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 per cent 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 neighbouring galaxies or how its internal structures form and evolve over time, among other important cosmological questions.
An international team of astronomers led by Laura Watkins of the European Southern Observatory in Garching, Germany, has now come up with a new approach to the conundrum. By combining data from two space telescopes—NASA’s Hubble and the European Space Agency’s Gaia—the researchers have measured the galaxy’s mass to new precision. The results, set to be published in a future edition of the Astrophysical Journal (pre-print here), posits a total mass of the Milky Way at 1.5 trillion solar masses, which extends out some 129,000 light-years from the centre of the galaxy.
“We were surprised that our value fell in the middle of the very wide range of previous estimates,” Watkins told Gizmodo. “A lot of the most recent studies had tended to favour lower values. So this value was on the high end of the most recent work.”
As noted, the vast majority of this galactic stuff is dark matter, about 84 per cent, or five-sixths of the total, according to the new research. The 200-billion-or-so stars in the galaxy account for around 60 billion solar masses, or around 4 per cent of the total. The remaining 12 per cent consists of non-stellar material such as clouds of gas, planets, comets, asteroids, and the stationary bike in your garage. As for the supermassive black hole in the centre of the Milky Way, it was measured to be around 4 million solar masses; it’s certainly heavy, but it represents a very small percentage of the total.
Compared to other galaxies, the Milky Way is on the heavier side as these things go, but it’s still an intermediate-mass galaxy.
“For some context, the lowest mass galaxies are around a billion solar masses and the most massive are around 30 trillion solar masses, so the Milky Way is on the higher end of this range—but we already knew that,” said Watkins. “Compared to other galaxies with similar brightness, the Milky Way’s mass is fairly typical.”
A computer-generated image of the Milky Way, with accurate positions of globular clusters (the bright yellow dots) in orbit around it. Image: ESA/Hubble, NASA, L. Calçada
To get around the dark matter problem, Watkins’ team measured the velocities and movements of globular clusters—dense and abundant concentrations of stars in orbit far from the galaxy’s centre. The combined mass and distance of globular clusters make them excellent tracers, or reference points, for measuring the mass of the Milky Way. N. Wyn Evans, an astronomer at Cambridge University and a co-author of the new study, explained the technique in an ESA press release:
The more massive a galaxy, the faster its clusters move under the pull of its gravity. 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.
Gaia provided measurements of 34 globular clusters to a distance of 65,000 light-years from Earth, while Hubble provided measurements of 12 distant globular clusters, the farthest at a distance of 130,000-light years from Earth. Data from Gaia was pulled from a 22-month period, while data from Hubble was recorded over a 10-year span, allowing the astronomers to see the movements of these objects across reasonably large timescales.
In the ESA press release, study co-author Roeland van der Marel, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, said his team was able to “pin down the Milky Way’s mass in a way that would be impossible without these two space telescopes,” so this is a good example of scientists pooling their resources to produce research that wouldn’t otherwise be achievable.
Watkins is happy with the new technique and the results, but she admits there’s still room for improvement. Gaia, for example, will scan the sky for a total of around nine or 10 years before the mission is complete, and that data will allow scientists to measure stellar motions more accurately, and in turn, develop a clearer picture of galactic mass. Also, Watkins’ team used only 46 clusters in this work.
“Having motions for more objects would give better accuracy,” she told Gizmodo. “Particularly at larger distances from the centre of the galaxy.”
Finally, Watkins is also anticipating modelling improvements.
“For example, we assumed that the dark matter halo of the galaxy is perfectly spherical. But the halo might be a different shape—it might be a little longer in one direction than other,” she said. “We don’t know!”
Future Gaia data could offer more insight into the shape of this halo, which will help to refine astronomical models. For now, we’ll have to be content with the new figure of 1.5 trillion solar masses and our slightly heavier-than-average Milky Way. [The Astrophysical Journal]