Measuring distances in astronomy.
We have seen that to understand our Galaxy and the rest of the universe,
we need to know how far away things are. How do we do that?
For distances within our Galaxy and beyond, we have two main methods.
- The foundation of distance measurements is stellar parallax.
- 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
- There is a relation between surface
temperature and luminosity for most stars -- those on the ``main
- surface temperature measured by color or by details of the star's
- details of the star's spectrum tell whether it is on the main
- There is a relation between the
period and luminosity for Cepheid variable stars.
- 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
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.
Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA