Snapshots of a Galaxy

The Milky Way’s Formative Years

How do you see a galaxy in all its stages of formation?  You can’t watch it in real time because galaxies take millions or billions of years to fully assemble themselves. They form by assimilating each other, colliding, and interacting.

The only way to see a galaxy like the Milky Way form (for example) is to find galaxies at all stages of formation and take snapshots. Eventually, you get a series of views of how our own galaxy would have looked as it formed. (For what it’s worth, the Milky Way is still ingesting other dwarf galaxies, and it will likely gobble up the Magellanic Clouds and collide with the Andromeda Galaxy in the far future, so it’s really a work in progress.)

So, astronomers did just that: they found a lot of galaxies at different stages of their evolution, and put together this timeline of how our galaxy looked throughout its history. The galaxies are arranged according to time. Those on the left are nearby to our galaxy. Those at the far right existed when the cosmos was about 2 billion years old. The bluish glow from young stars dominates the color of the galaxies on the right. The galaxies at left are redder from the glow of older stellar populations.

To create this timeline,  astronomers traced 400 galaxies similar to our Milky Way at various stages of construction over a time span of 11 billion years. They used Hubble Space Telescope and its sharp eye to peer out across the universe. The result taught astronomers a lot about how our galaxy built itself up. It turns out that our galaxy built up most of its stars (about 90 percent) between 11 and 7 billion years ago. It also means that our galaxy began forming only about a billion or two years after the Big Bang. All those billions of years ago, the Milky Way was likely a faint, blue, low-mass object. It had a lot of gas, which it used to create many new stars. When star formation began in earnest, it likely had a bluish color. The blue colors of the Milky Way ancestors are a signpost of rapid star formation. At the peak of star birth, when the universe was about 4 billion years old, the Milky Way-like galaxies in this timeline were pumping out about 15 stars a year, and our Milky Way would likely have been that active, too. By comparison, our galaxy today is creating only an average of one star a year.

Things will change in the future, when our galaxy collides with Andromeda. The gravitational interactions will fuel huge outbursts of star formation, and the night sky from our planet (if it’s still around), would be spectacular!