Scientists Weigh the Milky Way

As kids, we’re taught that to determine the volume of an unusual object, we can measure the amount of water it displaces. Now, scientists are using similar logic to determine the weight of a truly unwieldy subject: the Milky Way galaxy.

Led by researcher Andreas Küpper, a team at Columbia University's Astronomy Department developed a new way of determining the weight of our galaxy. The resulting study was published in The Astrophysical Journal.

To put things in perspective, the Milky Way contains roughly 100 billion stars in a disk with a diameter that’s anywhere from 100,000 to 200,000 light years across. That enormous range makes it difficult to get an accurate gauge on some of the basic stats of the galaxy.

The team at Columbia University worked around that problem by observing streams of stars outside the Milky Way—and more precisely, observing the way the galaxy affected them. These star streams are the result of dissolving globular clusters that originated during the time when the universe was still just a babe. As it turns out, the orbiting streams can be used to determine the heft of our galaxy as well as the location of our sun within it.

"Globular clusters are compact groups of thousands to several millions of stars that were born together when the universe was still very young," wrote Küpper on his blog. "They orbit around the Milky Way and slowly disintegrate over the course of billions of years, leaving a unique trace behind. Such star streams stick out from the rest of the stars in the sky as they are dense and coherent, much like contrails from airplanes easily stick out from regular clouds."

Using the Sloan Digital Sky Survey, which catalogued stars over the course of a decade by scanning the Northern Hemisphere, the team looked at a particular stream called Palomar 5, discovered in 2001. In examining this stream, co-author Eduardo Balbinot, of the University of Surrey in England, found density wiggles, which are patterns showing variations in the density of the star stream. These variations are caused by the gravitational influence of other objects with mass, such as our galaxy. In the case of Palomar 5, they were pronounced and regularly spaced.

Then, using the Columbia University supercomputer called Yeti, the team created several million models of the stream. Only a galaxy at a certain size and weight could have caused the particular Palomar 5 wiggles. By comparing the models to the observations, the researchers were able to calculate the mass of the Milky Way.

By their estimation, the mass of the galaxy within a radius of 60,000 light years is 210 billion times the mass of the Sun, with an uncertainty of only 20 percent.

If that seems a little...unfathomable, the mass of the sun is 1.989 x 10^24 kilograms, which is about 333,000 times more massive than Earth. Any way you slice it, the mass of the galaxy is difficult to wrap your head around.

Küpper wrote that in the future, other star streams like Palomar 5 can be used to get an even more precise measure of the Milky Way, its formation, and how it compares to other galaxies.