Celestial cartography is in for a dramatic upgrade. The Gaia spacecraft, which was launched in late 2013 by the European Space Agency, is on a mission to chart the heavens in unprecedented detail. By the end of its five-year-long run it will pinpoint the positions of one billion stars on the sky with an uncertainty as small as five micro–arc seconds—roughly twice the size of a quarter sitting on the moon as seen from Earth—hundreds of times better than today’s best catalogue. But to top it off, its one-billion-pixel camera will also chart each star’s distance and two-dimensional velocity, providing a five-dimensional map of the Milky Way Galaxy. (Three-dimensional velocities will be given for the brightest 100 million stars.). To Kathryn Johnston, an astronomer at Columbia University, creating this star chart is like mapping Earth’s continents for the first time, transforming an image of a vague green blob into a three-dimensional 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,” says Johnston, who finds it stunning that astronomers are still unclear on the precise distance to the Milky Way’s center.
But that is about to change. On Wednesday the Gaia team will release the positions of roughly one billion stars and the distances and proper (or sideways) motions of the two million brightest stars in the sky (with the exception of a few). Although those two million stars are not necessarily the closest, they will all be located somewhere in our galactic neighborhood. And with each subsequent data release (planned once a year until 2019, with a final release in 2022) astronomers will chart stars at greater and greater distances within the galaxy, creating successive maps that radiate outward from the sun like ripples on a pond. Therefore, David Hogg, an astronomer at New York University, speculates that this week’s data release will mostly impact stellar physics—that is, the internal dynamics within stars—whereas later data will affect galactic physics—the study of our entire galaxy. Or to frame it within Johnston’s analogy, it is like understanding those nearby mountains, rivers and valleys before later seeing the continent as a whole and understanding how it formed in the first place.
So although astronomers across the globe are anxiously awaiting the data, stellar astronomers just might get their hands dirty first. Hogg suspects, for example, that Gaia’s first data release will hold the resolution to a recent 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, is far from trivial. Without a correct distance, astronomers cannot determine the stars’ luminosities or radii with any certainty. “And those two things strongly constrain how stars work as machines,” Hogg says. To boot, the Pleiades is a benchmark cluster for understanding how stars form. “The theory for young stars is quite tricky,” says Timo Prusti, Gaia project scientist. “Because they're unstable there are many alternatives. So one really needs to have precise observations to constrain these models.”
But this first batch of data will also likely lead to surprises. “Astronomers think we understand how stars work pretty well,” says David Spergel, an astronomer at Princeton University. “That was one of the great accomplishments of the ‘50s, ‘60s and ‘70s. But I suspect that when our data gets better, we'll discover that while we have the basic picture right, there are things we thought we understood that we don't.”
Still, most astronomers are waiting in anticipation for Gaia’s future observations. With two primary mirrors that can collect 30 times the amount of light as Hipparcos and a camera that has a resolution over 50 times greater than the one onboard the Hubble Space Telescope, it is no wonder astronomers are excited. “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,” Hogg says. After its five-year run Gaia will release the three-dimensional positions and two-dimensional velocities for a full one billion stars, stretching out to the halo of the Milky Way—a large sphere of stars that enshrouds the galaxy’s disk and spiral arms. And it is here that the 2,000 kilogram-spacecraft will really shine.
Gaia’s main scientific goal is to unravel the structure and dynamics of the Milky Way in order to shed light on the violent history of our galaxy. Some of its 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. “It's like going to a crime scene,” Johnston says. “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.” But this history is far from clear, and many details—like how often the Milky Way munched on these dwarf galaxies—remain elusive. So Gaia will search for more streams in order to help astronomers piece together the biography of the Milky Way.
Ultimately, Gaia’s legacy is impossible to pin down in advance. “It's observing a huge number of stars and we're going to find all sorts of things among those stars that we didn't expect,” Hogg says. In addition to stars, the tiny satellite is expected to detect thousands of exoplanets and may even map the invisible dark matter that inhabits the Milky Way. Among its discoveries will certainly be surprises. Hogg plans to head to his office Wednesday morning to search for anomalous stars (those that appear excessively faint or bright or are moving too rapidly or sluggishly) within the new Gaia catalogue. “Those will be stars that have unusual origins or point to missing ingredients in stellar or galactic physics,” he says. “And so 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. People are excited because it's a new world. And the first data release is just a teaser for that new world.”