Hubble & Gaia accurately weigh the Milky Way

Astronomers weigh the galaxy with Hubble and Gaia

Hubble & Gaia accurately weigh the Milky Way

To estimate the overall mass, the researchers analyzed data obtained by the ESA's Gaia satellite on 34 globular clusters - clumps of hundreds of thousands of stars that orbit the galaxy's spiral disk - plus 12 distant clusters studied by the Hubble space telescope.

In a collaboration between NASA and the European Space Agency's Gaia observation craft, a team of experts calculated our galaxy to be around 1.5 trillion solar masses.

The new 1.5 trillion solar mass estimate fits right in with previous research on the subject that put the mass of our galaxy at 500 billion to 3 trillion solar masses.

"We just can't detect dark matter directly", explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis.

When the Hubble and Gaia measurements are combined as anchor points, like pins on a map, astronomers can estimate the distribution of the Milky Way's mass out to almost 1 million light years from Earth. This is exactly what an global team of astronomers have done and instead of looking for something that has escaped all detection so far, they measured the velocities of globular clusters to work out the mass of the Milky Way.

"The more massive a galaxy, the faster its clusters move under the pull of its gravity", said N. Wyn Evans of the University of Cambridge, UK.

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Scientists have previously been able to measure the speed at which a globular cluster approaches or recedes from Earth along our line of sight but this information alone is quite limiting. Beating the Uncertainties For a long time, scientists were divided about the actual weight of the galaxy, due to the varying efforts used to measure the distribution of dark matter.

Ashley Strickland at CNN reports that a surprisingly small amount of that mass comes from the 200 billion stars in the Milky Way and the large, 4-million-solar-mass black hole they all swirl around. The ESA's Gaia mission is designed to create the most precise 3D map of the astronomical objects in the galaxy and track how they move over time. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe. Its second data release includes measurements of globular clusters as far as 65 000 light-years from Earth.

The project involved worldwide collaboration between scientists working for NASA and ESA on the Hubble and Gaia telescopes. So the new data from the two spacecraft looked at our galaxy's brightest satellites: globular clusters.

That extrapolated figure gives a mass for the Milky Way as a staggering 1.5 trillion solar masses (with one solar mass being the mass of our Sun). Now, scientists have solved one of the riddle of the Milky Way's weight by combining fresh data from the Gaia mission and the Hubble Space Telescope. By better understanding the universe close by, we can better comprehend the galaxies a few more light-years away. Since Hubble has been observing some of these objects for ten years, it was also possible to accurately track the velocities of these clusters as well.

If you have ever been to a school fete and seen a large, glass jar filled with candies, labelled "guess how many jelly beans there are", chances are you have studied it meticulously to try and work out that magic number.

"We were lucky to have such a great combination of data", explained Roeland P. van der Marel (Space Telescope Science Institute, USA). 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.

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