Our galactic big brother might not be so big after all. Overturning 50 years of thinking on the subject, astronomers at the International Centre for Radio Astronomy Research (ICRAR) in Australia have calculated that the Andromeda galaxy has a similar mass to the Milky Way.

Lying around 2.5 million light-year away, Andromeda is the nearest major galaxy to our own. Astronomers have previously believed it to be two to three times more massive than the Milky Way, but the technique employed by the ICRAR team returned a very different result.

That technique uses observations of fast moving stars within the galaxy to determine the speed at which an object needs to be traveling to escape it (called escape velocity), which is in turn used to calculate the galaxy's mass.

"When a rocket is launched into space, it is thrown out with a speed of 11 km/s to overcome the Earth's gravitational pull," says astrophysicist Dr. Prajwal Kafle, from The University of Western Australia branch of ICRAR. "Our home galaxy, the Milky Way, is over a trillion times heavier than our tiny planet Earth so to escape its gravitational pull we have to launch with a speed of 550 km/s. We used this technique to tie down the mass of Andromeda."

The team concluded that Andromeda is 800 billion times heavier than the Sun – a figure comparable to the mass of the Milky Way.

According to Dr. Kafle, the findings also indicate that the amount of dark matter in the Andromeda galaxy is only a third of that uncovered in previous observations.

With the mass of the two galaxies now thought to be similar, new simulations are needed to find out what will happen when they eventually collide in around 5 billion years.

"We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed," says Dr Kafle. "It's really exciting that we've been able to come up with a new method and suddenly 50 years of collective understanding of the local group has been turned on its head."

The study was published in the journal Monthly Notices of the Royal Astronomical Society.

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