The Milky Way formed at the dawn of the universe, with the first stars being short-lived and massive. The galaxy has a supermassive black hole at its center and is surrounded by a halo of old stars. In the future, it will collide with the Andromeda Galaxy.
Our Milky Way is believed to be one of the original galaxies that formed at the dawn of the universe, more than 13.5 billion years ago. The universe itself began 13.7 billion years ago with the Big Bang. For about 200 million years, the universe was dark, as matter had not yet condensed into stars. During this time, the matter slowly condensed into progressively denser pockets.
In some areas the density and temperature necessary to initiate nuclear fusion were reached and the first stars were born. These primordial “Population III” stars are thought to have been short-lived and extremely massive – hundreds of times the mass of our Sun. Being so massive, they were also very unstable, producing supernovae within a few tens of millions of years, leaving behind black holes. From their remnants, Population II stars with more typical masses formed.
Scientists have put the age of the Milky Way at 13.6 billion years ago, plus or minus 800 million years. More studies will be needed to further narrow down that number.
Early in its history, the Milky Way may have undergone an active galactic phase, in which a central supermassive black hole formed from its condensed core, and gas infalling from the galactic debris disk released large amounts of energy. It’s hard to tell, because the supermassive black hole at the center of the galaxy has already gobbled up the evidence. Astronomers believe that the Sagittarius A* radio source is the Milky Way’s supermassive black hole. Sagittarius A* is a compact object containing 2.6 ± 0.2 million solar masses, confined in a volume with a radius not exceeding 17 light-hours (120 AU).
As the Milky Way aged, numerous stars experienced supernovae, decreasing the amount of light elements in the galaxy and increasing the amount of metals. This gave rise to metal-rich Population I stars like our Sun. Many of the stars in the galaxy have condensed into a rapidly spinning galactic disk, 100 times longer than it is thick. This disk is surrounded by a halo of old stars, called the galactic halo.
In about three billion years, the Milky Way is expected to collide with its similarly sized neighbor, the Andromeda Galaxy. When this happens, it won’t be particularly noticeable, as the stellar collision rate will still be practically zero, but the average stellar density will increase by a factor of about 2.
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