The European Space Agency's Gaia mission has released its first data release, mapping a billion stars across the Milky Way and beyond
It’s a gold mine of data that astronomers will be sifting through for decades to come, and it’s just the beginning. At a press briefing in Madrid, Spain, European Space Agency (ESA) scientists presented the long-awaited first data release of the Gaia mission, containing accurate positions of roughly a billion stars in the Milky Way and nearby galaxies. Never before have astronomers measured such a quantity of stars to such a high precision.
“The DR1 data are an important point in our mission, but it’s just a starting point”, says Timo Prusti, ESA Gaia Project Scientist, addressing and inviting the scientific community: ”Please enjoy them with us!”
Mapping the Milky Way and Beyond
Gaia’s main goal is to create an extraordinarily precise three-dimensional map of more than a billion stars throughout our galaxy and beyond. To do this, Gaia will monitor each of these stars about 70 times over a five-year period. It’s looking for stellar parallax, the tiny apparent annual looping motion caused by our moving viewpoint on Earth.
Anthony Brown (Leiden University) presented the first version of the billion-star atlas, which gives star positions to a precision of a few microarcseconds for stars as faint as magnitude 20.7. That’s half a million times fainter than what the naked eye can see.
The catalog’s visual representation is an all-sky map with a spatial resolution comparable to that of the Hubble Space Telescope. “It’s like taking a Hubble image – just of the entire sky”, Brown explains.
But there’s more. The data release also contains three-dimensional data showing the motion of the 2 million stars in our neighbourhood bright enough for Gaia to measure their spectra and determine their speeds toward or away from Earth. The release also includes measurements of 3,200 RR Lyrae stars, variable stars vital to calculating the cosmic distance scale.
Gaia: Years in the Making
Almost three years after Gaia's launch on December 19, 2013, the mission is starting to deliver on its promise. Nevertheless, the full work will take years. The final batch of data is expected to be released in 2023, about three years after the space mission has ended. For a selection of stars, though, scientists have already combined Gaia data with older parallax measurements, to make more accurate distance measurements available today.
To measure stellar parallaxes, it’s necessary to pinpoint a star’s position to a very high degree of accuracy. In fact, the fact the technical infeasibility of measuring the minuscule movements of the apparently “fixed” stars in the 16th century, led Danish astronomer Tycho Brahe to reject the Copernican system of an Earth that orbits the Sun, rather than the other way around.
It wasn’t until the 1830s that German astronomer Friedrich Wilhelm Bessel published the first reliable stellar parallax of a nearby star, 61 Cygni. Even in 1990, astronomers still had only measured parallaxes for 800 stars. Then Gaia’s predecessor, ESA’s Hipparcos satellite, was launched. Undisturbed by Earth’s atmosphere, Hipparcos boosted the number of known parallaxes to about 50,000. But efforts stalled after Hipparcos’ mission ended in 1997 — a NASA-designed Full-Sky Astrometric Mapping Explorer was scrapped in 2002 for budget reasons.
More Than Stars
Now the long wait has ended. Gaia outclasses Hipparcos in measurement accuracy and quantity by a large margin. With future releases scientists also expect to discover thousands of exoplanets, tens of thousands of asteroids, and even hundred thousands of white and brown dwarfs.
Gaia shows that astrometry is more than a dull accountant’s job. Distances to the stars lay the foundation for virtually every other field of astronomy, astrophysics, and cosmology. Stellar parallaxes form the first and most fundamental step of the cosmic distance ladder. Without knowledge of the an object’s three-dimensional position in space, its physical properties can at best only be guessed at.
Under ESA’s policy, Gaia data are immediately available to the public. While raw data might only be of interest to professional astronomers, amateurs will enjoy Gaia Sky, real-time, 3D astronomy visualisation software that runs on Windows, Linux, and OS X. The Gaiaverse website also hosts an archive for Gaia’s big data. It’s a huge task: Gaia is expected to produce more imaging data in its five-year lifetime than the Hubble telescope did in its first 21 years!
Moreover, combining Gaia’s bonanza with other surveys, such as the recently released GALAH (Galactic Archaeology with HERMES) spectra of more than 250,000 Milky Way stars, will provide new insights for astronomers trying to understand the composition and evolution of our galaxy. Once completed, Gaia’s census of our galactic neighbourhood will be used by generations of astronomers to come.