The exact origins of Milky Way are shrouded in mystery. But astronomers believe our original galaxy began over 13 billion years ago and was much smaller than it is today. How did it grow so big to its current size? For that, we can probably thank eternities of galactic cannibalism.
Astronomers don’t know exactly how the first galaxies formed, because the early ages of the universe are incredibly difficult to observe. (Observatories like the James Webb Space Telescope are designed to study exactly that time.) That said, scientists have a few clues.
The modern universe includes places of very high density, like galaxies, and places of very low density, like the voids between galaxies. But all observations indicate that the early universe was very different: there was virtually no difference 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 tiny clump of matter that had a density slightly above the cosmic average. This clump consisted almost entirely of black matter, the form of matter that does not interact with light. Because this small cluster had a little more density than average, it had a slightly stronger gravitational pull relative to its surroundings. This greater attraction allowed it to pull more dark matter into the block, which gave it even more gravity, which drew even more dark matter, and so on, according to “The Milky Way: An Autobiography of Our Galaxy (opens in a new tab)(Grand Central, 2022) by astrophysicist Moiya McTier.
But baby Milky Way was not alone. It was surrounded by several nearby clusters of dark matter. Eventually, these early clumps of dark matter grew large enough to attract normal matter, which accumulated in dense pockets and formed the first stars. These clusters remain today in and around the Milky Way and are known as globular clusters. They contain the oldest stars in the galaxy, some of which are nearly 13 billion years old, according to the Harvard Smithsonian Center for Astrophysics (opens in a new tab).
A violent youth
The initial clusters of dark matter, along with their collections of stars, eventually coalesced to form the proto-Milky Way around 12 billion years ago. Once this merger was achieved, the Milky Way appeared as a distinct entity in the cosmos, separate from its surroundings. Its massive gravity pulled in more and more dark matter and gas, causing it to grow rapidly.
As it grew, most of the gas collected in the center. As the gas collapsed, it formed a thin, rapidly rotating disc. This disc began to produce stars rapidly. After a few billion years, the Milky Way experienced a period of rapid star formation never before seen in the galaxy, according to the California Institute of Technology Encyclopedia of Astronomy and Astrophysics (opens in a new tab).
But the mergers were not over. Using observations from the Gaia satellite, astronomers have identified more than a dozen collections of stars in the Milky Way that look a little different from their neighbors. These collections feature stars with similar ages, element composition, and velocities.
Astronomers believe these clusters represent the remnants of smaller galaxies that fell into the Milky Way billions of years ago. Our galaxy’s strong gravity tore these unlucky intruders apart, cannibalizing them and leaving only small remnants behind, according to EarthSky.org (opens in a new tab).
The Modern Galaxy
The Milky Way hasn’t given up on its cannibal ways: it’s tearing apart its closest satellites, the Large and Small Magellanic Clouds. Interestingly, the Milky Way has not undergone a merger with a galaxy of similar mass in its 13 billion year history. These mergers are catastrophic: the collision triggers 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 turn “red and dead”, meaning they are filled with nothing but small, dim red stars.
However, the Milky Way is on a collision course with its nearest 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 be gone.