How is the distance to stars measured?

24th September 2009
How is the distance to stars measured? (iStock)

Asked by: Lachezar Yanchev, Stockport

For stars a few thousand light-years away the standard method exploits an optical effect called parallax. Hold a pen in your hand and watch its tip as you move your head from side to side: it appears to move relative to objects in the background. Move it further away and it appears to move less. The same is true of stars, whose distance can be calculated by observing their shifting position seen at different points in the Earth’s orbit.

For more distant stars, it’s sometimes possible to estimate their intrinsic brightness, allowing a comparison with their observed brightness in the sky
to give an estimate of their distance