The first key, the period-luminosity scale discovered by Henrietta Leavitt, allowed astronomers to calculate the distance to variable stars from Earth. Hubble had already used this knowledge in ...

The first key, the period-luminosity scale discovered by Henrietta Leavitt, allowed astronomers to calculate the distance to variable stars from Earth. Hubble had already used this knowledge in ...

Instead, they suggest that the Kardashev Scale establishes upper limits on the amount of stellar energy a civilization can harness (a "luminosity limit") and that civilizations might circumvent ...