How the Milky Way Was Built

Milky Way and Andromeda
The Milky Way was built up through collisions of smaller galaxies over 13.5 billion years of time. In the future, it will collide with the Andromeda Galaxy, mingling stars and clouds of gas and dust. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

When you look up into the night sky and see the Milky Way from our vantage point inside it, you might wonder just how it all got built. Our galaxy is incredibly ancient. Not quite as old as the universe, but close. Some astronomers suggest it began to piece itself together within a few hundred million years after the Big Bang.

Galactic Pieces and Parts

What are the building blocks of our Milky Way? The pieces and parts started out with clouds of hydrogen and helium some 13.5 billion years ago. There were clouds with different amounts of mass and varying mixtures of the two primordial gases. The very first stars to form were hydrogen-rich and very massive. They lived very short lives of a few tens of millions of years (at most). Eventually they died in tremendously huge supernova explosions, which seeded the infant galaxy with other gases and chemical elements. Smaller clouds eventually ended up in the center of the galaxy (tugged there by the pull of gravity) while their larger star-forming regions continued the star birth process over several generations of stars. These "dwarf galaxies" too, ended up merging together to continue building up the Milky Way we know today.

The most ancient part of the Milky Way still exists as the Halo System. It's a cloud of star clusters that swarm around in orbits circling the central region of the galaxies. They contain most of the oldest stars in the galaxy. Some very old stars also exist in the central region of the galaxy, while the younger stars — such as our Sun — orbit much farther away. They were born much later in the galaxy's development.

How Do Astronomers Know the Details?

The story of the Milky Way's origin and evolution is told by the stars (and clouds of gas and dust) it contains. Astronomers look at the colors of stars to tell their approximate ages. Color is one way to determine a star's type: how old it is; hot young stars are more likely to be blue-white, while older stars are cooler and reddish-orange. Stars like our Sun (which is middle-aged) are more likely to be yellowish. The colors of stars tell us about their ages, evolutionary history, and much more. If you look at a map of the galaxy using star colors, some very distinct patterns show up, and those patterns help tell the story of Milky Way evolution.

To determine the ages of stars in the galaxy, astronomers looked at more than 130,000 of the oldest ones in the Halo, using data from the Sloan Digital Sky Survey, which has mapped hundreds of thousand of stars in the galaxy. These very oldest stars — called blue horizontal-branch stars — have long since stopped fusing hydrogen in their cores and are fusing helium. They're a very different color from younger, less massive stars.

Their placement throughout the galaxy's halo section has been used to come up with a hierarchical model of galaxy formation that involves multiple collisions and mergers. In it, the Milky Way formed as many smaller groupings of stars along with clouds of gas and dust (called mini-halos) merged together. As the infant galaxy got larger, its strong central gravity pulled the oldest stars to the center. As more galaxies merged together in the process, more stars were pulled in, and more waves of star formation took place. Over time, our galaxy took shape. Star formation continues to take place in the outer arms, with less star birth occurring in the central regions.

The Future of Our Milky Way

The Milky Way continues to gather in stars from dwarf galaxies that are being drawn slowly into its core. Eventually, even some of its closer neighbors, such as the Large and Small Magellanic Clouds (seen from the Southern Hemisphere on our planet) could be drawn in as well. Each galaxy that collides with ours contributes its rich collection of stars to the mass of the galaxy. But, there's an even larger merger in the distant future, when the Andromeda Galaxy mingles its billions of stars of all ages with ours. The end result will be Milkdromeda, billions of years from now. At that point, astronomers in the far distant future will have an incredible mapping job to do!