| Measuring the distances to the galaxies is a difficult problem. Establishing the
distance scale required a sequence of approaches, each working over longer
and longer scales. The detailed story is much more complicated than that
presented here. This is just a sketch to give a flavor of the method. |
| The distances to the nearest stars is established by
parallax. |
| The distance to a star can be computed if we know its actual brightness. The
HR Diagram is
relation between spectral type (roughly, the color of the start) and
actual brightness of a star. With this the distances to much farther stars
were measured. |
| The distances to nearby galaxies was found by measuring the
brightnesses of some variable stars. |
| These distance measurements led to Hubble's
law and the discovery that the universe is expanding. |
|
| All these measurements involve uncertainties, perhaps the largest source
being interstellar and intergalactic dust. |
| Because each type of measurement sets the baseline
for the next, errors compound, reducing the certainty of our estimates of large distances. |