Gaia delivers a trove of data revealing secrets of the Milky Way

Astronomers are going gaga over Gaia.

The April 25 release of data from the European Space Agency’s Gaia spacecraft, which cataloged nearly 1.7 billion stars, has kicked off a scientific spree, with multiple papers published online in the last two weeks at arXiv.org.

Charting stars in the Milky Way and beyond, Gaia surveys the entire sky. The spacecraft can measure stars’ motions and distances (SN Online: 4/25/18), properties which haven’t been inventoried on such a large scale before. “It’s really opening new dimensions in how we view stars,” says astronomer Ana Bonaca of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
Because Gaia takes multiple images over time, “you’re not only getting a static picture of the sky at one instant, you’re looking at how it changes,” says astronomer Laura Watkins of the Space Telescope Science Institute in Baltimore. “We’ve never really had something like this before.”

Here are five new observations made with the unprecedented info.

1. Sizing up the Milky Way
   Pinning down the mass of our home galaxy is a hefty challenge. Much of the Milky Way’s mass is hidden in the form of a dark matter halo, a shroud of matter that is invisible except for its gravitational pull. But scientists can gauge the galaxy’s unseen bulk by observing objects moving at the outskirts of the galaxy.

Combining information from Gaia and the Hubble Space Telescope, Watkins and colleagues estimated the galaxy’s mass using the motions of clumps of stars called globular clusters. The Milky Way is about 1.7 trillion times the mass of the sun, the team reports in a paper submitted April 30.

2. Rescaling exoplanets
   Exoplanet updates are also on the agenda. Because NASA’s exoplanet-hunting Kepler telescope has limited ability in gauging how big stars are, the diameters of exoplanets passing in front of those stars were not well known (SN Online: 6/19/17). “Gaia has now completely changed the game and solved this problem,” says astronomer Daniel Huber of the University of Hawaii at Manoa.

Knowing both the brightness and distance of a star helps determine its size. So Huber and colleagues used Gaia’s data to better size up nearly 200,000 stars and more than 2,000 orbiting planets, the researchers report in a paper submitted May 1.

3. Expanding a cosmic debate
   A disagreement over how fast the universe is expanding persists (SN Online: 1/16/18). Gaia data reinforced the discrepancy in results between two methods for measuring the expansion rate.

One of those techniques involves estimating the distances of exploding stars, or supernovas, and measuring how their light is stretched by the expansion of space. Gaia improved distance estimates for variable stars called Cepheids, which scientists use to estimate how far away the supernovas are. The result: The expansion rate mismatch is now slightly worse, Adam Riess of the Space Telescope Science Institute and colleagues report in a paper submitted April 27.

4. Dipping into star streams
   The Milky Way is a violent beast, ripping up clumps of stars and stretching them into strands known as stellar streams. Bonaca and astronomer Adrian Price-Whelan of Princeton University study the longest thin stream in the Milky Way, known as GD-1, in a paper posted May 1.

Gaia’s measurements of stars’ motions, combined with information about their brightness and color from the Hawaii-based Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS, allowed the duo to pinpoint which stars were going with the flow of the stellar stream. The data also revealed gaps where stars seem to be missing. That could indicate the stream was disturbed in the past by a close encounter with a clump of dark matter.

5. Spotting speed demons
   Several teams used Gaia to pick out fast-moving stars, zipping through the galaxy at speeds of more than 1,000 kilometers per second. A team including Ken Shen of the University of California, Berkeley seized on this capability to look for clues to the origins of a kind of explosion called a type 1a supernova, thought to occur when a dead star known as a white dwarf explodes.

Scientists don’t know exactly what causes a white dwarf explosion. In one theory, two white dwarfs swirl around one another as one steals material from the other. The thief eventually explodes and its partner is flung away at high speed.

Shen and colleagues wasted no time in hunting for these fast-moving white dwarfs. Within an hour and a half of the Gaia data release, the team had the first of several ground-based telescopes taking a closer look at some of the speed demons. Three stars potentially fit the bill for coming from a type 1a supernova, the team reports in a paper submitted April 30.

This is only the beginning of the Gaiapalooza, though. The data is so rich, Watkins says, “it’s going to take us months and years to get to grips with what’s there.”