Known Milky Way satellite galaxies.

EARTHSKY // BLOGS // SPACE Deborah Byrd DEC 17, 2012

Estimate suggests Milky Way mass of 1.6 trillion suns

New estimate of Milky Way mass doesn’t mean our galaxy has 1.6 trillion stars. It includes the mass of the dark matter halo thought to surround our Milky Way.

You sometimes hear the estimate that the mass of our galaxy is equal to several hundred billion suns. Now some astronomers are suggesting a mass for the Milky Way of 1.6 trillion suns. The estimate isn’t just for the Milky Way’s stars. It also includes the mass of our Milky Way’s invisible dark halo. It’s based on the first-ever measurement of the proper motion, or sideways motion along our line of sight, of a small galaxy satellite galaxy to our Milky Way. Ken Croswell reported on the role of this small galaxy – called Leo I – on December 3, 2012 in Scientific American.

How do you weigh a galaxy? One way is by observing other galaxies near our Milky Way. Our galaxy has some two dozen known satellite galaxies. If you knew the size of the orbits of these small galaxies, their speeds in orbit around our galaxy, and how long they take to orbit the galaxy, you could use the laws of physics to determine precisely both the mass of our Milky Way and the mass of each satellite galaxy. Unfortunately, our human lifespans are short in contrast to the vast lifespans and motions of galaxies. And although the galaxies move at speeds that are mind-bogglingly fast in contrast to, say, the speed of a car or a train or a jet, space is so vast and the distances in space are so great that astronomers have to work long and hard to obtain precise measurements of these high speeds.

Read more: Estimate suggests Milky Way mass of 1.6 trillion suns | Space | EarthSky.

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