Measuring distances in astronomy.

For distances within our Galaxy and beyond, we have two main methods.

1. The foundation of distance measurements is stellar parallax.
2. Given the luminosity of a ``standard candle'', a measurement of its apparent brightness gives its distance.
   • You need a kind of object that always has the same luminosity -- the standard candle.
     • There is always some approximation here.
   • You need to know what that luminosity is.
     • Get a distance measurement to one of the standard candles by parallax or some other method.
     • Distance together with apparent brightness --> luminosity.

Two kinds of standard candles

1. There is a relation between surface temperature and luminosity for most stars -- those on the ``main sequence.''
   • surface temperature measured by color or by details of the star's spectrum.
   • details of the star's spectrum tell whether it is on the main sequence.
2. There is a relation between the period and luminosity for Cepheid variable stars.

Some history

• 1912, Henrietta Leavitt reports period-luminosity relation for Cepheid variable stars.
• 1917 Harlow Shapley determines the size of our galaxy from RR Lyrae variables in globular clusters.
  • The distance from the Sun to the galactic center based on modern data is 25 kly.
• 1924 Edwin Hubble determines the distance to the Andromeda Galaxy, M31, using Cepheid variables located there.
  • The distance based on modern data is 2.5 Mly.
  • This settles the debate over whether the ``spiral nebula'' are outside our Galaxy or not.