Keeping with the Milky Way

Author | Ignasi Ribas (Spain)

Translation | Jiang Yun, Chen Wei (Purple Mountain Observatory)

We used to think that the Milky Way was the center of the universe, but as we learn more, it is getting farther and farther away from the center of the universe.

Discovery of nearby galaxies

Until the 1920s, we thought that nebulae (faint objects composed of filamentary structures) were part of the Milky Way, but studies of Cepheid variables in the Andromeda Galaxy (M31) and other nearby galaxies revealed that they have extragalactic characteristics.

Andromeda Galaxy (M31)

According to the data, the first galaxy to be studied after the Milky Way was the Andromeda Galaxy, which was first observed in 961 AD, but it is certain that prehistoric humans in the southern hemisphere had seen the Magellanic Clouds. The next galaxy was not observed until the 17th century, when telescopes achieved considerable precision and sensitivity. In the 18th century, the French astronomer and comet hunter Charles Messier compiled one of the first astronomical catalogs of the time, which included 110 celestial bodies (30 of which were galaxies).

In the 19th century, the development of the astronomical telescope led to the discovery of thousands of galaxies, but it took another century to accurately measure the wavelengths of their spectra and finally determine that these galaxies were outside the Milky Way. American astronomer Edwin Hubble was one of the scientists who made the greatest contribution to the study of galaxies. He determined the distances between galaxies through his study of Cepheid variables.

The cosmic ruler - Cepheid variables are a special type of star whose temperature and diameter change regularly, resulting in changes in luminosity. From the pulsation period of Cepheid variables, its absolute luminosity and distance can be estimated.

The Milky Way Galaxy in the known universe

Today, the number of visible galaxies is estimated to be in the trillions. Their distribution suggests that, on scales of hundreds of millions of light years, the Universe is homogeneous, as is ordinary matter. However, on smaller scales, galaxies can be seen clustered into clusters and superclusters, all embedded in a network of filaments of low-density gas.

The Milky Way in the Universe

The Local Group consists of at least 54 galaxies, the most important of which are the Andromeda Galaxy, the Milky Way and the Triangulum Galaxy (M33). Most galaxies in the Local Group are directly influenced by larger galaxies or entire galaxy groups. The Virgo Supercluster consists of the Local Group and other galaxy groups.

As the largest galaxy in the Local Group, the Andromeda Galaxy is the most distant object visible to the naked eye. However, it pales in comparison to Virgo A (M87) and especially IC 1101, the largest known galaxy. These larger galaxies are located at the center of the cluster, surrounded by smaller galaxies that are constantly being cannibalized by the larger galaxies as they approach.

Neighbors in the Milky Way

Magellanic Cloud

The Magellanic Clouds consist of two dwarf galaxies near the Milky Way. The close relationship between the three leads to gravitational interactions that cause the Magellanic Clouds and the Milky Way disk to distort. Under favorable meteorological conditions, the Magellanic Clouds can be observed with the naked eye near the South Celestial Pole. The larger of the dwarf galaxies is called the Large Magellanic Cloud, and the smaller is called the Small Magellanic Cloud.

〇Andromeda Galaxy

The Andromeda Galaxy is a spiral galaxy 2.5 million light-years away from Earth (a light-year is the distance light travels in one year, and one light-year is equivalent to 9.46 trillion kilometers). It is one of the largest and brightest galaxies in the Local Group.

〇Other galaxies in the Local Group

Collision with the Andromeda Galaxy

Neighboring galaxies interact with each other, creating distortions as they exchange gas and dust. If they are close enough, they will meet in what is called a galactic collision. However, the stars in the colliding galaxies usually do not collide, instead, their interstellar dust interacts to produce flares and various different structures, such as bars and rings. If the galaxies "pass by" but do not pass through each other, they will merge into a larger structure.

Galaxy mergers: Larger galaxies are formed by the merger of smaller galaxies. Spiral galaxies are the result of mergers of smaller galaxies, while elliptical galaxies are the product of collisions of larger galaxies.

Thanks to observations of the evolution of many galaxies, we know that the Milky Way is a complex and dynamic galaxy, and that it will eventually merge with its largest neighbor, the Andromeda Galaxy.

〇New Galaxy Order

The Milky Way has been growing larger since its creation by merging with other galaxies, although this merging has essentially stopped in the past billion years. Currently, the Milky Way is growing in mass by accreting gas from the Magellanic Clouds. The Milky Way will eventually orbit parallel to its giant neighbor, the Andromeda Galaxy. The two galaxies are approaching each other at a speed of 300 kilometers per second (accuracy varies between observations) and are expected to collide in about 4 billion years and will undergo a long period of merging.

This image shows the hypothetical first collision of the Milky Way and Andromeda galaxies. Assuming we are standing on a fictitious planet, the sky looks like it is burning due to the presence of large emission nebulae, which usually indicates a high rate of star formation.

〇The future of the galaxy

In about 7 billion years, the Milky Way and Andromeda will merge into one giant elliptical galaxy. The new Milky Way and Andromeda will dominate this corner of the universe. When we stand on a hypothetical planet, we will see a night sky filled with new galaxies.

Imaginary image of the night sky filled with the Milky Way and Andromeda galaxies

Epic Collision

In fact, collisions between galaxies are common phenomena in the evolution of galaxies. This photo taken by the Hubble Space Telescope in 2005 shows the collision of two spiral galaxies, called the Antennae Galaxies, about 70 million light-years away from us. Both galaxies are "starburst" galaxies known for their high production of new stars.

Antennae Galaxy (also known as NGC 4038/NGC 4039)

(Slightly edited)

About the author/translator

Ignasi Ribas

Astrophysicist at the Spanish Institute of Space Sciences and director of the Institute of Space Studies of Catalonia. He also collaborates with the International Astronomical Union and has been director and member of several of its sections and scientific committees.

Jiang Yun

Associate researcher at the Astrochemistry and Planetary Science Laboratory of Purple Mountain Observatory, Chinese Academy of Sciences. Research interests: petrology, mineralogy, isotope geochemistry and chronology of Martian, lunar and asteroid meteorites.

Chen Wei

Senior engineer of the Key Laboratory of Dark Matter and Space Astronomy at Purple Mountain Observatory, Chinese Academy of Sciences. He is engaged in research on solar high-energy physics. His research interests mainly include theoretical calculation, observational analysis, and space detection of solar gamma rays and X-rays.

Rotating Editor-in-Chief: Du Fujun

Editor: Wang Kechao

Good book excerpt

The Complete Book of the Universe: A New Visual Guide from National Geographic

VISUAL GALAXY