For 10 points, parsecs are unit of measurement for what quantity, whose value for the Earth-Sun system is 1 astronomical unit?
One method for measuring this quantity involves plotting the apparent magnitude against temperature of a cluster's main sequence stars and comparing to a standard H-R diagram. Another method uses a power law between the rotational speed of a (*) spiral galaxy and its luminosity, known as the Tully-Fisher relation. The difference between apparent and absolute magnitude is this quantity's namesake modulus. It can be found by measuring the difference in position of stars due to the motion of the Earth, known as parallax. It is measured with the help of RR Lyrae variables, type Ia supernovae, Cepheid variables, and other standard candles. For 10 points, parsecs are unit of measurement for what quantity, whose value for the Earth-Sun system is 1 astronomical unit?
One method for measuring this quantity involves plotting the apparent magnitude against temperature of a cluster's main sequence stars and comparing to a standard H-R diagram.
distance [or absolute magnitude; do not accept or prompt on "magnitude" alone]