Gaia is creating an incredibly precise three-dimensional map of more than a billion stars in the Milky Way and beyond. It's mapping their motion, luminosity, temperature and composition. This massive stellar census will provide the data needed to answer a series of important questions about the origin, structure and evolutionary history of the Milky Way.

The European Space Agency (ESA) and Gaia collaboration have unveiled new data in the form of five key product releases. The results are very exciting and are also the first preview of the fourth version of the Gaia star catalog (GaiaDR4), which is expected to be released at the end of 2025.

The Gaia satellite has been mapping the sky since 2014, and its maps include stars that are a million times dimmer than what can be seen with the naked eye. The most complete and detailed third version of the star catalog (GaiaDR3) will be released in June 2022, which is a milestone in astrophysics research. The exceptionally precise stellar distances, motions and fundamental parameters of Gaia, as well as the classification of quasars and precise astrometry of asteroids, are now part of the daily work and research of most astronomers.

Artist's view of the Gaia satellite in front of the Milky Way. Image credit: ESA/ATGmedialab; Background: ESO/S: ESO/S.Brunier

The globular star cluster Omega Centauri is one of the highlights of the newly released data. The cluster contains about 10 million stars and appears very dense in the sky, posing a challenge for Gaia to resolve them. Consider that GaiaDR4 is expected to employ various technologies to process data in the densest and most interesting areas. Reprocessing of the Omega Centauri cluster restored astrometric and photometric measurements of an additional 526,587 stars at its core.

Although Gaia has observed the same stars multiple times over different epochs, ESA has so far only published average measurements. This does not prevent Gaia from identifying variable stars, but the characteristics such as instantaneous photometry and radial velocity used in its classification process have not yet been published. While we wait for Gaia DR4, all observational data (dural and average) will be released, with some release of the highest quality time series data acquired for 9164 long-period variable stars as part of these key product releases, which will help the scientific community prepare for the large amount of data Gaia will provide in 2025.

The ESA-Gaia spacecraft has repeatedly observed the spectra of an unprecedented number of cold giant stars, known as Mira variables, whose surfaces are known to expand and contract over long periods of time, sometimes more than a year. This artist's impression of the star Mira shows how the speed of motion on the star's surface and its atmosphere can be precisely measured by the movement of dark lines (Doppler) observed in Gaia's detailed spectrum. Source: Royal Observatory of Belgium

The space between stars is not completely empty. It is filled with low-density gas and dust composed of atoms, ions and molecules. This interstellar material absorbs and scatters light, causing the star's light flux to continue to redden and weaken. Other broad features appear in stellar spectra, known as "interstellar diffusion bands." They are caused by the absorption of very complex molecules found in the interstellar medium in certain directions. Such interstellar dispersions exist within the wavelength range of the Gaia Radial Velocity Spectrometer, which tracks the formation of the galactic disk and its spiral arms.

For three gravitationally lensed quasars (from left to right: H1413+117, J2240+0321 and J1310-1714), we show a comparison of the position of GaiaDR3 (without special treatment for the quasar environment) and the ground-based PanSTARRS image in the upper row. The next row shows a composite image reconstructed after the launch of the Gaia Focus product (which performed quasar environment analysis). Observation data taken from the ground may be somewhat blurry due to the influence of the atmosphere. With Gaia's ultra-high resolution, images of these gravitational lensing sources become clearer.

At one end of the distance scale, near the limit of our observable universe, Gaia detects quasars. Some of these quasars may be so close to massive galaxies in the sky that their light paths are bent by the galaxy's gravitational well, like passing through a lens. The gravitational mirage produced by lensing can be used to directly estimate the age and expansion rate of the universe. Over the past few months, the Gaia team has discovered 381 new candidate lensed quasars and mirages.

At the other end of the distance scale, the Gaia team reprocessed 156,764 asteroids, but using 66 months of data instead of DR3's 34 months. As a result, observations of most main-belt asteroids now cover a full circle around the Sun, closing the orbit and thus greatly improving orbit accuracy.

Contributions and future prospects

The expertise of astrophysicists at the Université Leuven in Belgium, the Royal Observatory of Belgium, the Université Libre de Bruxelles, the University of Antwerp and the University of Liège played an important role in processing and analyzing Gaia data and, in particular, in implementing key product releases. Their work was supported by the Belgian Federal Science Policy Office (BELSPO) through the ESA PRODEX programme. In addition to providing new data to complement the third version of the Gaia Star Catalog, the key product launch also provides a proof-of-concept for various new features implemented in the data analysis pipeline, doubling the data volume of DR4. They provide an essential and useful overview of all the promises that the Fourth Edition of the Star Catalog will deliver.