We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

We can measure the speed of a distant star from the shift in the frequency spectrum of its light using the same technique (the Doppler effect) that the police use to give you speeding tickets. The faster a star is moving away from us, the redder its light will be. Knowing how far away things are required some clever sciencing.14 For example, how do you tell the difference between a dim star that’s close by and a bright star that’s far away? Through a telescope, they look the same: like little dim points of light in the night. That was true until scientists identified a special kind of star, one that very predictably did the same thing everywhere in the universe. Because of their size and composition, these special stars grow at the same rate, and when they reach a certain size, they always do the same thing: they explode. Or to be more accurate, they implode, but the implosion is so violent, it generates a corresponding big explosion.15 This type of explosion is called a type Ia supernova. What’s useful about these supernovae is that, generally speaking, they all explode in a similar way. This means that, after some calibration, if you see one that’s dim, you know it’s far away, and if you see one that’s bright, you know it’s nearby.