(Updated 7 Dec. 2012)
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)
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.
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.
The AAVSO Website has a
fine article concerning the history of the discovery of variability in Delta Cephei.
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
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
17-4 Gravitational lensing (pp. 504 - 506 in 6th ed.)
Quasars; clusters of galaxies. See also:
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