The colors indicate the "average" speed at which 33.8 million stars (mostly belonging to our Galaxy, which contains about 200 billion in all) are approaching or moving away from us.
Our Galaxy, the Milky Way
The Milky Way is a "barred" spiral galaxy : it is shaped like a disk about 130,000 light-years in diameter, with a bulge in the central part, the bar. Our solar system is part of the disk and is located about 27,000 light-years from the Galactic center. Since we are inside the Milky Way, we can study it in much more detail than any other galaxy.
The Gaia satellite was launched nearly 10 years ago by the European Space Agency (ESA). Since then, it has revolutionized the discipline of understanding the history of our galaxy, its functioning and its interactions with its environment.
Gaia is above all an "astrometric" satellite : it repeatedly measures the positions of stars on the celestial sphere and deduces their distances and transverse velocities (i.e. their speeds in the plane perpendicular to the line of sight). The third Gaia catalog contained the distances of nearly 1.5 billion stars - in comparison, the Hipparcos satellite, a pioneer in space astrometry, had determined the distances of about 118,000 stars in the 1990s.
Some stars are moving away from us, others are approaching
To fully describe the speed of a star, it is necessary to measure, in addition to the transverse velocity, its speed of distance or approach with respect to the observer, also called "radial velocity".
Gaia performs this measurement using a spectrograph, an instrument that separates the collected photons according to their wavelengths (their color). Indeed, stellar spectra present "spectral lines", produced by the chemical elements present in the atmospheres of stars. When a star moves away from the observer, its lines are shifted towards the longer wavelengths, while when it approaches, its lines are shifted towards the shorter wavelengths (this is the Doppler-Fizeau effect). We measure the radial velocity by measuring the shift of the spectral lines.
What does this map of our Galaxy tell us ?
First of all, that the stars in the disk of the Milky Way are rotating around the Galactic center (this is called revolutionary motion). It is this overall motion that produces the alternation of dark and bright areas in the middle part of the figure : the stars located in the innermost areas of the Galaxy are moving at about the same speed as the Sun on its orbit around the Galactic center (about 250 kilometers per second), but on shorter orbits. Those behind us seem to be catching up with us, while those in front of us seem to be distancing us.
In the outer zones, the orbits are longer and the phenomenon is reversed. The stars on more internal orbits take on average less time to complete a revolution than those on more external orbits.
This image also tells us that the Milky Way is not an isolated system, but that it is surrounded and interacts with other nearby galaxies. For example, the two bright areas in the lower right of the image are two galaxies called the Large and Small Magellanic Clouds. Located in our "near suburbs", at "only" 160,000 and 200,000 light-years away, their radial velocities (they move away from the Sun at about 260 and 150 kilometers per second), very different from those of the foreground stars, make them stand out clearly by contrast.
The galactic rotation as well as the diversity of objects surrounding the Milky Way were of course known before this image, even if the rotation is represented here with a much larger number of stars than before. However, there are still many other questions that we hope this new catalog will answer. How do the bar and the spiral arms perturb the displacements of the stars ? How many stars, now captured by our Galaxy, were born in other galaxies ? The exploitation of the third Gaia catalog is just beginning.
To learn more
Waltzing with the black hole at the center of our galaxy