Gaia in the UK

Taking the Galactic Census

Space velocities, radial velocities and proper motions

All objects we observe in space are moving, including the Sun and the solar system with it. The main movement for objects observed by Gaia is around the core of our Galaxy, but there is considerable variation in those movements, resulting from the conditions during the formation of the object and encounters it had during its lifetime so far. We measure motions in space in two ways. Along the line-of-sight we can measure the radial velocity of an object by examining the Doppler shift in its spectral lines. For an object moving away from us the lines in its spectrum (usually absorption lines) will be shifted towards longer, redder wavelengths. If the object is moving towards us, the shift will be towards shorter, bluer wavelength. Two components of the space velocity show as a displacement on the sky, which is referred to as the proper motion of the object. This proper motion scales with the distance. For the same space velocity the proper motion will be smaller when the object is seen at a larger distance.

Creating a 6D map of the Galaxy

When we combine the measurement of the proper motion with that of the parallax, we can get a measure of the real space velocity perpendicular to the line-of-sight. Combine this with the radial velocity and we have a measurement of the space velocity. Similarly, the position on the sky and the parallax combined give the position in space of the object. This way, Gaia will produce a 6-dimensional map of the sky (3 dimensions of the position in space and 3 dimensions of space velocity).

Velocities in space are determined by the distribution of mass in space. For the Earth the most important mass is that of the Sun, around which it describes an orbit. For stars it is the mass of the Galaxy, around the centre of which those stars describe each their own orbit. Combining the 3-dimensional positions with the space velocities will  provide us with a unique source of information on topics such as the distribution of mass in the Galaxy.


Page last updated: 07 December 2013