No Winner in Milky Way-Andromeda Clash

Our Milky Way and the Andromeda galaxy—two giant galaxies in our local patch of the universe—are heading for an immense collision with each other in only a few billion years’ time.

Our Milky Way and the Andromeda galaxy. Image credit: tasnimnews.com (CC BY 4.0)

Both Andromeda and the Milky Way appear to have about the same total mass, about 800 billion times that of our sun, suggesting that the result of this intergalactic gravitational battle may actually be a tie.

The Andromeda and the Milky Way galaxies are very similar, giant spirals containing hundreds of billions of individual stars. But astronomers have struggled to work out which of these two galaxies is the more massive.

And knowing the masses of the two giant galaxies will help to reveal the details of our ultimate fate.

The answer to this question is vitally important if we are to understand the dynamic history of all nearby galaxies, both large and small, as the gravitational field of the most massive will command the action.

Until now, astronomers have been unable to pin down the galactic masses, with a lack of data and complex calculations yielding very uncertain answers.

Simply counting the number of stars in any galaxy, and adding their individual masses, won’t give you its total mass. Not even close. The mass of a galaxy is dominated by its dark side, an immense amount of matter that is unseen by telescopes.

The light side of a galaxy, the glowing stars and gas that we can see, accounts for only a couple of tens of percent of the total mass. The rest, the significant majority, is this elusive dark matter that dominates all of the mass in the universe.

But it is the gravitational pull of this dark matter that holds the stars in their orbits, meaning we can measure its presence.

As dark matter holds stars in their orbits, we can use their motions to measure the overall mass of the Andromeda galaxy, including the unseen dark matter.

While the calculation was complicated, the result was unequivocal: previous estimates had overstated the total mass of Andromeda. It and the Milky Way are equals in the local universe.

We know that the collision with Andromeda is coming in only a few billion years. Previously we were unsure who would be the major player in this battle, and who would dominate the gravitational battle ahead.

There will be no winner in this cosmic clash, but at least the Milky Way will be on an equal footing with its cosmic rival. (Tasnim News Agency)

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