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Honors Astronomy Lab, The H-R Diagram
In this lab we will learn about stars using the Hertzsprung-Russell (H-R) diagram. Traditionally the
H-R diagram is a graph of the absolute magnitude of a star versus its spectral class; however, a graph of
luminosity and temperature will have an identical form. From a star's position on the graph, we can
immediately read its absolute magnitude and spectral class. With a little work we can also determine its
luminosity, temperature, distance and radius; and with a little more work we might be able to also
determine its mass and age. Your lecture class textbook and the handout Magnitude and Distance
Modulus Review and Summary will be helpful. Look over the section on in your textbook on the H-R
diagram and then answer the questions below.
H-R diagram: General Questions
1.
Define absolute magnitude, apparent magnitude, and luminosity.
2.
Star A and star B are both 10 pc from the earth. Which is more luminous, star A with an apparent
magnitude of +4.0 or star B with an absolute magnitude of +4.0? Explain.
3.
What characteristic of a star is directly related to spectral class?
4.
What does luminosity class tell you about a star?
5.
Where (in general-not specific stars nor any specific H-R diagram) would the following types of
stars be plotted on the H-R diagram: hottest, coolest, brightest, dimmest, most red, most blue,
largest, smallest, least massive, most massive?
H-R diagram: The 25 Nearest Star Systems
Figure 1 is an H-R diagram from the data in Table I for the RECONS 25 (the 25 nearest star systems
according to RECONS, the Research Consortium on Nearby Stars). Use this H-R diagram, Table I and
the radius, temperature and mass overlays and the table of nearest stars to answer the following
questions. Note that spectral class D indicates white dwarfs and T indicates brown dwarfs.
6a.
Are there any giants near us?
6b.
Are there any supergiants near us?
6c.
Why do you suppose this result (6a and 6b) is the case?
7a.
Which star among the nearest stars is the hottest?
7b.
Which star is the biggest?
7c.
Which star is the coolest?
7d.
Which star is the smallest?
8a.
Among the nearest stars, does the sun seem to be average in terms of its brightness?
8b.
Does the sun seem to be average in terms of its luminosity?
8c.
Does the sun seem to be average in terms of its temperature?
9a.
How many of the nearby stars are bright enough to be visible without a telescope? (brighter than
6th magnitude
9b.
How many are bright enough to be visible without a telescope if they were 10 parsecs away?
9c.
How many are bright enough to be visible without a telescope if they were 100 parsecs away?
10a. Look at the table of nearest stars. Multiple star systems have the members labeled with the letters
A, B and so forth. What is the ratio of the number of multiple star systems to single stars? Ignore
brown dwarfs.
10b. What is the ratio of the number of total multiple star members to single stars? Again ignore brown
dwarfs.
10c. Can you think of (or discover through research) a reason for the number of single stars as opposed
to the number of multiple stars?
HR Diagram: The Stars in the Direction of Orion
Now we shall see how our local nearby stars compare with other stars we can see? The stars in
Orion are not some special group of stars but only those stars which happen to be bright enough to be
seen when we look in the direction of the constellation of Orion. Begin making an H-R diagram for the
stars in Orion (Table II). Plot the MV values (MV again is simply absolute magnitude, M, with the
subscript indicating that the magnitude is measured in the visual part of the spectrum) against the
spectral classification for the first 8 stars in Table II. Make carefully positioned round points just like the
previous H-R Diagram you worked with. If your plot is accurate and completely correct, your instructor
will trade your graph for a completed H-R diagram of the stars in Orion.
11a. Compare the stars in Orion to the nearest stars. How do the stars in Orion compare to the nearest
stars in terms of average true brightness?
11b. How do the stars in Orion compare as to the nearest stars in terms of average size?
11c. How do the stars in Orion compare as to the nearest stars in terms of average temperature?
12a. Which star in Orion is the hottest and what is its temperature?
12b. Which star is the biggest and what is its size?
12c. Which star is the coolest and what is its temperature?
12d. Which star is the smallest and what is its size?
12e. Which star is the brightest and what is its absolute magnitude?
12f. Which star is the faintest and what is its absolute magnitude?
13a. How many magnitudes brighter than the sun is the brightest star in Orion?
13b. How many magnitudes brighter than the Sun is the dimmest star?
14.
Which do you think is probably more representative of the stellar population of any random
sample of this part of our galaxy, the nearest stars or the stars of Orion? Explain your reasoning.
Distance and Magnitudes
The difference between apparent magnitude and absolute magnitude (mV - MV), called the distance
modulus, allows us to calculate the distance to stars. You can use the distance modulus table to get an
exact distance if the distance modulus is an integer number of magnitudes and you can use it to estimate
an approximate distance if the modulus is not an integer. The relationship between distance in parsecs
and the distance modulus is shown below:
" m−M +5 %
$
'
5
#
&
d = 10
15a. What is the most distant star in Orion? How far away is it?
15b. What is the closest star in Orion? How far away is it?
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