The Milky Way. Credit: NASA
It is a galactic challenge, to be sure, but Gwendolyn Eadie is getting closer to an accurate answer to a question that has defined her early career in astrophysics: what is the mass of the Milky Way? 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 2 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,” Eadie says.
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.
Eadie, a PhD candidate in physics and astronomy at McMaster University uses 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 2 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, however, 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 center of the galaxy, with uncertainties, which makes her results easy to compare with other studies.
Eadie and 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.”Every so often I think, ‘I’m measuring the mass of the Milky Way.’ That’s pretty neat.”
http://www.eurekalert.org/pub_releases/2016-05/mu-mtm053016.php
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