Milky Way is 50 percent heftier than thought

January 21st, 2009 - 1:11 pm ICT by ANI  

London, Jan 21 (ANI): A new measurement of the Milky Ways mass has suggested that the galaxy is 50 percent heftier than thought and about as heavy as our nearest large neighbour, Andromeda.

Astronomers have attempted to measure the mass of the Milky Way since the 1920s.

But, the measurement turns out to be exceedingly tricky, not least because some 90 percent of the galaxys mass is thought to be made of dark matter a mysterious, invisible substance that only reveals its presence by its gravitational tugs on stars and gas clouds.

For a number of other galaxies, astronomers can circumvent the dark-matter problem by observing how a galaxy bends the light of more distant galaxies.

The larger the distortion, the stronger the gravitational tug of the intervening galaxy’’s ordinary and dark matter.

But, because of Earths position within the Milky Way, astronomers are limited to directly measuring objects they can see: the galaxys gas and stars, as well as distant satellite galaxies and star clusters.

In principle, the motion of these objects can be used to estimate the galaxys mass.

The faster they move, the more mass must lie within their orbits to keep them from escaping into intergalactic space.

Now, according to a report in New Scientist, Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, examined masers dense star-forming regions that naturally emit microwaves, to find out the Milky Ways mass.

By observing the same regions at different times of year, Reid and colleagues were able to discern slight changes in the masers position, which revealed their distance from Earth.

The team found that the masers seem to be orbiting the galactic centre faster than expected at those distances, meaning more mass is needed to keep them in orbit.

The revised speed suggests the Milky Way weighs some 3 trillion times the mass of the Sun.

Masers can only be used to weigh the Milky Way out to the edge of its visible disc, which ends some 60,000 light years from the galactic centre.

However, most of the Milky Ways mass is thought to lie further away, out to a distance of some 650,000 light years.

Though measuring the mass of the Milky Way out to the very edge may be impossible, the motion of distant satellite galaxies and globular clusters dense clumps of ancient stars could offer some clue. (ANI)

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