 You'll recall how we used the inverse square law when we covered star luminosity in the nearby star segment. We measured the brightness and calculated the distance from parallax measurements to get the luminosity. But if we had a way to know what the intrinsic luminosity of a star was, we could use that, along with the apparent brightness, to get the distance. For example, if we measured the brightness of a 10 watt candle from some distance away to be a tenth of a watt per square meter, we could calculate that the candle is just under three meters away. The celestial object with a known luminosity is called a standard candle. But until 1912 there were no known standard candle stars. That changed when Henrietta Leavitt published her findings on CVD Variable Stars.