http://en.wikipedia.org/wiki/Gaia_(spacecraft)
ESA’s Gaia spacecraft arrived at its Lagrangian orbit slot just a few days ago and is currently undergoing calibration and shakedown tests. In a few months, its five year mission of gathering precise data on the one billion brightest stars will begin. This effort is a follow-up to earlier missions, such as Tyco and Hipparchos, and will provide us for the first time with a truly statistically valid sample of the Milky Way’s stellar population.
The faintest stars visible in our telescopes may only appear that way because they are so far away, or dimmed by the dust haze of the galactic disk, they may be intrinsically extremely bright. At the same time, most stars are extremely faint, and cannot be easily detected even though they may be very close to earth, and if detected, their distances or other properties cannot be easily determined.
The vast majority of all stars are brown, red or white dwarfs, yet not even one of them is visible to the naked eye. The stars we see on a dark night are mostly giants at great distances. Our much more numerous faint neighbors are invisible. By being able to see the faint stars nearby as well as the brightest ones across the galaxy, Gaia will have access to a sufficiently large sample to accurately determine the stellar Luminosity Function, the mathematical curve or histogram that tells us how many stars there are at each brightness level. This relationship is only poorly understood, and is most uncertain at the fainter magnitudes where most stars shine. For example, although our own sun is of average brightness, it is brighter than most stars. The Luminosity Function is also intimately related to the Mass Function and to the ages of stars, too, so this spacecraft will give us a good statistical sample valid for stars across the galaxy, as well as for the huge swarm of faint ones that are relatively nearby.
The galaxy is a 100,000 light years across, and about a thousand light years thick, and masses approximately 100 billion solar masses. Because of uncertainty in the Mass Function, we don’t know how many stars that adds up to. We’re about to find out.
Gaia will be the most ambitious and detailed survey of the galactic stellar population ever made, but it will only see less than one percent of the galactic population.