Annu. Rev. Astron. Astrophys. 1999. 37: 445-486 Copyright © 1999 by Annual Reviews. All rights reserved

6. THE EXTRAGALACTIC DISTANCE SCALE

6.1. The First Step from the Local Group to NGC 2403

By the end of Baade's 200-inch campaign on M31, Baade & Swope (1963) had determined its moduli to be (m - M)_AB = 24.84 and (m - M)_AV = 24.68, fully 2.7 mag larger than Hubble's (1929a) value.

In a program parallel to Baade's, Hubble started an observing program to discover Cepheids in galaxies just beyond the Local Group. The targets were galaxies in the M81 and M101 groups (Holmberg 1950), eventually narrowing to M81, NGC 2403, and M101. The principal observers were Hubble, Humason, and the writer, with occasional plates taken by Arp, Baum, and Minkowski.

By 1963, 59 variables had been found in NGC 2403, of which 17 were later confirmed to be Cepheids. Some 10 Cepheid candidates were found in M81 together with 24 normal novae and a number of luminous blue variables (LBVs) of the kind discussed by Hubble & Sandage (1953) in M31 and M33. No Cepheids were found in M101, although nine LBVs were discovered (Sandage & Tammann 1974c, Sandage 1983b) using the total plate material that extended from 1909 (with the Mount Wilson 60-inch) to 1963. The variables in M81 were considerably less conspicuous than those in NGC 2403. Consequently the data for NGC 2403 were the first to be analyzed.

The long collaboration between G.A. Tammann and the writer began in 1963, starting with the analysis of the 200-inch plates of NGC 2403. By 1968 Tammann had obtained light curves for the 17 Cepheids in NGC 2403, with periods between 87 and 20 days, and light curves based on a photoelectric sequence that had been set up in the field of the galaxy. The distance modulus was (m - M)_o = 27.56 (Tammann & Sandage 1968), based on the Cepheid calibration of the P-L relation discussed in Section 4.

The result was a major shock at the time because of its implied consequences for the revision of Hubble's "remote" distance scale, and therefore for the value of the Hubble constant. As late as 1950, Hubble's distance modulus for NGC 2403 was (m - M) = 24.0 (D = 0.6 Mpc). If we were right that the modulus was (m - M)_o = 27.56 (D = 3.2 Mpc), then even at the very local distance of NGC 2403, Hubble's distance scale would be too small by a factor of five. This was much larger than Baade's original factor of two, as well as the final factor of three from Baade & Swope (1963). Therefore, by 1965 it was clear that a much larger attack on the distance scale was necessary than had been anticipated in 1948. The program was expanded into what ultimately became the series of ten papers called "Steps Toward the Hubble Constant."