News of the WeekAstronomy

Milky Way Looks Like Big Kid on the Block

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Science  28 Apr 2000:
Vol. 288, Issue 5466, pp. 593-595
DOI: 10.1126/science.288.5466.593b

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Astronomers have previously estimated the mass of the Andromeda galaxy to be about twice that of the Milky Way, because it is larger and brighter and contains twice as many globular star clusters--spherical collections of hundreds of thousands of stars. But measurements of brightness ignore the spherical halo of dark, unseen matter that surrounds a galaxy. So two British astronomers have analyzed velocity measurements of 37 objects that orbit Andromeda, such as small satellite galaxies and outlying globular clusters and planetary nebulae; from the speeds, they have estimated that Andromeda's total mass is about half that of the Milky Way, they report in a paper slated for publication in the Monthly Notices of the Royal Astronomical Society.