Gaia: Census of the Milky Way

1.5 million kilometers from us follows one of the most important tools of astronomy on Earth: the European space observatory Gaia. Its goal is to map the billions of stars in our cosmic home, the Milky Way. Now astronomers have come a step closer to this goal: With its third data catalogue, Gaia provides data on the movement, temperature, chemistry and age of almost two billion stars. Also included in the package is the most comprehensive chemical map of our galaxy to date, revealing the elemental composition of stars and hence the galaxy’s ‘chemical DNA’. The new data provide unique new insights into the structure and evolution of our Milky Way.

The Milky Way is our cosmic home, our sun is one of more than 100 billion stars in this galaxy. However, so far we only know parts of their structure, composition and eventful history. Because from our position many parts of the Milky Way are difficult to see, they are obscured by clouds of dust and gas or by the galactic center. In addition, the sheer number of celestial bodies and other cosmic phenomena makes it difficult to get an overview. To provide more clarity, the European Space Agency ESA sent the Gaia space observatory into space in 2013. Since then, this satellite, positioned at Lagrange point 2, around 1.5 million kilometers behind Earth, has been surveying our home galaxy with the help of several telescopes and sensors. The observatory targets hundreds of cosmic objects per second and collects information about their distance, spectrum and movement. The aim of the mission is to map the Milky Way in three dimensions as accurately and completely as possible.

Features and positions of 1.8 billion stars mapped

Now ESA has completed and published the third data catalog of the Gaia mission. It contains new details about the roughly 1.8 billion stars, including data on their chemical composition, stellar temperatures, colors, masses, and the age and speed at which stars are moving. “This data release represents a major step forward in our journey towards a detailed ‘census’ of the Milky Way and already characterizes a substantial part of its stellar inhabitants,” says Nicholas Walton of the University of Cambridge. “Similar to the 100,000 Genomes project in biology, we can now characterize hundreds of millions of stars and this allows us to trace their life cycles and better understand the incredible history and future of our Milky Way.” Also thousands of Solar System objects such as asteroids and moons of planets and millions of galaxies and quasars outside the Milky Way are included in the new dataset.

One of the most surprising discoveries in the new data, even for astronomers, is that the Gaia satellite can even detect starquakes – tiny movements on a star’s surface that change its shape. The observatory has previously detected radial oscillations that regularly cause stars to swell and shrink. Meanwhile, Gaia has also discovered non-radial oscillations that deform the entire star. They pass through the star like a mighty tsunami. “Starquakes teach us a lot about the stars, especially their inner workings. Gaia is a gold mine for ‘asteroseismology’ of massive stars,” says Conny Aerts of the Catholic University of Leuven in Belgium.

A chemical map of our galaxy

Also part of the new Gaia data set is the largest chemical map of our galaxy to date. It shows the elemental composition of stars and their distribution from the Sun’s neighborhood to the farthest outskirts of the Milky Way. Because the interstellar medium in the early days of the cosmos contained almost only hydrogen and hardly any heavy elements, and larger atoms only gradually appeared as a result of supernovae, the so-called metal content of the stars also allows conclusions to be drawn about the time of their formation and their origin. “Gaia’s chemical mapping is comparable to the sequencing of the human genome,” explains George Seabroke of University College London. “The more stars we can characterize chemically, the better we can understand our galaxy as a whole. The current chemical catalog contains six million stars, ten times larger than any previous Earth-based catalogue, which is truly revolutionary.”

(Video: ESA/Gaia/DPAC)

In addition to their characteristics, the new data set also shows how stars move across the sky, as well as their radial velocity – the speed at which they are traveling towards or away from us. This allows astronomers to analyze the three-dimensional motion of these stars in detail. This allows conclusions to be drawn, for example, about their place of origin, their migration or also close passages to other star systems. Also included in the Gaia dataset is a new catalog of more than 800,000 binary star systems in the Milky Way, as well as new asteroid mapping that captures the position and movement of 156,000 rocky objects in our solar system. The data package also contains more detailed information about ten million variable stars, the dust in the interstellar medium and about quasars and galaxies outside our galaxy.

“Unlike other missions that target specific objects, the Gaia mission is a survey mission. This means that Gaia will inevitably make discoveries that other, more specialized missions miss,” says Timo Prusti, of the European Space Agency ESA. “It’s one of its strengths and we can’t wait for astronomers to dive into the new data and find out even more about our home galaxy and its surroundings.”

Source: European Space Agency (ESA); Gaia Data Release 3