How heavy is the Milky Way in comparison to its neighbor galaxy, Andromeda? Well, depends how you calculate it. In 2006 one group of astronomers figured the Milky Way was twice as massive as Andromeda. A 2009 estimate evened the score, calculating that the two galaxies are equal in mass.
Now a new formula suggests we're only half the mass of Andromeda. To come up with that number, astronomers first measured the speed and distance of galaxies between three and 10 million light-years away. That allowed them to calculate the deformation of spacetime, and thus the mass, of our stellar neighborhood, aka the Local Group—a group of more than 50 galaxies, in which the Milky Way and Andromeda dominate.
They deduced the center of mass between the two heavyweights and found that Andromeda may indeed be twice as massive as the Milky Way. The results are in the Monthly Notices of the Royal Astronomical Society. [Jorge Peñarrubia et al, A dynamical model of the local cosmic expansion]
The researchers say this new model might be more accurate than past ones, because it accounts for dark matter far from each galaxy's center—assuming the data's right, that is.
"Well, I never bet money on anything. Because I'm aware of all the uncertainties and approximations." Lead author Jorge Peñarrubia of the University of Edinburgh. "One year from now, two years from now, some clever guy will come and say, ‘Hey, you forgot to include this, actually when you do everything changes,’ and then you will have to accept that."
For now, we'll just have to accept that we live in the wimpier of the two galaxies. At least until the next weigh-in.
[The above text is a transcript of this podcast.]