Astronomers are about to unfold a new map of cosmic reality. The Gaia spacecraft, launched in late 2013 by the European Space Agency, is on a five-year mission to chart the heavens in unprecedented detail—and the first set of coordinates has been released. By the end of Gaia's run, it will have pinpointed the positions of approximately one billion stars in the Milky Way and nearby galaxies with a resolution so high it can spot objects as small as five microarcseconds—roughly half the size of a dime sitting on the moon as seen from Earth. Its billion-pixel camera will also record each star's distance and two-dimensional velocity, providing a fresh understanding of our galactic neighborhood.
To Kathryn Johnston, an astronomer at Columbia University, creating this star chart is analogous to mapping Earth's continents for the first time—transforming an image of a vague green and blue blob into a world with mountains, rivers and valleys. “In a strange way, we almost don't know what our galaxy looks like as accurately as we know other galaxies,” Johnston says, explaining that it seems nearly impossible to take a picture of a whole galaxy when you are stuck inside of it. But that is exactly what Gaia will do.
The updated map has been highly anticipated; on the first day of the initial data release in September at least 10,000 people accessed the archive, says Gaia project scientist Timo Prusti. The dataset includes the preliminary positions of one billion stars (future data releases will improve these numbers) and the distances and sideways motions of the two million brightest stars in the sky. With each subsequent release, the distances and motions of stars at greater and greater distances within the galaxy will be revealed, creating successive maps that radiate outward from the sun like ripples on a pond.
Findings are already pouring in. For instance, Gaia scientists have used the preliminary results to resolve a controversy over the distance to the Pleiades star cluster—the famous “Seven Sisters.” The debate, which was sparked by the final data release from Gaia's precursor, the Hipparcos mission, was far from trivial: without a correct distance, astronomers could not determine the stars' luminosities or radii with any certainty. A precise measurement (Hipparcos' was wrong) is also important because the Pleiades embody a benchmark cluster for understanding how stars form. “The theory for young stars is quite tricky,” Prusti says. “Because they're unstable, there are many alternatives. So one really needs to have precise observations to constrain these models.”
Other research groups are using the new data to investigate unusual stars (those that appear excessively faint or bright or are moving too rapidly or sluggishly). “Astronomers think we understand how stars work pretty well,” says David Spergel, an astronomer at Princeton University. “But I suspect that when our data get better we'll discover that while we have the basic picture right, there are things we thought we understood that we don't.” Planetary physics is also involved as astronomers search for stars that host interesting planets. Although Gaia has not yet discovered any such worlds, scientists hope the spacecraft will ultimately detect thousands or even tens of thousands.
Despite the September bounty, astronomers are still eagerly awaiting Gaia's next observations (there will be four more releases). “Although there is lots of science we can do with the first data release, it's nothing like the science we're going to be able to do at the end of the mission,” says David Hogg, an astronomer at New York University. With the full dataset in 2022, researchers will be able to tackle the mission's main scientific goal: to unravel the structure and dynamics of the Milky Way and to illuminate its violent history. For instance, some of the Milky Way's stars were born within smaller galaxies, which were later cannibalized by our monstrous galaxy. Today the remnants of those puny galaxies can be seen in the form of faint streams of stars that stretch across the sky, providing clues about the timeline of our neighborhood's evolution. “You're finding galaxies that were alive in the past, you're finding the orbits that they were on and you're finding the stars associated with them. So you can build up the history of our galaxy eating other galaxies,” says Columbia's Johnston.
Ultimately Gaia's full legacy is impossible to pin down in advance. On top of its main mission, the satellite will also observe thousands of nonstar objects in the solar system, possibly map the distribution of dark matter in the Milky Way, and chart the positions of hundreds of thousands of quasars—the blazing cores of ancient galaxies. In the long term, Gaia will also improve observations from other telescopes because the instruments will know exactly where to look, Prusti says. In the meantime, Hogg has organized “Gaia Sprints” in New York City and Heidelberg for all types of astronomers to come together and explore the data in a collaborative environment. “I think that the right way to think about it—and the real reason everyone is excited—is that it's an opportunity for discovery,” Hogg says. “People are excited because it's a new world. And the first data release is just a teaser for that new world.” —Shannon Hall, with additional reporting by Sara Goudarzi
By the numbers
1.5 million kilometers
The Gaia spacecraft's distance from Earth
Resolution of Gaia's camera compared with the one onboard the Hubble Space Telescope
Amount of light Gaia's primary mirrors can collect compared with the star-mapping satellite Hipparcos
Number of times Gaia will observe each of its one billion targets