Distances

Controversy arose, circa 1920: are the "spiral nebula" local, or are they outside the Milky way? Resolution: measure their distance. The first successful distance measurements exploited the 'period-luminosity' relationship, obeyed by stars of the 'Cepheid variable' type. This relationship was discovered by Henrietta Leavitt in 1912 during studies of the southern sky object called the Small Magellanic Cloud. The longer the period of the Cepheid's brightness oscillation, the brighter the star. So, if the period can be observed, M (M = absolute magnitude of star) can be determined, and thus used to find D. This was the first method used; among the methods others are:

1. The brightest supernovae are about equally bright (M about -18). Find one in a galaxy, use m with M and find D.

2. Choose the brightest galaxy in a group of galaxies. Assume its M (generalize M to describe the entire galaxy, not just a star) is that of the brightest nearby galaxies. Use that M with its m to find D.

3. Choose the largest galaxy in a group. Assume its size is the same as the largest of the nearby galaxies; use its apparent (angular) size together with the assumed (linear) size to find D.

4. Find a globular cluster in the galaxy. Assume its brightness is typical of local globular clusters. Use m to find D.

5. Tully and Fisher discovered a strong correlation between the widths of galactic spectral lines and the absolute luminosity of the galaxy. Since the widths may be measured without knowledge of distance, the width can be used to find M and then M and m can be used to find D, as usual.

6. etc.