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Take that, Andromeda!

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. No more.

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

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. The new findings were presented Monday at the American Astronomical Society's convention in Long Beach, Calif.

That difference means a lot, said study author Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The slight 5-foot-5, 140-pound astrophysicist said it is the cosmic equivalent of his suddenly bulking up to the size of a 6-foot-3, 210-pound NFL linebacker.

”Previously we thought Andromeda was dominant, and that we were the little sister of Andromeda,” Mr. 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 Andromeda sooner than predicted – though still billions of years from now.

Mr. 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 Mr. Reid said had not been done before.

With that, he was able to determine the speed at which the spiral-shaped Milky Way is rotating. That speed – about 914,000 kilometres an hour – is faster than the 792,000 km/h that scientists had been using for decades. That's about a 15 per cent jump in spiral speed, Mr. Reid said.

Once the speed of the galaxy's spin was determined, complex formulas that end up cubing the speed determined the mass of all the dark matter in the Milky Way. And the dark matter – which we can't see – is by far the heaviest stuff in the universe. So that means the Milky Way is about 1½ times the mass astronomers had previously calculated.

The paper makes sense but is not the final word on the size of the Milky Way, said Mark Morris, an astrophysicist at the University of California Los Angeles, who was not part of the study.

Being bigger means the gravity between the Milky Way and Andromeda is stronger.

So the long-forecast collision between the neighbouring galaxies is likely to happen sooner and less likely to be a glancing blow, Mr. Reid said.

But he said that will not happen for at least two billion years.