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Less to the Milky Way than previously thought

The Milky Way 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 at the University of Edinburgh have found that the Milky Way is approximately half the mass of a neighbouring galaxy – known as Andromeda – which has a similar structure to our own. They publish their results in the journal Monthly Notices of the Royal Astronomical Society.

Andromeda Galaxy smallAn image of the Andromeda galaxy, Messier 31. Credit: Adam Evans. Click for a full size versionThe 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 Edinburgh astronomers 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.

Dr Jorge Peñarrubia, of the University of Edinburgh’s School of Physics and Astronomy, who led the study, said: “We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging. Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible.”


Media contacts

Corin Campbell
Press & PR Office
University of Edinburgh
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Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 / 4582
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Science contact

Dr Jorge Peñarrubia
University of Edinburgh
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An image of the Andromeda Galaxy is available from

Caption: An image of the Andromeda Galaxy, M31. Credit: Adam Evans


Further information

The research appears in J. Peñarrubia, Y-Z. Ma, M. G. Walker and A. McConnachie, 2014, "A dynamic model of the local cosmic expansion",  Monthly Notices of the Royal Astronomical Society, vol. 443, pp. 2204-2222, published by Oxford University Press.

The work was supported by the UK’s Science and Technology Facilities Council.  The study was carried out by University of Edinburgh scientists in collaboration with the University of British Colombia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics.

Findings from the study are supported by separate research carried out by astronomers at the University of Cambridge, which used different data and methods and produced very similar results. A preprint of the Cambridge paper is available on the arXiv.


Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc

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