From Sky & Telescope: “The Milky Way Contains the Mass of 1.5 Trillion Suns”

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From Sky & Telescope

March 18, 2019
Monica Young

Astronomers are using Gaia and the Hubble Space Telescope to make the most precise measure of the Milky Way’s mass to date. The new result puts our galaxy on par with — if not more massive than — Andromeda Galaxy.

ESA/GAIA satellite

NASA/ESA Hubble Telescope

The mass of the Milky Way has long been debated, to the point that we don’t even know where it stands in the Local Group of galaxies.

Local Group. Andrew Z. Colvin 3 March 2011

Is it the heavyweight champion, or does our sister galaxy, Andromeda, outweigh us?

Andromeda Galaxy Adam Evans

Laura Watkins (Space Telescope Science Institute) and colleagues have used data recently released by the European Space Agency’s Gaia satellite, as well as roughly ten years of Hubble Space Telescope observations, to peg the motions of 46 tightly packed bunches of stars. Known as globular clusters, their orbits help pin down the Milky Way’s mass.

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This artist’s impression shows a computer generated model of the Milky Way and the accurate positions of the globular clusters used in this study surrounding it.
ESA / Hubble, NASA / L. Calçada

Milky Way NASA/JPL-Caltech /ESO R. Hurt

Our galaxy’s gravitational pull determines the clusters’ movements, explains coauthor N. Wyn Evans (University of Cambridge, UK). If our galaxy is more massive, the clusters will move faster under the stronger pull of its gravity. The key is to understand exactly how fast the clusters are moving.

Many previous measurements have measured the speed at which a cluster is approaching or receding from Earth. “However,” Evans says, “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 team finds a mass equivalent to 1.5 trillion Suns. The results appear in The Astrophysical Journal.

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The Milky Way’s disk of stars (labeled here as “thin disk”) are relatively insignificant to the galaxy’s massive dark matter halo.
NASA / ESA / A. Feild

Milky Way Dark Matter Halo. Jürg Diemand, UCSC/UCO/ Lick

A Tricky Scale

Astronomers have been fussing over the mass of the Milky Way the way parents fuss over their newborns. Understandably so: Just as a baby’s weight serves as an indicator of more important things, like their growth and well-being, the heft of our galaxy affects everything from our understanding of its formation to the nature of dark matter.

But while the pediatrician will usually tell you your baby’s weight to within a percent (equivalent to a tenth of an ounce if you’re in the U.S.), the Milky Way’s mass is known only to within a factor of two. Imagine putting your newborn on the scale, only to have the needle waver between 5 and 10 — is baby failing to thrive? Or doing just fine? The uncertainty would render the result meaningless.

On the galactic scale, of course, there are a few more zeroes involved: Over the years, astronomers have found that the Milky Way’s mass is somewhere between 0.5 trillion and 3 trillion Suns. There are plenty of reasons for the large range. First, studying our galaxy is difficult because we’re inside of it; things like dust or the galactic plane of stars can block our view. Second, even when astronomers trace the orbits of objects — such as globular clusters — measuring their motion across the sky is trickier. It takes many years of observations to nail down their so-called proper motions. That’s what Watkins and her colleagues have done, using dedicated Hubble programs that have monitored stellar motions over roughly 10 years, as well as the second data release from the Gaia mission that has been monitoring stars since 2014.

By far the trickiest part of the problem, though, is that much of the mass astronomers are trying to measure can’t be seen. The bulk of the Milky Way is in dark matter, not stars. Moreover, the Milky Way’s dark matter halo may extend 1 million light-years out from the galaxy’s center. Even if astronomers follow the orbit of a globular cluster around the galaxy, it will only reveal the mass inside its orbit. The farthest globular cluster in Watkins’s study is out at 130,000 light-years. To measure the mass beyond that distance, the astronomers must make some assumptions about the nature and shape of the dark matter halo.

A More Exact Mass

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The globular cluster NGC 4147 is about 60,000 light-years from Earth.
ESA / Hubble / NASA / T. Sohn et al.

Nevertheless, the new measurement is so precise that it has helped narrow things down. “Together with another analysis of similar data by Posti & Helmi, this [Astronomy and Astrophysics] has tipped the scale towards a heavier Milky Way,” says Ana Bonaca (Harvard-Smithsonian Center for Astrophysics), who was not involved in the study. “Thanks to these studies, we now know that a very low value for the mass of the Milky Way is unlikely.”

For astronomers, this new mass estimate will be most relevant for understanding the Milky Way’s swarm of satellite galaxies. For the rest of us: Phew — we’re not smaller than Andromeda after all!

There’s still work to be done, though. The ideal tracer would be in the outer halo, Bonaca notes, out beyond 300,000 light-years. The trick is finding something that far out that we can still see, such as globular clusters, dwarf galaxies, or even streams of stars that the Milky Way’s gravity has torn from an infalling cluster or dwarf. Watkins and colleagues for their part think it’s likely that Gaia will continue to estimate the motions of many more globular clusters. No doubt, researchers will continue to narrow down the Milky Way’s mass using this and other methods for some time to come.

Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA

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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”