Astronomers keep trying to measure the mass of the Milky Way and they keep coming up with different numbers. But it’s not that they’re bad at math. Measuring the mass of something as enormous as the Milky Way is confounding. Plus, we’re embedded in it; it takes some very clever maneuvering to constrain its mass.
The Milky Way’s mass is a fundamental scientific question that astronomers have been trying to answer for decades. The problem is, even astronomers’ best estimates vary wildly. The difficulty arises not from measuring the mass of the stars themselves. It comes from the challenge of measuring dark matter.
Don’t know what dark matter is? Okay, Universe Today is here to help. (If you do know what it is, then you can skip the next section.)
The Obligatory “What is Dark Matter?” Part
First of all, dark matter is hypothetical. We don’t really know what it is. But we know it’s there, or rather we know something’s there.
The things we can see and interact with are made of what’s called ‘baryonic matter.’ It’s made of atoms and it’s all the stuff we’re familiar with: Our bodies, the planets, stars, Kim Jong-un’s eyeglasses, etc. But baryonic matter only makes up about 10-15% of matter in the universe.
We think that dark matter makes up about 85-90% of the matter in the universe. It’s distinct from regular matter because it doesn’t interact with light and we can’t see it. That’s why it’s called dark matter.
But we know it’s there because galaxies behave as if they have way more mass than we can see. The hint that it’s there is in the gravity. Galaxies must have more mass, and hence more gravity, than we can see in their regular matter, or they would just fly apart. Their mass and gravity hold them together.
The short version is that things just couldn’t be the way they are unless there was a lot more mass than we can measure.
It’s Really Hard to Measure
Laura Watkins, European Southern Observatory
“We just can’t detect dark matter directly.”
“We just can’t detect dark matter directly,” explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. “That’s what leads to the present uncertainty in the Milky Way’s mass – you can’t measure accurately what you can’t see!“
So how can we measure something we can’t see? Astronomers busy themselves measuring the effect of dark matter and then kind of work backwards. But even with all the effort put into it, estimates vary wildly, from as low as 500 billion times the mass of our Sun, up to 3 trillion times the mass of our Sun. That’s a huge discrepancy, and a real nagging problem in astronomy. And it’s because of the difficulty of measuring all the dark matter.
Now a new study led by Laura Watkins from the European Southern Observatory thinks they’ve come closest yet to measuring the dark matter, and hence the entire mass, of the Milky Way. Their number?
They say the Milky Way contains 1.5 trillion times as much mass as our Sun, or 1.5 trillion solar masses, within a radius of 125,000 light years from the galactic centre.
Let’s get into the guts of how astronomers measure the Milky Way’s mass.
Astronomers can’t just take sample measurements of stars and then extrapolate. That doesn’t work because they can’t see all the dark matter. So they measure other things. And thanks to the Gaia mission, a bunch of measuring has already been done for them.
Enter Gaia and Globular Clusters
Gaia is the ESO’s mission to create a 3D map of the Milky Way. It’s an ambitious mission, but it has yielded great results. Gaia has measured the positional and radial velocity of about one billion of the stars in the Milky Way, and in the Local Group. This is about one percent of the stars in our galaxy. That may not sound like a lot, but the accuracy of the measurements is also really important, especially when it comes to measuring dark matter.
Some of the approximately one billion stars that Gaia measured are in the globular clusters that are near the Milky Way. Globular clusters are spherical collections of stars, and there about 150 of them orbiting the Milky Way. Most importantly, the more massive the galaxy is, the faster the globular clusters orbit. And Gaia has given us more accurate measurements of their velocity than ever before.
N. Wyn Evans, University of Cambridge, UK.
“The more massive a galaxy, the faster its clusters move under the pull of its gravity.“
“The more massive a galaxy, the faster its clusters move under the pull of its gravity” explains N. Wyn Evans (University of Cambridge, UK). “Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated.“
The Hubble Helps Out
The further away the globular cluster, the more they tell us about the Milky Way’s mass. Although Gaia provided the extremely accurate velocity measurements of the clusters, it was the venerable Hubble Space Telescope that measured clusters as far away as 130,000 light years from Earth, adding a great deal of accuracy to the new mass measurement for the Milky Way.
“Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy” said Tony Sohn (Space Telescope Science Institute, USA), who led the Hubble measurements.
“We were lucky to have such a great combination of data,” explained Roeland P. van der Marel (Space Telescope Science Institute, USA). “By combining Gaia’s measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way’s mass in a way that would be impossible without these two space telescopes.“
Why It Matters
So now what?
The mass of the Milky Way is more than just a curiosity, it’s an intrinsic and important part of much larger questions. A galaxy’s dark matter content is linked to the formation and growth of structures in the Universe.
This more accurate measurement of the Milky Way’s mass helps us understand our home galaxy and its place in the cosmos.