Milky Way (Meaning and Explanation)

We explain what the Milky Way is, how it originated and its dimensions. Also, what are its characteristics and structure

What is the Milky Way?

The Milky Way is the galaxy where our solar system is located, along with billions of other stars and their planetary systems. It is located in a cluster of about forty different galaxies called the Local Group. The Milky Way and Andromeda are the two brightest galaxies in the group.

From our planet, the Milky Way is observable with the naked eye as a band of blurry white light that is found throughout the celestial sphere. It is brightest towards its center, towards the constellation of Sagittarius. This has allowed its observation since ancient times, when it was thought to contain all the stars in the universe.

In the 17th century, Galileo Galilei was the first to observe it with a telescope. However, the American astronomer Edwin Hubble was the one who delimited it and studied it in greater depth in the 20th century.

Where does the name “Milky Way” come from? The name “Milky Way” comes from Latin and means “way of milk” or “milky path”. The designation is linked to the white, luminous appearance of the band of stars seen in the night sky. The association with milk derives from Greek mythology. According to legend, Hera was to breastfeed Heracles (better known by his Latin name, Hercules), a son of Zeus who had been born as a result of infidelity. However, Hera was not willing to do it, but one day she woke up and discovered that Heracles had been made to suckle at her breast. Faced with this, she abruptly removed it, so that the milk continued to come out, causing some drops to spill out and form the bright stripe in the sky known as the Milky Way.

Characteristics of the Milky Way

The Milky Way has the following characteristics:

It is the galaxy where the solar system and our planet Earth meet.
It was formed 13.6 billion years ago.
It has a diameter of approximately 100,000 light years.
It is estimated that it has between 100,000 and 400,000 million stars.
Galileo Galilei was the first to observe it from Earth with a telescope.
It is part of a group of galaxies called the Local Group.
It is made up of three parts: halo, disc and bulb.
It is believed that at its center there is a large supermassive black hole.

Origin of the Milky Way

The Milky Way, like other galaxies, formed after the Big Bang, approximately 13.6 billion years ago.. As the universe expanded and cooled, larger structures formed. In this context, the Milky Way began to take shape when the denser regions attracted additional matter around them through gravitational force.

Subsequently, the matter continued to accumulate in a rotating disk known as the galactic disk. The spiral structure of the galaxy, with its characteristic arms, developed over time due to gravitational interactions and internal dynamics.

Throughout its history, The Milky Way has experienced mergers with smaller galaxies and the continued formation of stars and planetary systems.

Structure of the Milky Way

The Milky Way has a fairly common structure among known galaxies, most of which are spiral-shaped. Its components can be organized into six different sections, which are:

The galactic core. It is the heart of the galaxy, composed of a supermassive black hole (Sagittarius A) and an accretion disk that surrounds it, that is, a cloud of gas at very high temperatures. It is a region of high energy activity and powerful emissions of infrared and X-ray radiation, due to the rapid movement of gas clouds.
The central bulb. It is the spherical region close to the galactic nucleus, densely populated by population II stars, that is, by old stars of low metallicity. The galaxy’s major globular clusters are found in this region, and can be observed optically as they protrude from the galaxy’s stellar dust cloud.
The galactic disk. It is the region that extends from the galactic nucleus to about 75 thousand light years away, and is the most visible and obvious region of the galaxy. This is the massive and bright set of stars that precede the appearance of the spiral arms, often divided into a thick disk, of old stars and a high density of matter, and a thin disk, of low density and younger stars.
The arms of the spiral. It is the outer set of “arms” or appendages of the galaxy, which give it its spiral shape. This shape is not easy to observe with the naked eye and was not confirmed until 1953, although it remains difficult to understand from a practical point of view. Each spiral has a different name: the Perseus arm, the outer arm, the scutum-centaurus arm, among others.
The galactic spherical component. It is an extension of the central bulge above and below the galactic disk, remotely spherical in shape and populated almost exclusively by outer globular clusters, scattered stars and dwarf stars devoid of heavy materials.
The galactic halo. It is the least understood component of the galactic structure, exterior to its visible portion, and consisting of a spheroidal portion of space surrounding the galaxy. It has an immense amount of dark matter, the mass of which has a notable effect on galactic rotation. It is thought to extend 100,000 light years from the center and to have an amount of mass equivalent to that of the rest of the galaxy.

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Formation of the Milky Way

The Milky Way is thought to be one of the oldest galaxies in the known universe, as many of its globular clusters contain some of the oldest known stars. The formation of the galaxy took place around 12 or 13 billion years ago an age very close to that estimated for the entire cosmos, and had its beginning in the material of the globular clusters that make up the stellar halo.

However, the Milky Way It gained greater complexity and density through its encounter with other smaller galaxies, which ended up merging with it. In fact, the galaxy is currently partially devouring the mass of other satellite galaxies, such as the Large Magellanic Cloud and the Small Magellanic Cloud.

Dimensions and location of the Milky Way

The Milky Way has an estimated average diameter of 100,000 light years.. It is estimated that there are between 100,000 and 400,000 million different stars. Our solar system is located on the outskirts of the galaxy, about 28,000 light years from the center.in one of the arms of the spiral of the galactic disk.

Movement of the Milky Way

Galaxy — The Milky Way completes one rotation on its axis approximately every 225 million years.

The Milky Way has two main movements in space:

Galactic rotation. The Milky Way rotates on its axis, just like a rotating disk. It takes between 200 and 250 million years to complete an orbit around the galactic center. This movement is what gives shape to the galaxy’s disk.
Translation around the galactic center. In addition to rotation, the Milky Way moves in an orbit around the galactic center. This orbit is complex and is thought to involve gravitational interactions with other Local Group galaxies, such as the Andromeda galaxy.

Parts of the Milky Way

The Milky Way is divided into three parts: halo, disk and bulge.

Halo. It is a spherical-shaped structure that surrounds the galaxy and has a low concentration of stars and gas clouds. Instead, it contains collections of old stars that circle the galaxy like satellites. It also has abundant dark matter, which gravitationally influences the movement of the galaxy.
Disk. It is where the majority of young stars are contained, since it is the region with the highest concentration of gas and where star birth processes still occur. The arms of the galactic spiral are also found there.
Bulb. It is the central region of the galaxy, with the highest stellar density. It has a flattened spherical shape and has a rotating movement similar to the rotation of the planets.

The galactic center

Milky Way - black hole — It is believed that a large black hole exists at the galactic center.

At the center of the Milky Way is a dense and complex region called Sagittarius A* (Sgr A*). It is believed that at the heart of this region there is a supermassive black hole.

Observations indicate that Sgr A* has a mass equivalent to millions of times the mass of the Sun, which is consistent with the typical characteristics of a supermassive black hole. However, due to the amount of dust and gas in the central region of the galaxy, the light is obstructed, making it difficult to observe.

Evidence of the presence of said black hole has been collected by studying the movement of stars near that region. These stars orbit an invisible point with an extremely large mass at the center, which would be consistent with the presence of a supermassive black hole in the center of the Milky Wayalthough there are still debates about it.

The Milky Way in culture

The birth of the Milky Way - Rubens — Rubens painted *The birth of the Milky Way* based on the myth of Hercules and Hera.

The Milky Way has been present in the human imagination about the sky and space since Antiquity. The Aztecs saw in it the cloud serpent Mixcóatl, and the Vikings thought it was the door to Valhalla, the kingdom of warriors fallen in combat.

On the other hand, Spanish pilgrims have used the position of the Milky Way in the sky as a reference to reach Santiago de Compostela, which is why it is often named the “Camino de Santiago”.

Likewise, the Milky Way has been represented many times in fine arts. An emblematic case is the painting The birth of the Milky Way, by Rubens, who used the myth of Hercules and Hera to create it.

References

European Space Agency. (2004). The Milky Way. ESA Kids. https://www.esa.int/
Flores, J. (2022). 11 things you probably didn’t know about the Milky Way. National Geographic Spain. https://www.nationalgeographic.com.es/
NASA Science. (2022). What is a galaxy? https://spaceplace.nasa.gov/