![]() ![]() By the end of Gaia’s mission, the Hipparcos data will be superseded by Gaia’s more precise measurements. For now, it contains 2 million stars, measured twice as precisely as by Hipparcos alone. The Tycho-Gaia Astrometric Solution (TGAS) is the first step in an updated map that will eventually contain a star’s position in 3-D, plus its motion, more accurately than ever before. So the Gaia team combined the final Hipparcos catalog, called Hipparcos-Tycho 2, with Gaia’s data thus far. That extended timeline was just what Gaia needed. The Hipparcos catalog knows where these stars were 20 years ago, and Gaia knows where they are now, to even better precision. But the Hipparcos satellite has already done this work once, for millions of Gaia’s target stars. From only the first year of data, it’s impossible to untangle the motion of the stars from the telescope’s own motion as it follows Earth’s orbit around the Sun. Gaia measures the motions of stars to determine their distance. It will measure the positions and motions of more than 1 billion stars across the whole sky, down to 20th magnitude, with an accuracy of a few millionths of an arcsecond, crushing previous surveys.įor this first data release, though, Gaia needed some help. For 10 million objects, its margin of error will be less than 1 percent. (For an in-depth overview of Gaia’s design and mission, check out Astronomy’s December 2014 story, “How Gaia will map a billion stars.”)Įventually, Gaia’s goal is to plot the distance of 100 million stars to better than 10 percent accuracy. Finally, the spectrometer measures the Doppler shift of particular chemical signatures to reveal whether stars are moving toward or away from Earth. Its photometric camera measures the brightness of stars at both red and blue wavelengths, producing data about their temperatures and compositions. Its astrometric instrument charts positions and motions of stars with pinpoint precision by observing how they appear to move over the course of Gaia’s five-year mission. Gaia, orbiting around 932,000 miles (1.5 million kilometers) away and mapping the whole sky, carries three instruments. Gaia’s mission statement is to “map a billion stars,” but even this bold promise undersells the space observatory’s true abilities. ![]() The results are already changing astronomers’ perspective of our galaxy. But in September 2015, the Gaia science team released a first round of data from 14 months of observing, and welcomed scientists around the globe to jump in. Gaia won’t complete its mission until 2019, and the final data analysis won’t be available until years afterward. The ESA mission originally stood for “Global Astrometric Interferometer for Astrophysics.” Though many of the parameters changed, ESA kept the name for mission continuity. Gaia was a Greek goddess who was regarded as a sort of Mother Earth. In 2013, ESA launched Gaia, which will return even higher-precision data on over a billion stars. In 1989, the European Space Agency (ESA) launched the Hipparcos satellite to measure the positions of 2.5 million stars, a catalog that wasn’t released in full until 2000. And even tracking a star’s motion across the sky in two dimensions is difficult without years of data. We’ve come a long way from thinking of the stars as a two-dimensional projection on the sky, but measuring a star’s distance remains quite tricky. Over millennia, huge advances have been made in astronomy, but some surprisingly basic questions remain: Where exactly are the stars? Where, in the grand scheme of things, is Earth? What is the shape and structure of our home galaxy? Since ancient times, humans have stared at the sky, cataloged its residents, marked new arrivals, and charted the constant stars and wandering planets. Astronomy is often called the oldest branch of science.
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