@article{10902/14560,
year = {2018},
month = {8},
url = {http://hdl.handle.net/10902/14560},
abstract = {Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. 
Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. 
Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. 
Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. 
Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.},
organization = {This work presents results from the European Space Agency (ESA) space mission Gaia. Gaia data are being processed by the Gaia Data Processing and Analysis Consortium (DPAC). Funding for the DPAC is provided by national institutions, in particular the institutions participating in the Gaia MultiLateral Agreement (MLA). The Gaia mission website is https: //www.cosmos.esa.int/gaia. The Gaia Archive website is http://gea.
 esac.esa.int/archive/. The Gaia mission and data processing have financially
 been supported by the Spanish Ministry of Economy (MINECO/FEDER, UE) through grants ESP2014-55996-C2-1-R, ESP2014-55996-C2-2-R, ESP2016-80079-C2-1-R, and ESP2016-80079-C2-2-R, the Spanish Ministerio de Economía, Industria y Competitividad through grant AyA2014-55216.},
publisher = {EDP Sciences},
publisher = {Astronomy & Astrophysics, 2018, 616, A10},
title = {Gaia Data Release 2: Observational Hertzsprung-Russell diagrams},
author = {Babusiaux, Carine and Leeuwen, F. van and Barstow, Martin Adrian and Jordi, C. and Vallenari, Antonella and Bossini, D. and Bressan, A. and Cantat-Gaudin, T. and Leeuwen, M. van and Brown, Anthony G.A. and Prusti, Timo and De Bruijne, Jos H.J. and Bailer-Jones, Coryn A.L. and Biermann, Michael and Evans, Dafydd Wyn and Eyer, Laurent and Jansen, F. and Klioner, Sergei A. and Lammers, Uwe and Carballo Fidalgo, Ruth},
}