Milky Way is Andromeda galaxy's fraternal twin

Washington: For decades, astronomers thought when it came to the major galaxies in Earth's cosmic neighbourhood, our Milky Way was a weak sister to the larger Andromeda. Not anymore.

The Milky Way is considerably larger, bulkier and spinning faster than astronomers once thought, Andromeda's equal.

Scientists mapped the Milky Way in a more detailed, three-dimensional way and found that it is 15 per cent larger in breadth.

More important, it is denser, with 50 per cent more mass, which is like weight. The new findings were presented on Monday at the American Astronomical Society's convention in Long Beach, California.

The danger

That difference means a lot, said study author Mark Reid of the Harvard-Smithsonian Centre for Astrophysics.

"Previously we thought Andromeda was dominant, and that we were the little sister of Andromeda," Reid said. "But now it's more like we're fraternal twins."

That is not necessarily good news. A bigger Milky Way means that it could be crashing violently into the neighbouring Andromeda galaxy sooner than predicted - though still billions of years from now.

Reid and his colleagues used a large system of 10 radio telescope antennas to measure the brightest newborn stars in the galaxy at different times in Earth's orbit around the sun.

They made a map of those stars, not just in the locations where they were first seen, but an additional dimension of time - something Reid said has not been done before.

With that, Reid was able to determine the speed (914,0000km/h) at which the spiral-shaped Milky Way is spinning around its centre.