Download Surveying the Stars CH11 The H-R Diagram and Stellar Evolution

ASTRO 110, Sec. 4, Spring 2008
Surveying the Stars CH11
• What is stellar parallax? How do we measure distance with parallax? (pp.303-304) How far out can
we accurately measure stellar distances with this technique? What is a parsec (pc) (also, 1 pc =
3.26 light years, or 206,265 AU)?
• What is the difference between apparent brightness and luminosity? Review how the brightness
of a star falls off with distance (Fig.11-2; inverse square law; p.302), and how combining the apparent
brightness measured for a star with its distance gives you its luminosity. Notice we can express the
intrinsic brightness of stars either in terms of the luminosity [energy emitted per second], or in units
of L , the luminosity of our Sun. This is labeled on the H-R diagram (Fig.11-10).
• What is an H-R diagram (Fig.11-10)? Equivalent representations of temperature and luminosity can
also be shown by color (p. 117-119, thermal radiation, and Wien’s Law (A-3) and magnitude of
stars; you will sometimes see H-R diagrams referred to as ‘color-magnitude’ diagrams. What is the
sequence of spectral types of stars (Table 11-1, pp.305-306)? Recall this is primarily a temperature sequence. For nearby stars, we can determine distances (by stellar parallax) and plot observed
luminosities and temperatures. These show typical properties for different types of stars.
• How do we know the size of a star (pp.311-312)? What property of thermal radiation helps us with
this (the Stefan-Boltzman law, A-3)?
• What are luminosity classes (p.312)? Using a spectral classification scheme and luminosity classification, we can estimate the intrinsic brightness, or absolute magnitude of a star. Then, by comparing
this with its apparent brightness, we can estimate distances to stars which are too far away to make
reliable stellar parallax distance determinations. This is the method of spectroscopic parallax.
• What is the Doppler effect (pp.119-121)? How does it work? For binary stars, how can we use the
Doppler effect with Kepler’s 3rd Law to estimate masses (pp.308-309)? What types of binary stars
are there?
• What is the mass-luminosity relation for stars? (pp.313-314) Which types of stars obey this
relation?
The H-R Diagram and Stellar Evolution.CH11,12,13
• Where are the major stellar types located on the H-R diagram? [Main Sequence, White Dwarfs,
Red Giants, Supergiants] What physical properties are associated with each of these types? [Relative
sizes, masses (along the main sequence), luminosities, densities] Where does our Sun fall in the H-R
diagram (its spectral type and luminosity class)? Consider the sequence of spectral types of stars
along the H-R diagram: Which stars are most luminous? Which are most common?
• How do stars generate their energy? What is the Solar neutrino problem?
• What determines the evolutionary path and fate of a star? Its main sequence lifetime and rate of
evolution? What is the expected fate of our own Sun? Its main sequence lifetime and current age?
In which region of the H-R diagram will it spend most of its life?
• What evolutionary track will a star like our Sun follow in the H-R diagram ? Identify where the
various stages lie in the H-R diagram (include the proto-star to pre-main-sequence phase; possible
variable phase as a red giant; and planetary nebula phase with subsequent cooling of the central
white dwarf) What processes are taking place in the core (gravitational collapse? hydrogen burning?
helium or heavier elements burning? degenerate core?) and how is the outward appearance changing
(cooling and swelling of outer envelope during red giant phase out to the orbit of Mars, ejection of
ASTRO 110, Sec. 4, Spring 2008
mass in the planetary nebula stage around a central white dwarf roughly the size of the Earth) in
each phase?
• How does this picture change for a much more massive star? (Supergiant phase, burning of heavier
elements in higher temperature core, explosive ending in supernova, possible neutron star or black
hole)
• What is degenerate matter? What kinds are there? What conditions will produce them? Are there
mass limits for the different types of degenerate matter?
• What is the significance of supernovae in the formation of elements? (Most of the elements that we
are made of, apart from hydrogen, were probably made in supernovae) What might remain of a star
after a supernova explosion? Do we detect any of these? (Neutron stars or black holes; rotating
neutron stars as radio or X-ray pulsars; possible black hole candidate Cygnus X-1)
Star Clusters, Stellar Populations, and the Milky Way Galaxy.
• Open clusters containing O and B stars are typically associated with young stellar populations. What
is the rough range of ages for these objects (pp. 316-318)? How do we determine this? Why is there
a turn-off point for main sequence stars ?
• How do the ages and appearance of globular clusters contrast with that of open clusters and associations? How do the H-R diagrams of globular clusters differ from those of open clusters (in the masses
of stars at the turn-off point or in the relative numbers of main sequence and giant stars?
• What kinds of objects and clusters belong to Population I, the ‘Disk’ population? to Population II ?
How are they distributed in the Galaxy? Compare their ages and composition. How does this picture
of their distribution, ages, and composition match our picture of how the Galaxy formed?
• What is the importance of Cepheid (and RR Lyrae) variables? What is the period-luminosity relation,
and how is it used to determine distances to nearby galaxies? Where are these variables located on the
H-R diagram? (Note that these are giant stars, and very luminous, hence visible to large distances)?
• How do we know that spiral galaxies rotate? Do all parts rotate at the same angular speed? Will the
arms eventually wind up ? Why or why not?
• What is the size of our Galaxy? Where is the Sun located in this picture ? What type of galaxy is
it? How old ? How massive?
External Galaxies
• What are the types of galaxies? (Hubble ‘tuning fork’ diagram) Compare these types (spiral, elliptical,
irregular) according to their relative mixture of population I and population II objects. Spirals and
irregulars have more dust and gas, ellipticals are generally lacking in these. Note the difference in
relative prominence of ‘bulge’ and ‘disk’ components along the sequence of spirals.
• What is the typical distance between galaxies? What are their masses?
• What do we mean by the redshift of a galaxy ? How is it measured in terms of velocity of recession?
Do more distant galaxies have larger redshifts? (Hubble Relation) What is the Hubble constant?