Our Galaxy Is Much Lighter Than We Thought

Milky Way viewed in infrared wavelengths. Credit: NASA/Spitzer

Andromeda is our closest galactic neighbor, 2.5 million light-years away. Previous studies have indicated that Andromeda has twice as many stars as the Milky Way, though some astronomers have estimated that our galaxy is more massive. A new method for calculating the mass of a galaxy indicates that Milky Way is actually half as massive as Andromeda. This new calculation was developed by an international team of astronomers and has been published in the journal Monthly Notices of the Royal Astronomical Society.

This new method is surprisingly precise, as it takes into account the relative distance of all 54 galaxies and the plethora of dwarf galaxies that are in our galactic Local Group. Galaxies just outside of our Local Group were also measured, because gravity from outside the group have the same effect as gravity from within the group.

Though the Universe is expanding, galaxies within a Local Group are being pulled together due to gravitational forces; a fact that would ultimately lead to this new calculation. Once the list of galaxies and their respective distances was compiled, the researchers were able to identify the center point of the Local Group in order to add up all of the regular visible matter as well as dark matter. 

Walker’s team was able to account for the forces of gravity within the Local Group as well as external expansion forces in order to precisely determine the mass of our galaxy and our closest neighbor. Ultimately, they were able to disprove other studies that had claimed the Milky Way to be more massive than Andromeda. In fact, our galaxy only has half as much mass as Andromeda. What’s more, they found out that only 10 percent of each of these galaxies are comprised of regular matter. The rest of the mass comes from dark matter.

"Historically, estimations of the Milky Way's mass have been all over the map," said co-author Matthew Walker in a press release

"By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy. This is the first time we've been able to measure these two things simultaneously.”

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