MILKY WAY: ORIGIN, CHARACTERISTICS, PARTS, COMPONENTS - ARTE - 2025

The Milky Way is the barred spiral galaxy to which our solar system belongs. It is made up of roughly 300 billion stars in star systems, plus planets, gas, and cosmic dust.

From the Earth we can see a fraction of it, as a band of whitish light that crosses the sky, very visible during the summer of the northern hemisphere, in the constellations of Scorpio and Sagittarius.

Figure 1. View of the Milky Way from Earth. Source: Pixabay.

For the ancient Greeks the milky appearance of this luminous strip was the milk spilled from the breast of Hera, wife of Zeus, god of light, sky and lightning. That is why they called it the "Milky Way" or milk road.

Other ancient cultures also associated the Milky Way with a road. In the Iberian Peninsula it is known as El Camino de Santiago and for the Scandinavians it led to Valhalla or abode of the gods.

Democritus, the extraordinary ancient Greek thinker, had already suggested that the Milky Way contains thousands of stars within it. When Galileo pointed his telescope at it, he realized that it was indeed full of stars.

In time, the astronomers who followed him realized that the solar system was also part of that strip that envelops the night sky.

The English astronomers William Herschel (1738-1822), discoverer of Uranus, together with his sister Caroline Herschel (1750-1848), created a kind of three-dimensional map of how the stars are distributed in the galaxy.

They concluded that they were arranged in the shape of an irregular disk, with the Sun in the center, although they could not determine their true size.

Only at the beginning of the 20th century did astronomers realize that the solar system was only a tiny part of a much larger grouping: a galaxy. And later that the universe contained billions of them.

Characteristics of the Milky Way

The Milky Way is a very extensive structure. To establish distances at this level, other units of measurement are required. That is why in literature they are used:

- The light year, which is the distance that light travels in a vacuum during one year. The speed of light is constant and in a vacuum it is 300,000 km / s. Nothing in the universe moves faster.

- The parsec, abbreviated pc, is equivalent to 3.2616 light years, while a kiloparsec is 1000 parsecs or 3261.6 light years.

The shape of the Milky Way is that of a barred spiral about 60,000 pc in diameter. It is difficult to define precise limits, since the edges are not clearly defined, since the galaxy has a halo of stars and interstellar matter.

Figure 2. An artist's concept of the Milky Way made from theoretical models and from the observation of other spiral galaxies. From the central bar sprout two main arms that branch later. Source: NASA.

The galactic center is located towards the constellation of Sagittarius, as noted by the astronomer Harlow Shapley at the beginning of the 20th century, the first to estimate the size of the galactic disk.

The solar system, for its part, is in one of these spiral arms: the arm of Orion, on the outskirts of the galaxy. Interstellar dust prevents us from seeing the center, however in radio and infrared frequencies it is possible.

Thanks to them, it is known that the stars there rotate at high speed around a supermassive black hole, equivalent to about 3.7 million solar masses.

As for the origin of the Milky Way, cosmologists believe that it is almost as old as the Big Bang, the explosion that gave rise to the entire universe.

The first stars to form galaxies must have formed about 100 million years later. That is why cosmologists estimate its age at 13.6 billion years (the Big Bang occurred 13.8 billion years ago).

The age of the Milky Way

Figure 3. The galaxy UGC 12158 at 118 Mpc from Earth looks a lot like what scientists think the Milky Way looks like. It is in the constellation of Pegasus. The image was taken by the Hubble Space Telescope. Source: NASA via Wikimedia Commons.

To establish the age of the Milky Way, astronomers look for the oldest stars.

The age of the stars is known through their light, which provides information about their temperature and the elements that compose it.

Stars have a nuclear reactor inside them, which needs a supply of material to function. This material is initially hydrogen, the lightest element of all, which fuses into helium. A star with a lot of hydrogen is young, and one that is poor in this element is old.

By analyzing the light of a star with spectroscopic techniques it is possible to know the amount of hydrogen it has, because each element absorbs certain wavelengths of light and emits others.

The absorbed wavelengths are reflected in the spectrum in the form of dark lines with a characteristic arrangement. This indicates abundance of the element in question, and in this way it is possible to know if a star has a lot of hydrogen and to roughly estimate its age.

Therefore, the age of the Milky Way is that of its oldest stars plus the age of their predecessors, if any. And if there were, they had to contain only hydrogen, helium and lithium, the lightest elements.

The oldest stars in the Milky Way are known to be at least 13.5 billion years old, but inside they contain some heavy elements that were unable to fuse on their own.

This means that they must have acquired them from the predecessor stars, the first generation stars, whose lives were very short because of their great mass and exploded as supernovae.

By adding these ages, cosmologists estimate that the Milky Way formed 13.6 billion years ago.

Parts of the Milky Way

The spiral of the Milky Way has three well-defined regions, which rotate at different speeds (the closer to the center, the faster the rotation):

- The disk, a region abundant in gas and dust that measures approximately 40,000 pc in length and 2,000 pc in thickness: Most of the stars in the galaxy are found there, almost all of them very hot and recently formed blue stars.

- The bulb is a spherical thickening around the center, above and below the disk, with a radius of 6000 pc. This region, unlike the disk, is sparse in dust and gas, with an ancient stellar population.

- The halo, a huge faint sphere that surrounds the galaxy and whose center coincides with that of the disk. The stars here are clustered in globular clusters, and like the bulb, there is little interstellar material here, so the population of stars is also largely ancient.

Figure 4. Parts of the Milky Way. The Sun, located in one of its arms, performs various movements, apart from moving around the galactic center. PNG / PSG are the directions of the galactic north and south poles. Source: Wikimedia Commons.

Spiral structure

The Milky Way is shaped like a barred spiral. Astronomers still do not know why the matter of the galaxy is arranged in this way. Not all spiral galaxies have bars, and many are not even spirals, but rather elliptical.

Figure 5. The spiral structure of the Milky Way seen directly from above. The Sun is the yellow point on the arm of Orion (Ori). The names of the arms correspond to the constellations and are abbreviated. Source: Wikimedia Commons. User: Rursus / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

One theory is that density variations in matter can propagate in space, much like ripples in a pond do when a stone is thrown. This is the so-called density wave theory, but it is not the only one that has been proposed to explain the presence of the spiral arms.

Components

Satellite galaxies

There are several smaller galaxies that accompany the Milky Way, the best known of which are the Magellanic Clouds.

Figure 6. The Large Magellanic Cloud. Source: Wikimedia Commons.

Recently the Sagittarius dwarf galaxy was found and one more, on which scientists still cannot agree on whether it is a satellite galaxy in its own right or a part of the Milky Way: the Canis Major dwarf galaxy.

There may even be other satellite galaxies of the Milky Way that cannot be seen from our location, on one of the spiral arms. The powerful gravity of the Milky Way attracts them and surely in millions of years they will become part of it.

Central black hole

Thanks to infrared telescopes, astronomers have been able to track the movement of stars in the vicinity of the center of the galaxy.

There is an intense X-ray source known as Sgr A (Saggitarius A), which is believed to be the supermassive black hole that all galaxies, including our own, have at the center.

The black hole in Saggitarius A is estimated to be about 4 million solar masses. A glow emanates from it, a product of interstellar matter that continually drains into it. Occasionally a violent glow indicates that a star has entered its interior.

Stars

The splendor of the Milky Way is due to the stars that populate it: between 200 and 400 million. Our Sun is a mid-life average star, located in the arm of Orion, 7900 pc from the bustling galactic center.

There are many types of stars, classified according to their mass and their temperature. They are also classified according to their content of light elements, hydrogen and helium or heavier elements, which astronomers generically call metals.

The latter are younger stars, called population I, while the former are older and are known as population II.

In galaxies like the Milky Way there are stars from both populations. In the spiral arms and in the galactic disk those of population II prevail, while in the halo and bulb those of population I.

Planets

Until relatively recently the only star system with known planets was the Solar System. In it there are two kinds of planets; rocky ones like Earth and giants like Jupiter.

Starting in the 90s of the 20th century, planets were discovered in other star systems: extrasolar planets or exoplanets.

So far there are more than 3000 discovered and their number does not stop. The vast majority are jovian-type planets, that is, gas giants, but some rocky ones like Earth have been found.

Interstellar matter

The space between the stars is filled with interstellar gas and dust. When the Milky Way is observed from the Earth, lines and darker areas are seen, where gas and dust abound. It is made up mainly of light elements: hydrogen and helium, with traces of heavier elements.

Interstellar matter has a fundamental role in the galaxy and in the universe, as this is the raw material for stars and planetary systems.

References

1. CDS. How do we know the age of the Milky Way? Recovered from: Cienciadesofa.com.
2. Kutner, M. 2003. Astronomy: a physical perspective. Cambridge University Press.
3. NASA Space Place. What is a satellite galaxy? Recovered from: spaceplace.nasa.gov.
4. Pasachoff, J. 2007. The Cosmos: Astronomy in the New Millenium. Third Edition. Thomson-Brooks / Cole.
5. Seeds, M. 2011. Foundations of Astronomy. Seventh Edition. Cengage Learning.
6. Views of the Solar System. The Birth and Formation of galaxies. Recovered from: solarviews.com.
7. Wikipedia. Milky Way. Recovered from: es. wikipedia.org.
8. Wikipedia. Milky Way Galaxy. Recovered from: en.wikipedia.org.