“Gaia – The Dynamic Sky in 3D“ with Anthony Brown (Leiden Observatory, Leiden, The Netherlands)

The talk will consist of an overview of the Gaia mission and how it maps the Milky Way in 3D. This is followed by scientific highlights from the second Gaia data release, which took place in April 2018 and has resulted in over 3500 peer-reviewed publications to date. 

Gaia is global space astrometry mission, it will make the largest, most precise three-dimensional map of our Galaxy by surveying more than one billion stars. Gaia will monitor each of its target stars about 70 times over a five-year period. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500 000 distant quasars and will provide stringent new tests of Einstein’s General Theory of Relativity.

Gaia will create an extraordinarily precise three-dimensional map of more than a billion stars throughout our Galaxy and beyond, mapping their motions, luminosities, temperatures and compositions. This huge stellar census will provide the data needed to tackle an enormous range of important problems related to the origin, structure and evolutionary history of our Galaxy.

For example, Gaia will identify which stars are relics from smaller galaxies long ago ‘swallowed’ by the Milky Way. By watching for the large-scale motion of stars in our Galaxy, it will also probe the distribution of dark matter, the invisible substance thought to hold our Galaxy together.

Gaia will achieve its goals by repeatedly measuring the positions of all objects down to 20th magnitude. For all objects brighter than 15th magnitude, Gaia will measure their positions to an accuracy of 24 microarcseconds. This is comparable to measuring the diameter of a human hair at a distance of 1000 km. It will allow the nearest stars to have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic centre, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%. The vast catalogue of celestial objects expected from Gaia’s scientific haul will not only benefit studies of the Solar System and the Milky Way, but also address fundamental physics questions that underpins our entire Universe.

At its heart, the Gaia satellite contains two optical telescopes that work with three science instruments to precisely determine the location of stars and their velocities, and to split their light into a spectrum for analysis. During its originally planned five-year mission, the spacecraft spins slowly, sweeping the two telescopes across the entire celestial sphere. As the detectors repeatedly measure the position of each celestial object, they will detect any changes in the object’s motion through space.

Gaia is mapping the stars from an orbit around the Sun, at a distance of 1.5 million km beyond Earth’s orbit. This special location, known as the L2 Lagrangian point, keeps pace with Earth as we orbit the Sun. It offers a clearer view of the cosmos than an orbit around Earth, which would result in the spacecraft passing in and out of Earth’s shadow and causing it to heat up and cool down, distorting its view. Free from this restriction and far away from the heat radiated by Earth, L2 provides a much more stable viewpoint. 

Gaia is a fully European mission. The spacecraft is controlled from the European Space Operations Centre (ESOC, Darmstadt, Germany) using three ground stations in Spain, Argentina and Australia. Science operations are conducted from the European Space Astronomy Centre (ESAC, Villafranca, Spain). The Gaia Data Processing and Analysis Consortium (DPAC) process the raw data to be published in the largest stellar catalogue ever made.


Anthony Brown is an associate professor at Leiden Observatory and has been involved in the ESA Gaia mission since 1997. He currently chairs the Gaia Data Processing and Analysis Consortium, a team of over 400 European astronomers and IT specialists who are in charge of turning the raw measurements from the Gaia spacecraft into a three dimensional map of over one billion stars in our home galaxy, the Milky Way. Anthony is very broadly interested in the astronomical research that can be done with the aid of Gaia data, from studies of our own solar system to understanding the formation history of the Milky Way.

Seminar recorded on November 12, 2020