Measuring the Milky Way

Finding our place in the Galaxy has been no easy feat

Mapping the Milky Way from inside it is like trying to draw a plan of your home town using only the view from the window. But it can be done. All you need is to measure the position of as many stars as you can, then combine them all together to create your cosmic atlas.

Measuring a star’s position on the night sky is easy, but to make the map 3D requires knowing how far away it is too. Astronomers do this using something called parallax. Precisely measure the position of a star twice, with a six-month interval, and you should notice its apparent position shifts a tiny amount. This is because Earth has moved from one side of its orbit to the other, so is now placed 300 million kilometres to one side relative to the star. Measure the angle of the change and you can then use trigonometry to calculate the star’s distance.

The fullest view of our home Galaxy comes courtesy of ESA’s Gaia spacecraft, which has been measuring the parallax angle of over two billion stars since 2013. Gaia has an accuracy of 24 microarcseconds, precise enough to measure stars right up to the galactic centre 30,000 lightyears away. To see what’s happening on the other side of our Galaxy, though, you need radio telescopes that cut through the thick dust of the galactic disc. They also use interferometry to measure down to miniscule angle sizes. This isn’t much good for individual stars, but can be used to trace radio-bright star-forming regions, hinting at our Galaxy’s other spiral arms wrapping behind its back.