How Heavy Is the Milky Way?

By combining data from the Hubble and Gaia space telescopes, an international team of astronomers has come up with the most accurate estimate yet of our galaxy’s mass.

From stars, planets, and asteroids through to black holes and invisible dark matter, our galaxy is packed with a lot of stuff. The total mass of all this celestial stuff, however, is not something astronomers have been able to agree upon.

The mass of the Milky Way has been estimated to be as low as 500 billion solar masses (where one solar mass equals the mass of our Sun) to as high as 2 to 3 trillion solar masses. This dramatic uncertainty has a lot to do with the different approaches used by astronomers to “weigh” our galaxy, not to mention the uncertainty caused by a rather enigmatic variable: dark matter. This invisible, and still hypothetical, form of matter accounts for as much as 90 percent of our galaxy’s total bulk, but as it cannot be seen or measured directly, dark matter presents serious problems for astronomers.

Not being able to settle on an agreed-upon mass for the Milky Way is not good. Without an accurate sense of how much mass our galaxy encompasses, astronomers can’t fully understand how it interacts with neighboring galaxies or how its internal structures form and evolve over time, among other important cosmological questions.

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