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Gaia Maps 1.7 Billion Stars, Widens Cosmic Census.

ON APRIL 25TH, European astronomers published the second release of data (DR2) from the European Space Agency's Gaia satellite. The resulting catalog is the most extensive and precise yet, containing data on 1.7 billion stars.

Based on 22 months of data collection, Gaia's DR2 consists of precise parallaxes, and thus geometric distances, to more than 1.3 billion stars, as well as positions and brightnesses of almost 1.7 billion stars. That's a huge leap compared to the mission's first data release in 2016 (S&T: Jan. 2017, p. 10), which contained 2 million stellar distances. Moreover, the newly published distances rely solely on Gaia's own measurements--in DR1 Gaia's measurements had to be augmented by data from the 1990s-era Hipparcos satellite observations.

The satellite spins continuously around its axis as it orbits the Sun, enabling its two telescopes to scan great circles on the sky, observing about 100,000 stars every minute. The optics feed three instruments: one for astrometry (to determine stars' positions and proper motions across the sky), one for photometry (to measure the stars' colors and effective temperatures), and one for spectroscopy (to measure bright stars' radial velocity toward or away from Earth and to assess their composition). At Gaia's heart is a CCD with 848 million pixels, the largest digital camera ever used in space.

Thanks to spectrometry, Gaia's DR2 contains estimates of the effective temperature, radius, and luminosity of 76 million stars, as well as time-dependent measurements for more than 550,000 variable stars. Closer to Earth, Gaia also observed about 14,000 known solar system objects, most of them asteroids. On the other end of the distance scale, the database contains positions and brightnesses for more than half a million quasars. Their near-zero parallax means they serve as useful references for Gaia's celestial coordinate system.

More data releases will follow DR2, with the final Gaia catalog--the definitive stellar catalog for the foreseeable future--scheduled to be published in late 2022.

For additional images and an animated view of the Hyades star cluster, visit https://is.gd/gaiaDR2.

Caption: This graphical representation of Gaia's all-sky data is based on measurements of 1.7 billion stars.

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Title Annotation:STARS
Author:Hattenbach, Jan
Publication:Sky & Telescope
Date:Aug 1, 2018
Words:360
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