trillion galaxies in the observable universe similar to the Milky Way.

At the end of the 18th century, the German astronomer William Herschel made systematic observations of nebulae in space and proposed that some of them were “universal islands,” that is, independent galaxies outside the Milky Way. It was in the 20th century, with the work of astronomers such as Edwin Hubble, that stronger evidence accumulated that the nebulae Herschel mentioned were actually other galaxies.

See also: Big Bang Theory

Features of galaxies

Galaxies have the following characteristics:

• They are collections of stars, planets, gases, cosmic dust and dark matter.
• They have various sizes and shapes.
• There are billions of galaxies in the universe.
• They can be red or blue.
• They can group together and form galaxy clusters.

Composition of galaxies

Galaxies are made up of:

• stars. They are spheres of gas that generate light and heat. They constitute the main source of energy and luminosity in a galaxy. They vary in size, mass and temperature.
• Planets. They are rocky or gaseous bodies that orbit around stars. They can have atmospheres and, in some cases, host conditions suitable for life. Planets represent a small fraction of a galaxy's total mass.
• Gases. Hydrogen is the most abundant gas in the universe and plays a relevant role in the formation of stars. The second most abundant gas is helium. Other gases found in galaxies, in a much smaller proportion, are carbon, oxygen and nitrogen.
• cosmic dust. They are tiny particles composed of silicates, carbon and other elements that are dispersed in interstellar space. Like gases, they act as precursor material in the formation of planets and stars.
• dark matter. It is an invisible form of matter that does not emit, absorb or reflect light. Its presence is inferred by gravitational effects on stars and visible matter. Although its exact nature is still unknown, it is believed to constitute a significant part of the total mass of galaxies.

Types of galaxies

Galaxies can be classified according to their shape into different types:

• Elliptical galaxies. They have a defined internal structure with little interstellar matter. Since the stars that make them up are in a very advanced phase of evolution, they are considered the oldest type of galaxy. The largest known galaxies are elliptical. An example is the M87.
• Spiral galaxies. They have a central core from which arms emerge that form a spiral. In the core there are a large number of stars. On the contrary, in the arms the stars decrease and interstellar matter is abundant. Within the observable universe, 75% of galaxies are spirals. A subtype of spiral galaxy is the barred spiral galaxy, which has only two arms, as is the case with the Milky Way.
• Lenticular galaxies. It is estimated that they were spiral galaxies that lost their interstellar matter and their arms, so they only preserve the ellipse-shaped nucleus. They are galaxies that are in a transition state between an elliptical galaxy and a spiral galaxy. An example is the Sombrero galaxy.
• Irregular galaxies. They are those that do not have a defined configuration. Many of these galaxies were originally spiral-shaped, but were distorted by the gravity of another larger nearby galaxy. An example is the Large Magellanic Cloud.

When galaxies are considerably smaller in size than the rest, they are called “dwarf galaxies.” These galaxies usually orbit close to larger ones, and have much fewer stars.

The formation of galaxies

The time and size scales referred to galaxies are so large that the study of their origins can only be carried out with the use of numerical simulations. Instead of relying exclusively on observations, astronomers and cosmologists use simulations to model the complex physical processes that explain galactic formation and evolution.

According to these models, the most accepted theory is the one called “by hierarchical formation.” The theory of hierarchical formation maintains that small fluctuations in the density of matter after the Big Bang generated regions that were denser than the rest. These regions attracted more matter through gravity and merged with other smaller structures to form protogalaxies and star clusters. Over time, merger continued from smaller structures to larger, more massive galaxies.

Simulations suggest that the largest and most complex galaxies, such as the spirals and ellipticals we observe today, formed in this way.

The color of galaxies

Galaxies can have different colors. The color of a galaxy provides information about its properties and its star formation history. The galaxies that are observed from Earth can be:

• blue galaxies. Blue galaxies indicate increased star formation activity. These galaxies contain young, massive stars that emit primarily blue light.
• red galaxies. Red galaxies have a low rate of star formation and are composed of older stars. The further away a galaxy is, the redder its color appears.

galaxy clusters

Galaxy clusters are large groupings of galaxies and filaments of matter that extend across vast distances in the universe. They are the largest and most massive structures in the observable universe.

These clusters contain many galaxies interconnected by gravity and are enveloped in large amounts of dark matter. In this way, they can contain thousands of galaxies and are distributed over a wide area of ​​space.

• Planets
• Universe
• stars
• Sun
• Asteroid Belt
• Solar system
• Astronomy

References

• European Space Agency. (2014). the universe. ESA Kids. https://www.esa.int/
• Astrophysical Institute of the Canary Islands. (sf). Formation and evolution of galaxies. https://www.iac.es/
• NASA Science. (2021). What is a galaxy? https://spaceplace.nasa.gov/
• Ropero Portillo, S. (2023). Types of galaxies. EcologyGreen. https://www.ecologiaverde.com/