The most accurate map of the Milky Way gets even richer

The European Space Agency (ESA) has just released the latest version of the most accurate map of the Milky Way.

In addition to the position of the stars, their movement, their brilliance and their color, it contains more and more details on their physical properties such as their surface temperature, their chemical composition, their age in particular, which make it possible to address major scientific questions. .

This information makes it possible to understand the history of our galaxy, and in particular the impact of the accretion (or "fusion") mechanisms of dwarf galaxies on the formation and evolution of the Milky Way. This new catalog also offers the largest catalog ofbinary stars, gives the properties of millions of variable stars, information on the interstellar matter, but also the characteristics ofasteroids in the solar system and those of galaxies and quasars in the very distant Universe.

It thus offers a harvest of useful data to all disciplinary fields of astrophysics.

The European Space Agency's Gaia astrometric satellite was launched on December 19, 2013. It has since been performing a systematic scan of the sky with the aim of mapping it, for an initially planned duration of 5 years. The technological conditions allowing the operation of the mission being still operational, it will be continued until 2025.

Astrometry is the oldest branch of astronomy, which aims to measure the positions and movements of the stars. On the celestial vault, we obviously do not measure distances using a meter, everything translates into an angle measurement, between two stars, or between a star and a reference position defined on the sky. Gaia's strength is its ability to measure tiny angles. Equipped with two telescopes whose relative position is very stable, as well as a detector with 1 million pixels, and located outside the Earth's atmosphere which blurs the observation, the Gaia satellite can thus resolve such small angular details only three billionths of a degree (the size of a one euro coin seen from the Moon). This unrivaled precision allows Gaia to measure the position of the stars and their displacement on the celestial vault, and to estimate their distance by the parallax method, for nearly two billion stars in the Milky Way.

The movement of the Earth around the Sun in one year induces an apparent motion of the stars, called the parallax effect. This apparent displacement is inversely proportional to the distance of the star: the closer it is, the greater its displacement seems, in the same way that the tree near the railway seems to move more than the distant mountain for the traveler. by train. The nearest star, Proxima Centauri, has an apparent displacement which covers a very small angle: take the degree, the small graduation of a protractor, and divide it by 5140, and we will have an idea of ​​its apparent displacement. during the year.

An extraordinary harvest of data

Satellite data is processed on the ground by the Data Processing and Analysis Consortium (DPAC), in which many French teams are involved at all levels of the data processing chain. It emerges from successive catalogs, called DR for "data release", which are published during the mission: DR1 in September 2016, DR2 in April 2018, then DR3 in June 2022.

The first three catalogs have already had a strong impact on all fields of the astrophysical discipline, with nearly 5 results published in specialized scientific journals (discoveries of new stars by the thousands such as white dwarfs, brown dwarfs, asteroids, etc.). Each new catalog offers the promise of new discoveries. It brings precision, accuracy and homogeneity that constitute major advances in the knowledge of the Milky Way, and beyond.

A long way between observations and published data

The production of each catalog is a project in its own right. It brings a new level of complexity that requires the design and implementation of innovative methods in data processing. The reasons for this are the increase in the number of observations to be processed, the production of new astrophysical parameters with each version, as well as the improvement in the precision of the measurements which requires the consideration of ever finer effects.

Finally, each catalog requires the validation of more and more precise data, for example by comparing with other observations or with simulations calculated from models. Thus, more than two years pass between the moment when the last observations are acquired and the delivery of the catalog to the scientific community (to be more precise, the catalog is free of access to all, not only scientists, even if it is they above all who will exploit it).

The adventure goes on

The DPAC is now working on the production of DR3 which is scheduled for release in the spring of 2022. It will be followed by two others, DR4 at the end of 2025 and DR5 at the end of 2030. A further jump will be made with these last catalogs, with new products added. DR3 will thus be completed by physical parameters such as temperature, radius, mass, etc. of 300 million stars, light curves of seven million variable stars, orbital parameters of binary stars, morphological classifications of two million galaxies and quasar, a catalog dedicated to the Magellanic Clouds. DR4 meanwhile will be accompanied by the highly anticipated catalog of tens of thousands of exoplanets, mainly gas giant planets.

Celine Reylé, Astronomer at the UTINAM Institute, Observatory of Sciences of the Universe THETA Franche-Comté Bourgogne., Burgundy Franche-Comté University (UBFC)

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