milky way mass
July 31, 2014

First-Ever Precise Mass Measurements Reveal Smaller-Than-Expected Milky Way

redOrbit Staff & Wire Reports - Your Universe Online

For the first time, astronomers have been able to precisely measure the mass of the Milky Way, and have discovered that our solar system's home is actually smaller than previously believed.

As reported this month in the journal Monthly Notices of the Royal Astronomical Society, researchers from the University of Edinburgh, the University of British Colombia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics found that the Milky Way is approximately half the weight of the neighboring galaxy Andromeda.

Andromeda and the Milky Way are the two largest members of a group of galaxies known by scientists as the Local Group, and they have similar structures. So why is Andromeda so much more massive? The authors attribute the extra weight to the presence of dark matter, an invisible and mysterious substance found in many outer galaxies.

“They estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be twice as heavy,” the University of Edinburgh explained in a statement Wednesday. “Although both galaxies appear to be of similar dimensions, until now scientists had been unable to prove which is larger. Previous studies were only able to measure the mass enclosed within both galaxies’ inner regions.”

The researchers believe their work could shed new light about how the outer regions of galaxies are structured. In their study, they were able to determine the mass of invisible matter found in the outer regions of both galaxies, thus revealing their total weights. In total, 90 percent of each galaxy’s matter is invisible, they added.

“We don't know much about dark matter so this discovery means we'll get a chance to study it from within our own galaxy,” explained Yin-Zhe Ma, a postdoctoral fellow in the University of British Columbia Department of Physics and Astronomy. He and his colleagues were also able to measure the expansion of the universe within the Local Group of galaxies for the first time by observing the movement of the smaller satellite galaxies orbiting Andromeda and the Milky Way.

“Historically, estimations of the Milky Way's mass have been all over the map,” added Matthew Walker, an assistant professor of physics at Carnegie Mellon University. “By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy.”

Galaxies located within the Local Group are bound together by their collective gravity, the study authors explained. Thus, unlike most galaxies, they are moving closer together instead of further apart. This study marks the first time that scientists were able to take both cosmic expansion and gravity-related data to complete precise calculations of both the Milky Way and Andromeda, using computer models to infer the mass and structure of both galaxies.

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