The Milky Way, our home in the universe

When we look at the night sky away from city lights, we often see a faint, whitish band of light crossing the sky. This band is, in fact, the plane of our galaxy, the Milky Way. While every star we see in the sky, no matter the direction, is part of it, this spectacular band of light is the view of the densest and most populated region of stars, gas and galactic dust.

Main Image: Ahmad Basem

What is the Milky Way?

The Milky Way is our galaxy, a barred spiral with a bar-shaped central bulge and spiral arms containing between 100 and 400 billion stars, trillions of planets, clouds of gas, dust and dark matter, which accounts for much of its mass of 1.5 trillion solar masses.

Its diameter is about 100,000 light years, with a thickness of 1,000 light years at the arms. The Solar System, located in the Orion arm 27,000 light years from the center, orbits the core, where the supermassive black hole Sagittarius A* (4 million solar masses) resides.

Surrounded by a halo with globular clusters, it is part of the Local Group and will merge with Andromeda in 4.5 billion years. From Earth, it appears as a bright band in dark skies, and its study reveals clues about galactic evolution and the search for life.

Structure

Disk: This is the most visible part and the one we see in the night sky. The disk is a thin, flat structure that contains most of the galaxy's stars, including the youngest ones, as well as interstellar gas and dust. Here are the spiral arms, which are regions of higher density where star formation is more active.

The Central Bulb: At the heart of the galaxy is the bulge, a spherical region densely populated with stars. These stars are usually much older than those in the disk. At its center is a supermassive black hole known as Sagittarius A*, with a mass equivalent to millions of suns.

The Galactic Halo: The halo is a spherical and diffuse structure that surrounds the entire disk and bulge. It is composed mainly of globular clusters (groups of hundreds of thousands of very old stars) and scattered individual stars. The halo is believed to contain most of the galaxy's dark matter, a mysterious substance that emits no light but exerts a strong gravitational pull. The halo extends far beyond the disk, with a diameter of about 300,000 light years.

The Spiral Arms: The spiral arms are the "stellar highways" of the galaxy. Although they are difficult to see from our position, we know that the Milky Way has four main arms: Norma, Perseus, Sagittarius and Centauri. Our solar system resides in the Orion arm, a minor arm or spur that lies between those of Perseus and Sagittarius.

How was the Milky Way formed?

The Milky Way formed more than 13 billion years ago from clouds of gas and dark matter that collapsed under the force of gravity.

In its beginnings, it was a much smaller galaxy. It grew and became larger by "cannibalizing" other dwarf galaxies that collided with it, such as the Gaia-Enceladus galaxy about 10 billion years ago.

Over time, the gas and dust flattened to form the galactic disk and spiral arms we see today. The central bulge and the galactic halo are the oldest parts of the Milky Way, formed from the first stars and the remains of those cosmic collisions.

Today, the Milky Way continues to interact with smaller galaxies around it. These galaxies, known as satellite galaxies, orbit the Milky Way due to its immense gravitational force, similar to how the Moon orbits the Earth.

The most famous are the Magellanic Clouds (Large and Small), visible from the southern hemisphere. However, astronomers have discovered dozens of smaller, fainter satellite galaxies. It is believed that some of these galaxies will eventually be absorbed by the Milky Way in the future, thus continuing the process of galactic growth and "cannibalism" that marked their formation.

Our location in the Milky Way

Our location in the Milky Way puts the Solar System, with the Sun and its planets, far from the bustling galactic center.

We are in the Orion arm, one of the secondary spiral arms of this barred spiral galaxy, about 27,000 light years from the nucleus, where Sagittarius A* resides. This position puts us approximately halfway between the center and the edge of the galaxy.

From our perspective, immersed within the galactic disk, the Milky Way appears as a luminous band in the night sky, brighter in directions towards the galactic plane, where most of the stars, gas and interstellar dust are concentrated. This band varies in intensity depending on where we look, being fainter towards the edges and denser towards the center, although our horizon is partially obscured by dust clouds.

We orbit the galactic center at about 828,000 km/h, completing a revolution every 225-250 million years, in a continuous journey within this vast cosmic structure that contains between 100 and 400 billion stars and trillions of planets.

Characteristics of the Orion Arm

The Orion Arm is estimated to be about 10,000 light years long and about 3,000 light years thick.

Unlike the main arms, which are regions of high stellar density, the Orion Arm is less dense, making it less prominent on maps of the galaxy. However, numerous young stars, star clusters and nebulae are found inside it, indicating that it is an area of ​​active star formation.

The Orion Arm is famous for hosting some of the best-known and most beautiful star formations in the night sky. For example, it contains the Gould Belt, a band of bright stars that has an inclination of about 18 to 20 degrees with respect to the galactic plane, and is believed to be a region of recent star formation.

It is also home to the famous Orion Nebula (M42), one of the brightest nebulae and a stellar nursery much studied by astronomers. Despite being a smaller arm, the Orion Arm is a fascinating cosmic place full of wonders. It is, after all, our galactic home.

The future of our Milky Way

The Milky Way is not alone in the universe. It is part of a group of galaxies called the Local Group, which includes the Andromeda galaxy, our largest and closest neighbor. Currently, the Milky Way and Andromeda are approaching each other at about 110 kilometers per second.

In about 4.5 billion years, these two galaxies will collide. It will not be a catastrophic event for the individual stars, since the distances between them are enormous, but it will lead to the formation of a gigantic new elliptical galaxy that astronomers have named Lactomeda.

The Milky Way is a wonder of nature, a reminder of our small but significant existence in the vast cosmos. The next time you see that band of light in the sky, remember that you are looking at the heart of your galactic home.