How distances to stars are measured with parallax.

Measuring distances in space is hard. Measuring them accurately is very hard, mainly because everything is so far away. The best method is to look at the stars from places that are very far apart. Like, say, the same place on Earth…six months apart, when the planet is on opposite sides of its orbit. If you can do that, a little trigonometry will tell you how far they are, a technique known as stellar parallax.

The trouble is that stars are so far away, only large telescopes placed in space, above the atmosphere, can measure their positions accurately enough to measure distances to anything but the nearest stars. We’ve made great progress. The European Space Agency’s Hipparcos satellite measured the distances to 100,000 stars out to distances of hundreds of light-years, but that is not enough to get a good picture of the Galaxy as a whole.

Well that’s about to change with the new Gaia satellite, also built by the ESA. Gaia lauched last week, lifting its billion-pixel camera to the heavens to measure distances to the stars to unprecedented accuracy. By carefully comparing the amount of light that falls on adjacent pixels from the same star, Gaia will be able to measure parallaxes of 200 million stars as far away as the center of our Galaxy.

These measurements will let us make the best map of our Galaxy ever compiled, but that’s not the only thing it will do. Watching the positions of stars over time will show us the overall motion of stars in the galaxy. Slight wobbles in position will help us find planets with the previously-unsuccessful astrometric method, which will provide rarely-obtainable true masses. And on the side, it will be able to detect new asteroids and quasars, and test General Relativity.

Expect big developments over the next six years as Gaia begins its observing campaign.