Milky Way galaxy is bigger than thought

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.

Not anymore.

The Milky Way is considerably larger, bulkier and spinning faster than astronomers once thought. It is 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 importantly, it is denser, with 50 per cent more mass, which is like weight.

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

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's 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 hasn't been done before.

With that, Reid was able to determine the speed at which the spiral-shaped Milky Way is spinning around its centre. That speed - about 914,0000 kilometres per hour - is faster than the 791,800 km/h that scientists had been using for decades.

That is about a 15 per cent jump in spiral speed. The old number was based on less accurate measurements and this is based on actual observations, 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.

The dark matter - the stuff we cannot see - is by far the heaviest stuff in the universe. So that means the Milky Way is about one-and-a-half times the mass had astronomers 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, Reid said.

But do not worry. That is at least 2 to 3 billion years away, he said.