Impossible STARS
For decades astronomers have relied on the uniform size of Type-Ia supernovae to help them measure the Universe. Colin Stuart looks at the impossible stars that cast doubt on the cosmic ruler
How do astronomers measure distances in space? We’re yet to send anything beyond the outskirts of the Solar System and yet we regularly talk with confidence about how far away distant stars and galaxies are.
Charting celestial distances relies on a tool called the ‘cosmic distance ladder’, which is actually a range of different interconnected techniques (see box, page 34). One of the main methods is to use standard candles – astronomical objects that have a consistent inherent brightness. The dimmer they appear to us compared to this true brightness, the further away they must be. Among the most common standard candles is a type of exploding star called a Type Ia supernova. Back in the 1930s, 19-year-old physicist Subrahmanyan Chandrasekhar was travelling by boat from his home in India to study in Europe. During his three-week voyage he passed the time by thinking about objects called white dwarfs, which form when stars like the Sun die. Chandrasekhar calculated that there is a limit to how heavy they can be: 1.4 times the mass of our Sun, a threshold now known as the Chandrasekhar Limit.