Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- Part I Stars and stellar evolution up to the Second World War
- Part II The large-scale structure of the Universe, 1900–1939
- Part III The opening up of the electromagnetic spectrum
- Part IV The astrophysics of stars and galaxies since 1945
- Part V Astrophysical cosmology since 1945
- 12 Astrophysical cosmology
- 13 The determination of cosmological parameters
- 14 The evolution of galaxies and active galaxies with cosmic epoch
- 15 The origin of galaxies and the large-scale structure of the Universe
- 16 The very early Universe
- References
- Name index
- Object index
- Subject index
13 - The determination of cosmological parameters
from Part V - Astrophysical cosmology since 1945
Published online by Cambridge University Press: 05 February 2015
- Frontmatter
- Contents
- Preface
- Acknowledgements
- Part I Stars and stellar evolution up to the Second World War
- Part II The large-scale structure of the Universe, 1900–1939
- Part III The opening up of the electromagnetic spectrum
- Part IV The astrophysics of stars and galaxies since 1945
- Part V Astrophysical cosmology since 1945
- 12 Astrophysical cosmology
- 13 The determination of cosmological parameters
- 14 The evolution of galaxies and active galaxies with cosmic epoch
- 15 The origin of galaxies and the large-scale structure of the Universe
- 16 The very early Universe
- References
- Name index
- Object index
- Subject index
Summary
Sandage and the values of H0 and q0
In 1952, Walter Baade announced that the value of Hubble's constant, H0, had been overestimated because the distance to the Andromeda Nebula, M31, adopted by Hubble was about a factor of 2 too small (Baade, 1952). The cause of the discrepancy was that there is a difference in the period–luminosity relations for Cepheid variables of Populations I and II (see Section 12.2). By using the same type of Cepheid variable in our own Galaxy, in the Magellanic Clouds and in M31, the distance to M31 increased by a factor of 2. Consequently, Hubble's constant was reduced to 250 km s−1 Mpc−1 and H−10 increased to 4 × 109 years.
In 1956, Humason, Mayall and Sandage showed that the expected redshift–magnitude relation, m = 5 log10z + constant, is observed for galaxies selected at random, but there is a large scatter about the mean relation because of the breadth of the luminosity function of galaxies (Humason et al., 1956). It had been known since Hubble's pioneering studies of the 1930s, however, that the brightest galaxies in clusters of galaxies follow a very much tighter relation which follows precisely Hubble's law υ = H0r (Figure 13.1). Thus, in order to estimate the value of H0, it was only necessary to calibrate the observed relation by measuring the distance of the nearest rich cluster of galaxies, the Virgo cluster of galaxies, by techniques independent of its redshift.
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- Information
- The Cosmic CenturyA History of Astrophysics and Cosmology, pp. 340 - 364Publisher: Cambridge University PressPrint publication year: 2006