Astronomers have arrived at what they believe to be the most accurate measure yet of the mass of the Milky Way: about 4.8 x 1011 times the mass of the sun, or “solar masses,” to use a standard unit of mass in astronomy.
This comes to about 9.5 x 1041 kilograms — that is, 95 followed by 40 zeros. The number, of course, is inexact, as obviously no direct measure of all the billions of stars and other objects in the Milky Way could be taken.
But in a paper to be published in The Astrophysical Journal, scientists used methods of measurement that involve complex mathematical and statistical techniques called hierarchical Bayesian analysis, as well as direct measurements of the velocity of globular clusters, the tightly packed spherical groups of 10,000 to 100,000 old stars that move through the galaxy.
Just as the mass of the sun can be calculated by measuring its gravitational pull on Earth, the mass of the Milky Way can be calculated by measuring its gravitational pull on the globular clusters.
The estimate includes everything within 125 kiloparsecs of the center of the galaxy — that is, within 3.9 x 1018 kilometers. And “everything” is not just stars: There are planets, moons, gases, dust and other objects, not to mention the immense amount of dark matter. It cannot be detected directly, but its mass can be inferred from its gravitational effect on other objects.
“The biggest thing is that we’re including measurement uncertainties that are carried through the analysis,” said the lead author, Gwendolyn M. Eadie, a doctoral candidate at McMaster University in Hamilton, Ontario. “So we have a good handle on the uncertainty in our mass estimate. The low end is 4.0 x 1011 solar masses, and the high end is 5.8 x 1011.”
Ms. Eadie said that the findings were important from an astronomer’s perspective. “The methods we’ve developed could be important in other studies that do other kinds of research,” she said. “These methods have been used in other fields, but they’re starting to become more useful in astronomy now that we have computers that can do these complex calculations.”
What does it mean for the rest of us? “It just satisfies curiosity about the world we live in,” she said.