Jenni Virtanen

TL;DR: Gaia as discussed by the authors is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach.

TL;DR: The first Gaia data release, Gaia DR1 as discussed by the authors, consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the Hipparcos and Tycho-2 catalogues.

TL;DR: The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS) component by means of the astrometric data for open clusters as discussed by the authors, which is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho 2 positions in 1991.

TL;DR: The first Gaia Data Release contains the Tycho-Gaia Astrometric Solution (TGAS) component by means of the astrometric data for open clusters as mentioned in this paper, which is a subset of about 2 million stars for which, besides the position and photometry, the proper motion and parallax are calculated using Hipparcos and Tycho 2 positions in 1991.

TL;DR: In this paper, the first parallax measurements of the primary standard candles of the cosmological distance ladder, that involve astrometry collected by Gaia during the initial 14 months of science operation, are published in Gaia Data Release 1 (DR1) as part of the Tycho-Gaia Astrometric Solution (TGAS).