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EarthSky // Science Wire, Space Release Date: Jul 30, 2014

Milky Way is about half as massive as Andromeda galaxy, say astronomers

Dark matter makes up 90% of the matter in both systems, the study finds.

An image of the Andromeda galaxy, Messier 31.  Image credit: Adam Evans

An image of the Andromeda galaxy, Messier 31. Image credit: Adam Evans

Our Milky Way galaxy is less massive than astronomers previously thought, according to new research.

For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our solar system. A team led by researchers have found that the Milky Way is approximately half the mass of a neighboring galaxy – known as Andromeda – which has a similar structure to our own.

The Milky Way and Andromeda are the two largest members of a cluster of galaxies which astronomers call the Local Group. Both galaxies have a spiral shape and appear to be of similar dimensions, but until now scientists had been unable to prove which is most massive as previous studies were only able to measure the mass enclosed within both galaxies’ inner regions.

The scientists used recently published data on the known distances between galaxies – as well as their velocities – to calculate the total masses of Andromeda and the Milky Way.

Revealing this for both galaxies, they also found that so-called ‘dark’ matter makes up 90% of the matter in both systems.

Dark matter is a little understood invisible substance which makes up most of the outer regions of galaxies and around 27% of the content of the universe. The researchers estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be about twice as massive in total. Their work should help astronomers learn more about how the outer regions of galaxies are structured.

The results were published on July 29 in the journal Monthly Notices of the Royal Astronomical Society.

Read more from the Royal Astronomical Society