Andromeda vs Milky Way - The Tale of the Tape

Jonathan Strickland

Our galaxy's heftier cousin, Andromeda.| Image courtesy of NASA/JPL-Caltech/UCLA

Sometimes, my brain comes up with weird questions. Examples include "how many dinosaurs would it take to create a single barrel worth of petroleum" or "how long could I live off of Frito corn chips?" I'm comforted that other people also come up with bizarre questions, like "how much does the Milky Way actually weigh?"

As you can imagine, it's tough to answer such a question. For one thing, there are no scales out on the market large enough to weigh a galaxy. For another, we've got the entire system floating out in space, which for reasons I won't get into here absolutely refuses to work within the framework of Earth's gravity.

Weight is a problem, then. A more meaningful question might be "how much mass is in the Milky Way?" As it turns out, according to scientists at the University of Edinburgh, the answer is "not as much as is in the Andromeda galaxy." These scientists collaborated with researchers at Carnegie Mellon University, the University of British Columbia and NRC Harzberg Institute of Astrophysics to study both the Milky Way and Andromeda.

Both galaxies have similar physical dimensions -- they take up similar amounts of physical space. But as it turns out, Andromeda has about twice as much mass as the Milky Way. So much for an obesity problem -- we're positively svelte from a galactic perspective!

The researchers estimate that 90 percent of the mass in both galaxies is actually invisible to us -- it's dark matter. So how does a scientist detect something undetectable? It's all through indirect observations, such as how quickly the celestial bodies inside the Milky Way and Andromeda galaxies move around their respective galactic cores.

These comparisons reveal that, based upon our understanding of how masses move on the cosmological scale, Andromeda is more massive than the Milky Way. This still doesn't answer my question about how much our galaxy weighs, however. And after doing lots of research on the subject, the only conclusion I can draw is that no one is really confident in a firm estimation of how much mass might be in our galaxy. Guesses range from 580 billion solar masses to more than 4.5 trillion solar masses.

A solar mass, by the way, is equivalent to the Sun. If you want a solar mass expressed in kilograms, it's just under two nonillion kg (using the short scale definition of nonillion -- that's a 1 followed by 30 zeros). If you think I'm going to calculate out the grand scale of mass of the Milky Way in kilograms, you're mistaken.

So while I'm seeing headlines like "How much does our galaxy weigh" or "Astronomers weigh up the Milky Way," I think it's more accurate to say that our galaxy appears to have half the mass of the Andromeda galaxy and leave it at that.