From Science Magazine: “European satellite reveals motions of more than 1 billion stars and shape of the Milky Way”

Science Magazine

Apr. 25, 2018
Daniel Clery

The Large Magellanic Cloud, one of the Milky Way’s nearest neighbors, may be more massive than previously thought. The image is not a photograph, but rather a map of the density of stars detected by Gaia in each pixel.

Large Magellanic Cloud. Adrian Pingstone December 2003

ESA/GAIA satellite

“It’s like waiting for Christmas,” said Vasily Belokurov, an astronomer at the University of Cambridge in the United Kingdom last week. Today, the gifts arrived: the exact positions, motions, brightnesses, and colors of 1.3 billion stars in and around the Milky Way, as tracked by the European Space Agency’s (ESA’s) €750 million Gaia satellite, which after launch in 2013 began measuring the positions of stars and, over time, how they move. On 25 April, ESA made Gaia’s second data set—based on 22 months of observations—publicly available, which should enable a precise 3D map of large portions of the galaxy and the way it moves. “Nothing comes close to what Gaia will release,” Belokurov says.

One might think that the galaxy is completely mapped. But large parts of it are obscured by gas and dust, and it is hard to discern structure from the vantage of the solar system. Gaia is not only expected to clarify the spiral structures of the galaxy today, but because the satellite traces how stars move, astronomers can wind the clock backward and see how the galaxy evolved over the past 13 billion years—a field known as galactic archaeology. With Gaia’s color and brightness information, astronomers can classify the stars by composition and identify the stellar nurseries where different types were born, to understand how chemical elements were forged and distributed.

Gaia isn’t only about the Milky Way. For solar system scientists, the new data set will contain data on 14,000 asteroids. That’s a small fraction of the roughly 750,000 known minor bodies, but Gaia provides orbit information 100 times more accurate than before, says University of Cambridge astronomer Gerry Gilmore, who heads the U.K. branch of Gaia’s data processing consortium. That should help astronomers identify families of asteroids and trace how they relate to each other, shedding light on the solar system’s past and how planets formed from smaller bodies.

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