Hubble and Gaia accurately weigh the Milky Way

Mindy Sparks
March 10, 2019

Using an airborne infrared telescope, the astronomers counted pixels of dust in the faraway galaxy to estimate that roughly 50 million to 60 million suns' worth of mass were caught up in the Cigar Galaxy's mighty celestial wind. Using data from NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite, astronomers have determined the most accurate measurement of its mass: Our vast galaxy clocks in at 1.5 trillion solar masses. Therefore measuring the sideway motions directly significantly reduces the size of the error bars for the mass.

The Milky Way contains an estimated 200 billion stars. And Watkins tells Dvorsky at Gizmodo that Gaia, which is expected to map the sky for another decade, will continue to reveal more globular clusters and help astronomers continue refining the weight estimate of the galaxy. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well.

ESA's Gaia satellite is a space telescope created to measure the positions of billions of stars with unprecedented precision.

The global team of astronomers in this study consists of Laura L. Watkins (European Southern Observatory, Germany), Roeland P. van der Marel (Space Telescope Science Institute, USA, and Johns Hopkins University Center for Astrophysical Sciences, USA), Sangmo T. Sohn (Space Telescope Science Institute, USA), and N. Wyn Evans (University of Cambridge, UK). The clusters move because they feel a gravitational force - they only know the total force they feel, not how much of that force came from one type of object and how much came from another.

Located in the constellation of Hercules, about 230 million light-years away, NGC 6052 is a pair of colliding galaxies. © ESA/Hubble & NASA, T. Sohn et al. Now, the results of the research on Milky Way's mass can open up to new answers to many astrological questions.

Plait reports that the team had to estimate the mass of the galaxy beyond the 130,000 light year mark, especially the halo of dark matter that is believed to surround it.

Researchers came up with the figure by combining data from 46 globular clusters, masses of hundreds of thousands of stars that swarm around the Milky Way's centre. A team of researchers from ESO, the Space Telescope Science Institute, the Johns Hopkins University Center for Astrophysical Sciences and the University of Cambridge combined observations from the NASA/ESA Hubble Space Telescope and ESA's Gaia satellite to study the motions of globular star clusters that orbit our Galaxy. The rest is composed of dark matter, the undetectable stuff that makes up 80 percent of the universe.

"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, head of the Wide Field Infrared Survey Telescope mission office at the Space Telescope Science Institute, said in a statement.

The Milky Way isn't exactly the easiest thing to study, even if it's our galaxy.

These findings would also help us understand the history and future of the Milky Way. But as Phil Plait at Bad Astronomy blog notes, measuring the mass of the Milky Way is much more hard because we are inside it and can't get the big picture, literally. "We're inside the Milky Way, stuck about halfway out from the center, and everything we learn about it we learn from right here".

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