The mass of the Milky Way? Scientists make 'most accurate' estimation to date

Using the largest dataset yet, they estimated our galaxy has a mass of roughly 805 billion solar masses.
Chris Young
An artist's impression of the Milky Way.
An artist's impression of the Milky Way.

alex-mit / iStock 

A team of scientists from China used data from the European Space Agency's GAIA observatory to estimate the mass of the Milky Way.

According to their calculations, our galaxy has a mass of roughly 805 billion solar masses. Though that is an unfathomably large size to process, it is actually substantially smaller than an estimation made by scientists in the U.S. back in 2019.

Estimating the mass of the Milky Way

Measuring the mass of our Milky Way is impossible with current technologies, though scientists have been able to make estimations with increasing accuracy in recent years.

Scientists from the National Astronomical Observatories of the Chinese Academy of Sciences detailed the methodology behind their new estimation in a paper published in The Astrophysical Journal. The authors used a larger dataset — consisting of 260,000 stars — than had ever been used for a study of this type.

A large part of that data came from ESA's GAIA observatory. GAIA was launched in 2013 from Europe's Spaceport in French Guiana. Its mission is to create a precise three-dimensional map of astronomical objects throughout the Milky Way. As such, its data serves as a comprehensive chart of massive swathes of our galaxy.

Using their large dataset, the scientists conducted a survey of the Milky Way and measured its rotation curve. The rotation curve refers to the orbital speed of an astronomical object in relation to its radial distance from the galaxy's center. They also aimed to account for dark matter in their calculations. In a press statement, the researchers said the new estimation is the most accurate measurement of the rotation curve of the Milky Way to date.

An impossibly enormous task?

Back in 2019, another team of scientists from the Space Telescope Science Institute (STScI) in Baltimore, Maryland, estimated that the Milky Way has a mass of 1.5 trillion solar masses. For their estimation, they used the Hubble telescope and GAIA data to measure the three-dimensional movement of globular star clusters. 

In a NASA blog post, one of the scientists behind that estimation explained the importance of estimating our galaxy's mass. "We want to know the mass of the Milky Way more accurately so that we can put it into a cosmological context and compare it to simulations of galaxies in the evolving universe," Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Maryland, explained. "Not knowing the precise mass of the Milky Way presents a problem for a lot of cosmological questions."

It's worth noting that, though the most recent estimation used the largest dataset to date, 260,000 stars is still a relatively small portion of a galaxy that is believed to be home to at least 100 billion stars of dramatically different masses.

Study abstract:

We present a sample of 254,882 luminous red giant branch (LRGB) stars selected from the APOGEE and LAMOST surveys. By combining photometric and astrometric information from the Two Micron All Sky Survey and Gaia survey, the precise distances of the sample stars are determined by a supervised machine-learning algorithm: the gradient-boosted decision trees. To test the accuracy of the derived distances, member stars of globular clusters (GCs) and open clusters are used. The tests by cluster member stars show a precision of about 10% with negligible zero-point offsets, for the derived distances of our sample stars. The final sample covers a large volume of the Galactic disk(s) and halo of 0 < R < 30 kpc and ∣Z∣ ≤ 15 kpc. The rotation curve (RC) of the Milky Way across the radius of 5 ≲ R ≲ 25 kpc has been accurately measured with ∼54,000 stars of the thin disk population selected from the LRGB sample. The derived RC shows a weak decline along R with a gradient of −1.83 ± 0.02 (stat.) ± 0.07 (sys.) km s−1 kpc−1, in excellent agreement with the results measured by previous studies. The circular velocity at the solar position, yielded by our RC is 234.04 ± 0.08 (stat.) ± 1.36 (sys.) km s−1, again in great consistency with other independent determinations. From the newly constructed RC, as well as constraints from other data, we have constructed a mass model for our Galaxy, yielding a mass of the dark matter halo of M200 = (8.05 ± 1.15) × 1011 M⊙ with a corresponding radius of R200 = 192.37 ± 9.24 kpc and a local dark matter density of 0.39 ± 0.03 GeV cm−3.

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