The Task of Measuring the Milky Way

Measuring the mass of our home galaxy, or any galaxy, is particularly difficult. A galaxy includes not only stars, planets, moons, gases, dust and other objects and material, but also a big helping of dark matter, a mysterious and invisible form of matter that is not yet fully understood and has not been directly detected in the lab. Astronomers and cosmologists, however, can infer the presence of dark matter through its gravitational influence on visible objects.

Milky-Way-Formed-From-the-Inside-Out

The short answer, so far, is 7 x 1011 solar masses. In terms that are easier to comprehend, that’s about the mass of our Sun, multiplied by 700 billion. The Sun, for the record, has a mass of two nonillion (that’s 2 followed by 30 zeroes) kilograms, or 330,000 times the mass of Earth.

“And our galaxy isn’t even the biggest galaxy,” says Gwendolyn Eadie, a PhD candidate in physics and astronomy at McMaster University.

The orbits of globular clusters are determined by the galaxy’s gravity, which is dictated by its massive dark matter component. What’s new about Eadie’s research is the technique she devised for using globular cluster (GCs) velocities.

The total velocity of a GC must be measured in two directions: one along our line-of-sight, and one across the plane of the sky (the proper motion). Unfortunately, researchers have not yet measured the proper motions of all the GCs around the Milky Way.

Eadie and her academic supervisor William Harris, a professor of Physics and Astronomy at McMaster, have co-authored a paper on their most recent findings, which allow dark matter and visible matter to have different distributions in space. They have submitted this work to the Astrophysical Journal, and Eadie will present their results May 31 at the Canadian Astronomical Society’s conference in Winnipeg.