Gaia

One of the greatest problems for astronomers is that they can’t go out and study their targets from close range — they’re just too far away. In fact, it’s hard to measure how far away the stars really are. And without knowing a star’s distance, you can’t know its size or true brightness — values that are critical to understanding how stars work.

The most accurate system for measuring stellar distances is parallax. In essence, you look at a given star when Earth is on opposite sides of the Sun. The change in position makes the star appear to shift a tiny amount compared to the background of more-distant stars. It’s like holding up your finger and looking at it with first one eye, then the other — it appears to move back and forth against the objects beyond it.

Even for the closest stars, though, the shift is tiny. And as you move deeper into space, it gets tinier still. Such angles are extremely difficult to measure — especially with ground-based telescopes, which must look through Ea