STUDY GUIDE FOR CHAPTER 11
The following is a list of topics and subtopics along with some questions and explanations.
I.
Basic stellar properties
A. Distance via parallax
1. If the baseline is 1 AU and you measure the parallax angle p in arcseconds then d = 1/p
where d is measured in parsecs. One parsec = 3.26 light years. You can only get
distances to relatively nearby stars (100 pc or so) by parallax.
B. Brightness vs luminosity and distance
1. What is the definition of brightness? Of luminosity?
2. The relationship between them is
b
L
4d 2
3. Apparent magnitude is directly related to brightness. If the apparent magnitude decreases
what happens to the brightness?
4. Absolute magnitude
a. is defined as the apparent magnitude an object would have if it was 10 pc away.
b. It is directly related to luminosity.
c. A smaller value of the absolute magnitude corresponds to a larger value of the
luminosity.
C. Binary Star Systems
1. How do we determine masses of stars?
2. What types of binary star systems are there?
3. Which kind allows us to directly measure the size of a star?
D. Spectral Type
1. You should know the sequence O B A F G K M and that it has gradations of 0 through
9. The Sun is a G2 star.
2. You should know it is a sequence in temperature from hottest to coldest.
3. You should know why the types of spectral lines and the strengths of spectral lines
differ for stars whose photospheres have different temperatures.
E. Luminosity Class
1. From subtle differences in the spectra of stars of a given spectral type you can
determine the size of the star.
2. The luminosity classes are I, II, III, IV, V . They correspond to supergiant stars, bright
giants, giants, subgiants, and main sequence stars. The Sun is luminosity class V.
3. It is called luminosity class because for a star
L  4R 2T 4
II.
H-R Diagram
A. It is a plot of luminosity versus temperature for stars
1. Which direction does temperature increase in? Luminosity?
2. What can we plot instead of temperature? Instead of luminosity?
B. Different types of stars are in different regions. You should know what types they are and
what regions of the H-R diagram they are found in.
1. Where are the supergiants?
2. Where are the white dwarfs?
3. The main sequence is where most of the stars are.
a. What does it look like?
b. For stars on the main sequence only, what is the relationship between mass and
luminosity?
C. Spectroscopic parallax
1. This is not given in the book.
2. By measuring the spectral type and luminosity class of a star you can determine its
approximate location on the H-R diagram.
3. Then you can read off its luminosity (or absolute magnitude).
4. You always know the brightness if you can see the star so this allows you to determine
the distance to the star.
5. This works for any star that we can see well enough to get an accurate spectrum. This
includes stars in our galaxy that are too far away to use parallax.
III. Star Clusters
A. Types
1. Open clusters: Approximately how many stars do they have? Where are they found in the
galaxy?
2. Globular clusters: Approximately how many stars do they have? Where are they found in
the galaxy?
B. Ages: It is reasonable to assume that all stars in a cluster were formed at approximately the
same time. We can measure the age of a star cluster using the main sequence turnoff. What is
this and why does it tell us the age of a cluster?