The History and Nature of Astronomical Discovery - AST 210 H1

Distances in the Universe

(Updated 7 Dec. 2012)
This section of the course is based on material about determining the distances of stars and galaxies. The material appears in several different chapters of the textbook.
In the outline below, I indicate the material you should read and which of the terms that are highlighted in bold face or italics you should know.

  • 12-8 Cepheid variables as distance indicators: light curve, period-luminosity diagram; Henrietta Leavitt's study of the Cepheids in the Small Magellanic Cloud (see notes here), the difference between Cepheid I and Cepheid II stars and RR Lyrae stars; Cepheid and RR Lyrae variables on the HR diagram (Fig 14-15 on page 402 in 6th ed., p. 459 in 5th)
  • The AAVSO Website has a fine article concerning the history of the discovery of variability in Delta Cephei.
  • Conclusion

  • Star clusters: globular cluster, galactic cluster (page 383 in 6th ed.; p.411 in chapter 13 5th ed.), two reasons clusters are important to astronomers (page 383 in 6th ed.; 411-412 5th ed.)
  • 14-2 Stellar Maturity: Stellar nuclear fusion, carbon (CNO) cycle; Main Sequence Life of Stars: zero-age main sequence, turnoff point.
    Note that Figure 14-6 illustrates how the H-R diagram of a cluster can be used to determine its age. It can also be used to determine its distance. In Figure 14-6, the vertical axis can be shown as the absolute luminosity because the distances of the two clusters are known. However, in cases where the cluster's distance is not known, the vertical axis is plotted as apparent magnitude, but the distance can be derived through a comparison with the H-R diagram as plotted in Figure 12-17 on page 348 6th ed., p. 373 5th ed..

  • 14-3, 14-4, 14-5, 14-6 Stellar Evolution (lower mass stars).
  • 14-7 White Dwarfs: Chandrasekhar limit, accretion, novae.
  • 14-8 Type I supernovae. 15-1, 15-2, 15-3, 15-6 Evolution of more massive stars; Type II supernovae. 15-4 Basic ideas concerning neutron stars.
  • 15-7, 15-8 general relativity (done earlier), black holes.

  • The Size of Our Galaxy (pp.451-461 6th ed.; pp. 511-516 5th ed., in chapter 16): Milky Way Galaxy, how William and Caroline Herschel mapped the Galaxy, how Jacobus Kapteyn sought to find the Sun's location in the Galaxy, how the presence of interstellar dust prevented them from seeing the most distant stars in the Galaxy, globular clusters, how Shapley demonstrated that the Sun was not near the centre of the Galaxy, the importance of Cepheids and RR Lyrae variables and the period-luminosity relation

  • The Recognition of the Andromeda nebula as an external galaxy: The Shapley-Curtis debate p.485 (page 517 in 5th ed.), Edwin Hubble's study of Cepheid variables in Andromeda (pp.477 and 481 6th ed.; pp. 540-541 and 545 5th ed. in chapter 17)

  • 17-1 Hubble Classification Scheme for Galaxies": Different kinds of galaxies, Tobing Fork diagram.

  • 17-2 Measuring Galaxies: distances measured by various indicators, how astronomers start with the period-luminosity relation of Cepheids and follow a chain of reasoning that allows them to determine distances to galaxies too far way for their Cepheids to be visible, the Hubble law, Milton Humason (p.487 6th ed.), the Hubble law used to measure distance; observations, assumptions and conclusions, the precision of science, the Tully-Fisher relation.

  • See section 14-8. Type Ia supernovae are produced by the destructive explosion of white dwarfs in binary systems. They can appear almost as bright as the galaxies to which they belong.

  • 17-3 Masses of Galaxies: This section brings together a number of topics we have discussed during the course regarding masses of galaxies and clusters of galaxies.

  • 17-4 Look-Back Time: For this section, you should read the central paragraph on page 497 (6th ed.).
    If a celestial object is at a distance of 1 million parsecs we are viewing that object today the way it appeared approximately 3 million years ago. (A distance of 3.26 light years is equivalent to 1 parsec.) This photograph which is a 10-day exposure obtained with the Hubble Space Telescope shows some extremely distant objects and illustrates that the Universe in the past looks different than it does at the current epoch. Another example of this phenomenon is the "radio" image of Cygnus A in Figure 17-19 on page 500 (6th ed.), 17-21 (a) 534 (5th ed.) of the textbook.

  • 17-4 Gravitational lensing (pp. 504 - 506 in 6th ed.) Quasars; clusters of galaxies. See also: Bullet Cluster

  • 18-1 The Search for Centers and Edges: review of the theories of Ptolemy, Copernicus, Galileo, Newton, Herschel, Shapley, Hubble; Einstein's Universe, closed universe, open universe, flat universe, critical density, cosmological constant

  • 18-2 The Expanding Universe: what is expanding and what is not, the cosmological redshift: the difference between a Doppler redshift and a cosmological redshift, the Hubble law, Olber's paradox and its resolution

  • 18-3 Cosmological assumptions: homogeneous universe, isotropy, cosmological principle, universality

  • 18-4 The Big Bang: evidence - cosmic microwave background radiation, the COBE results and WMAP; the Steady State Theory and the arguments against it (see the box on page 526 and top of page 527 6th ed.), the Early Universe (note, in particular, paragraphs 4 to 9 in the box on page 528 and the top of page 529), additional evidence for the big bang; the age of the Universe (similar sections in same chapter of earlier editions).

  • 18-5 The Future: Will Expansion Stop? Evidence: Distant galaxies and high-redshift supernovae, density parameter, dark energy. You should know and understand Figures 18-13 to 18-17 (18-14 to 18-18 in old ed.).
    The Density of matter in the Universe, evidence for the existence of non-luminous dark matter. (This came up earlier, too)

  • 18-6 The Inflationary Universe: The horizon problem and how the inflationary universe model can account for this.
  • 18-7 The Grand Scale Structure of the Universe. In this section, you should examine some of the diagrams. These are Figures 18-19 to 18-30 in 6th ed. (Figs 18-19 to 18-29 5th ed., 18-21 to 18-26 in 4th ed.). Figure 18-18 (18-21 in 4th ed.) shows the distribution of clusters of galaxies. It illustrates that the clusters are not uniformly distributed, but are arranged more like bubbles, with voids between the walls of the bubbles. Figures 18-19, 18-20 18-22 and 18-23 (18-22, 18-23, 18-24 and 18-26 in 4th ed.) show temperature fluctuations in the Cosmic Microwave Background (CMB) radiation measured by the COBE, BOOMERANG and WMAP missions. The CMB radiation represents the Universe as it was approximately 380,000 years after the big bang. The fluctuations correspond to the seeds that grew to become galaxies.
  • Ch. 18 Conclusion: all

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