The Milky Way, as seen in infrared light, extends for approximately 100,000 light years in diameter, but may be growing very slowly over time as well, according to a new study. (R. Crain (LJMU) and J. Geach (U. Herts))

The Milky Way Is Still Growing, Surprising Scientists

Even with nothing new falling into it, our radius is increasing with every second that goes by.

It’s no big secret that galaxies grow over time. The force of gravity is powerful enough to pull smaller galaxies, gas clouds, and star clusters into larger ones, even over distances of millions of light years. Our own Milky Way has likely devoured hundreds of smaller galaxies over its lifetime, and continues to absorb the dwarf satellites which surround us. But there’s a steadier, more subtle way that galaxies grow: by continuing to form stars from the gas already inside. While most of the stars that form will do so in the plane or central bulge of a spiral galaxy like our own, a new study has shown that galaxies also grow outward over time, meaning that their physical extent increases in space. The implication is that our own galaxy is increasing in size by 500 meters per second: growing by a light year every 600,000 years.

A map of star density in the Milky Way and surrounding sky, clearly showing the Milky Way, the Large and Small Magellanic Clouds (our two largest satellite galaxies), and if you look more closely, NGC 104 to the left of the SMC, NGC 6205 slightly above and to the left of the galactic core, and NGC 7078 slightly below. The full extent of the Milky Way is difficult to determine from within our own galactic plane.(ESA/GAIA)

When galaxies form stars, there are usually a few triggers we can identify. They include:

• the slow, monolithic collapse of a cool cloud of gas,