The exact origins of Milky Way shrouded in mystery. But astronomers believe that our home galaxy began more than 13 billion years ago, and that it was much younger than its current size. How did it grow so much to reach its current size? For that, we can probably thank eons of cannibals in the galaxy.
Astronomers aren’t entirely sure how the first galaxies formed, because it’s so hard to observe the early ages of the universe. (Observatories eg James Webb Space Telescope designed to study that exact era). However, scientists do have some clues.
The contemporary universe features places of very high density, such as galaxies, and places of very low density, such as the voids between galaxies. But all observations indicate that the early universe was very different: There weren’t any differences in density across the universe, according to European Space Agency (Opens in a new tab).
The Milky Way probably began life like any other galaxy – as a small collection of matter with a density slightly greater than the cosmic average. This block is made almost entirely of dark matter, the form of matter that does not interact with light. Since this small lump was slightly more dense than average, it had a slightly stronger gravitational pull compared to its surroundings. And that greater drag enabled it to attract more dark matter into the cluster, which gave it more gravitational pull, which then pulled in more dark matter, et cetera, according to “The Milky Way: A Biography of Our Galaxy (Opens in a new tab)(Grand Central, 2022) by astrophysicist Moya McTeer.
But the infant Milky Way was not alone. It was surrounded by several neighboring clumps of dark matter. Eventually, the first dark matter clumps grew large enough to pull out ordinary matter, which coalesced into dense pockets and formed the first stars. These clumps remain today in and around the Milky Way and are known as globular clusters. It contains the oldest stars in the galaxy, some of which are nearly 13 billion years old, according to Harvard-Smithsonian Center for Astrophysics (Opens in a new tab).
Violent young man
The initial clumps of dark matter, along with their clusters of stars, eventually coalesced to form the primordial Milky Way sometime about 12 billion years ago. Once this merger occurred, the Milky Way emerged as a distinct entity in the universe, separate from its surroundings. Its massive gravity attracted more and more dark matter and gas, causing it to grow rapidly.
As it grows, most of the gas collects in the center. As the gas collapsed, it formed a thin, rapidly spinning disk. This disc quickly began producing stars. After a few billion years, the Milky Way experienced a period of rapid star formation that has never been surpassed in the galaxy, according to The Caltech Encyclopedia of Astronomy and Astrophysics (Opens in a new tab).
But the mergers are not over yet. Using observations from the Gaia satellite, astronomers have identified more than a dozen groups of stars in the Milky Way that look slightly different from their neighbors. These groups feature stars with similar ages, elemental compositions, and velocities.
Astronomers believe that these clumps represent the remnants of smaller galaxies that fell into the Milky Way billions of years ago. Our galaxy’s strong gravity tore apart these unlucky interlopers, and ate them, leaving only a small remnant of them, according to the report. EarthSky.org (Opens in a new tab).
The Milky Way has not abandoned its cannibalistic ways: it is currently tearing apart its closest satellites, the Large and Small Magellanic Clouds. Interestingly, the Milky Way has not experienced a merger with a galaxy of similar mass throughout its 13 billion year history. These mergers are catastrophic: the collision leads to the rapid formation of so many stars that there is not enough gas left to form new generations. After a major merger, galaxies tend to become “red and dead,” meaning they are filled with only small, dim red stars.
However, the Milky Way is on a collision course with its closest major neighbor, the Andromeda Galaxy, according to NASA (Opens in a new tab). In about 4 billion years the two galaxies will begin to collide, and the Milky Way as we know it will disappear.