What is the mass of our Milky Way?

Toronto, June 1  What is the mass of our Milky Way? It is 7X1011 solar masses or, in simpler terms, the mass of our Sun multiplied by 700 billion, say researchers who have tried to measure this galactic challenge with a new method.

The Sun, for the record, has a mass of two nonillion (that's two followed by 30 zeroes) kg or 330,000 times the mass of the Earth.

“And our galaxy isn't even the biggest galaxy," said Gwendolyn Eadie, PhD candidate in physics and astronomy at Hamilton-based McMaster University.

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. 

Astronomers, however, can infer the presence of dark matter through its gravitational influence on visible objects.

Eadie has been studying the mass of the Milky Way and its dark matter component by using the velocities and positions of globular star clusters that orbit the Milky Way.

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). 

Eadie has developed a way to use these velocities that are only partially known, in addition to the velocities that are fully known, to estimate the mass of the galaxy. 

Her method also predicts the mass contained within any distance from the centre of the galaxy, with uncertainties, which makes her results easy to compare with other studies.

Eadie and her academic supervisor William Harris, professor of physics and astronomy at McMaster, have submitted their work for publication in the Astrophysical Journal.