RANDY L. PHELPS
This is the section for the module on "Stellar
Parameters III: The Hertzprung-Russell Diagram and Stellar Ages"
Procedure:
1. For this module, please look through the "lecture" notes
for: "Stellar
Parameters III: The Hertzprung-Russell Diagram and Stellar Ages". These
notes contain the material, in condensed form, that I will expect you to become familiar
with. I am sure you will have questions about the material, especially since it may
be new to you and it is presented in the form of lecture notes. To help you expand
upon the material, and fill in some blanks, check out the following web sites:
2. Additionally, please do the following
web-based exercise. It will, I hope, help to reinforce some of the concepts you will
be expected to be familiar with.
Upon completion of
this web assignment, you should be comfortable with the following material:
- What is spectroscopic parallax and why is it
important for the determination of stellar distances? Note: a part of the
significance is that this technique illustrates a powerful tool - using the limited amount
of information you have, and being clever to push your knowledge farther into the
Universe!
- The HR diagram. What is plotted in the
diagram, and what does the diagram look like? What are the various groupings of
stars in the diagram referred to, and why do we call this groups what we do? How
does the Stefan-Boltzmann Law help to determine properties of stars in the HR diagram?
- The Luminosity Function of stars. What is
it, and what can we learn from plotting the number of stars of a given mass, as we do when
we construct a luminosity function?
- Star clusters. What are they and why are the
important for determining the ages of stars, as well as the distances to rare stars?
Upon completion of
this assignment, you should be able to answer these, and similar, questions
General Concepts
- If the apparent
magnitude and absolute magnitude of a star are known, what additional information can be
obtained?
- In the HR diagram, what two
physical properties of stars are plotted on the axes?
- In what part of the HR diagram can
most stars be found?
- Were giants, supergiants and white
dwarfs once main sequence stars?
- Which type of star is more common,
a spectral-type "B" star, or a spectral type "F" star?
- Which type of star is more common,
a spectral-type "G" star, or a spectral type "M" star?
- Which spectral-type stars are the
least common?
- What is a star cluster?
- Which stars evolve more rapidly,
high-mass stars, or low-mass stars?
- What are isochrones, and how do
they allow us to determine stellar ages?
- How does a comparison of
theoretical HR diagrams to observed HR diagrams give us the distance to a star cluster?
Applications
One star (star #1) of spectral-type
"F" is located at a distance of 4pc, based on its parallax. A spectrum of
another star (star #2) of unknown distance is obtained, revealing it to also be of
spectral-type "F". Describe, briefly, what information can be obtained for
star #2, including a description of any observations that would need to be conducted
(i.e., what information must be found through observations, and what information can be
inferred).
Two stars have the same
temperature, but star #1 is more luminous than is star #2. Which star has the larger
radius?
Two stars have the same
luminosity, but star #1 is cooler than is star #2. Which star has the larger radius?
Spectral-type "O" stars are not found near the Sun, so their distances cannot
be found from parallax measurements. An astronomer, however, wants to find out what
the absolute magnitude (i.e., energy outpput) of an "O-type" star is, and knows
that there is one in a particular star cluster. Describe, briefly, how the absolute
magnitude of the "O" star might be obtained.
These questions, and similar ones, will form the basis of the exam
material for this section of the course. If you have problems with the material,
please see me during my office hours. If you are unable to answer some of the questions, I
will help you before the date specified on the syllabus, provided you show me the results
of your inquiry into the material.
That is, you must provide me the answers you
we able to obtain for all questions, including your attempts at problem questions, before
I will help you with any of them!
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