How Massive Is Our Milky Way Galaxy?

June 1, 2016 | Joanne Kennell

Artist's illustration of the Milky Way galaxy from afar
Photo credit: NASA

As Donald Trump would say, “It’s yuge!”

Have you ever wondered how massive our Milky Way Galaxy is? As you can image it’s pretty darn heavy. And even though scientists have a rough estimate of its mass, Gwendolyn Eadie, a doctoral student from McMaster University in Hamilton, Canada, is getting closer to a more precise and accurate answer.

Based on current measurements, the Milky Way Galaxy weighs in at around 700,000,000,000 solar masses, or in other words, the mass of our sun multiplied by 700 billion. To put that into perspective, the mass of the sun is two nonillion (2 followed by 30 zeros) kilograms, or 330,000 times the mass of Earth. Whoa.

SEE ALSO: Astronomers Create Giant Image of the Milky Way

And the crazy thing is, "our galaxy isn't even the biggest galaxy," Eadie said in a press release.

So how does one go about measuring something so enormous? First, you have to remember that galaxies not only include stars, planets, moons, gases, dust, and other objects and material, they also contain a good chunk of dark matter — the mysterious and invisible substance than makes up about 27 percent of the universe. And even though dark matter has remained elusive, astronomers know it exists based on its gravitational influence on visible objects, and they can use this information to infer a mass.

To measure the Milky Way’s mass, Eadie is using the velocities of globular star clusters — spherical collection of stars — that orbit our galaxy. The orbits of these clusters are determined by the Milky Way’s gravity, which is largely determined by how much dark matter it contains. But what’s new about Eadie’s research is the technique she developed for using the globular cluster velocities.

To determine the total velocity of a cluster, it has to be measured in two directions: one along our line-of-sight (a straight unobstructed view), and the other across the plane of the sky (perpendicular to the line of sight). Unfortunately, researchers have not been able to measure the exact orbits of all the globular clusters surrounding the Milky Way.

This is where Eadie’s new technique comes in. She developed a way to use partially known cluster velocities, in addition to the fully known ones, to estimate the mass of the Milky Way. The method considers that dark and visible matter have different distributions in space. "Every so often I think, 'I'm measuring the mass of the Milky Way.' That's pretty neat." Eadie said.

Eadie, along with her academic supervisor William Harris, a professor of Physics and Astronomy at McMaster, co-authored a paper on their work, which has been submitted to the Astrophysical Journal.

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