HOW TO MAP THE MILKY WAY
Gaia may not be a household name like NASA’s Hubble telescope, but as space-based observatories go it’s just as sophisticated and just as valuable to astronomers. It’s not designed to produce spectacular images of galaxies and nebulae like the ones Hubble is famous for, but instead it’s designed to measure the positions of stars and other astronomical objects with unprecedented precision.
Gaia was launched by the European Space position in the sky, but its distance away from us as well. In principle this can be determined by measuring the star’s parallax – the angular change in its position as the telescope moves around the Sun. The problem is, the angle involved is minuscule, and it gets smaller the further away the star is. Until the first astrometric satellite, Hipparcos – also operated by ESA – was launched in 1989, only about 8,000 stars had parallaxes large enough to be detected by ground-based telescopes. Hipparcos multiplied that figure by 15, increasing it to almost 120,000 – but that was only the first step. Gaia is going to measure the parallax of 1 billion stars – 8,000 times as many as Hipparcos. This huge figure represents a sample of around one per cent of all the stars in the galaxy. Equally important, the sample will be evenly distributed across the whole galaxy, rather than being limited to nearby stars, as
