Download Name: Astronomy Lab: The Hertzsprung-Russell (H

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Astronomy Lab: The Hertzsprung-Russell (H-R) Diagram
Adapted from Astronomy Through Practical Investigations No. 31
L.S.W. Publications Inc. © 1988
Sometimes the student of astronomy starts to become overwhelmed trying to
understand the many measurements and observations astronomers make. Data concerning
distance, brightness, color, spectral class, mass, temperature, motion, etc. all seem to be
gathered in an attempt to impress the student with the astronomer’s cleverness. This is a
false impression, however, for the gathering of such information is not the ultimate goal of
the astronomer. Astronomy’s major goal is an understanding of the nature of stars; the
large collection of stars called galaxies; and ultimately, the universe itself. For example, in
the 20th century, it has become apparent that stars change or evolve over time, and it is
appropriate to talk in terms of stellar birth, life, and death. The very atoms which we are
composed of were created in stars. Moreover, since our planet earth is subject to the
“whims” of a star called the sun, it becomes most practical to learn about the life cycle of
stars (which we will be discussing in class in the coming days).
Some of the greatest advances concerning the nature of stars have come about by
comparing their properties as expressed in graphs. In the early 1900’s, while studying the
spectral classification work of the Harvard Observatory, Ejnar Hertzsprung noticed some
systematic differences among spectra that were related to their proper motion (the
observed motion of a star in the sky over time as the star moves through space). In
particular, he noticed differences in the later spectral classes (K and M red stars) that
roughly separated into two groups: (1) one with small proper motions and (2) another with
large proper motions. Since on average the amount of proper motion is an indication of the
distance of the star, he suggested there were differences in luminosity among stars of the
same spectral class. That is, some stars were at very great distances (small proper motion)
but still visible; therefore, they must be much brighter than other stars of the same
spectral class that were nearby (large proper motion). Soon thereafter, both Hertzsprung
and a famous American astronomer named Henry N. Russell developed a graphical
representation comparing the absolute magnitude (luminosity) and spectral class
(temperature) of stars. This diagram is named in their honor as the Hertzsprung-Russell
diagram or the H-R diagram. It is in terms of this diagram that much of the insight and
proof of the modern theory of stellar evolution has developed.
On the original H-R diagram, the vertical axis represents stellar brightness in
absolute magnitude (M) and the horizontal axis, spectral class. This is one of three common
representations found in use today. The other two are (1) absolute magnitude on the
vertical axis versus color (as measured by the color index) on the horizontal axis and (2) the
luminosity in units of solar luminosity on the vertical axis versus effective temperature on
the horizontal axis. The magnitude-color diagram is used for stars too faint to record their
spectra and the luminosity-effective temperature diagram is used by theoretical
astronomers calculating the properties of stellar models. The important property in
common among all three is that they compare the luminosity or total energy radiated
(absolute magnitude) versus the surface temperature (spectral class or color index). It is
not unexpected to find these quantities related. The following diagram shows schematically
four of the most interesting regions in which stars are found. Notice stellar properties
tend to cluster in certain regions rather than form a random or scattered pattern all over
the diagram. This result tells us there exists a relation between luminosity and
temperature, although for each temperature, there may be more than one relation. The
roughly diagonal strip running across the diagram is called the main sequence and contains
most of the stars.
Answer the following questions in complete sentences. Use only the information
from the reading above. You may want to highlight information in the reading that
applies to the questions below.
1.
Why do astronomers collect data on stars such as their distance, brightness,
color, mass and temperature?
2.
When looking at the spectral class K and M stars, Ejnar Hertzsprung
observed the proper motion of these stars. What did he conclude about the
luminosity of these stars when observing their proper motions (and thus
their distance)?
3.
What two properties do all H-R diagrams compare?
4.
Do stars plot randomly or in specific locations on the diagram? Explain you
answer.
Directions:
1. The data in Tables 1 and 2 represent the apparent magnitude (m), absolute
magnitude (M), distance and spectral class for 25 of the 100 brightest stars
and 25 of the 100 nearest stars. Notice these represent two different
ways of selecting star samples for an astronomical study. Many of the stars
in the sample of nearest stars are double or multiple stars, and the
companions are indicated by using A, B, and C, by the names of the stars. An
H-R diagram has been provided to you in which 75 stars from each of the
samples have already been plotted
− Plot the stars in Table 1 and 2 in the diagram to complete the samples
of 100 brightest and 100 nearest stars.
− Use a dot (•) for the brightest stars and an open dot (○) for the
nearest stars.
− If two stars fall on the same spot, shift one slightly.
− BE NEAT, YOU WILL NEED TO REFER TO THIS GRAPH TO
ANSWER LATER QUESTIONS.
2. Run a smooth diagonal band (or outline) through the middle of the diagram
that approximates the largest number of stars. Label this region the Main
Sequence.
3. Circle the area in the upper right hand corner that includes the largest
number of stars and label this Group 1.
4. Circle the stars that cluster in the lower left hand corner and label this
Group II.
5. Along the bottom of the diagram above the spectral class, color in a band
corresponding to the appropriate color of each spectral class:
O - blue
B - light blue
A - blue/white
F - white
G - yellow
K - orange
M - red
Questions:
1. The luminosity of a star is a measure of the total amount of energy a star radiates
per second into space. The luminosity thus depends on how much surface area a star
has and how much energy the star emits from each square unit of the surface area.
The radius of a star will determine its surface area and the temperature will
determine how much each square unit of surface radiates. Therefore, if we
consider two stars with the same surface temperature but different luminosities,
they must have different amounts of surface area, or they differ in radius.
Complete parts a-e applying these ideas to interpreting the H-R diagram.
a.
Comparing Group I in the H-R diagram you completed with the same
temperature Main Sequence Stars, which group is most luminous?
b. Considering the two factors that determine luminosity, why are stars of the
answer in part a. most luminous?
c.
Comparing Group II in the H-R diagram you completed with Main Sequence
Stars of the same temperature, which group is least luminous?
d. Considering the two factors that determine luminosity, why are stars of the
answer in part c. least luminous?
e.
In what portion of the H-R diagram are hot, bright stars found?
Cool, faint stars?
2. It is common in astronomy to use descriptive terms for regions on the H-R diagram.
In particular, stellar sizes are grouped as Supergiants, Giants, and Dwarfs. Using
the labeled letters on the following H-R diagram, complete the parts a-I of this
question.
a.
Which lettered region would most likely represent the region of the Red
Supergiants?
b. Which lettered region would most likely represent the region of the White
Dwarfs?
c.
Which lettered region would most likely represent the region of the Red
Giants?
d. Which lettered region would most likely represent the region of the Blue
Supergiants?
e.
Which lettered region would most likely represent the location of the Sun, a
G2 Main Sequence star?
f.
Comparing regions Z and W, which region represents the hottest stars?
g.
Comparing regions Z and W, which region represents the largest diameter
stars?
h. Comparing regions S and X, which region represents the hottest stars?
i.
Assuming stars of S and X have the same luminosity, which region would have
the larger diameter stars?
3. Refer to the plotted H-R diagram you completed to answer parts a-e of this
question.
a. Considering the two factors that influence apparent brightness (luminosity
and distance), which is more important in determining why the 100 brightest
stars appear bright?
b. Why don’t most of the nearest stars have proper names?
c.
Why aren’t there any White Dwarfs among the 100 brightest stars?
d. Which sample, the 100 nearest or the 100 brightest, has more Main
Sequence Stars?
e.
Which star, a K0 Supergiant (KOIa) or a K0 Main Sequence Star (KOV),
would be visible the greatest distance from the Earth?
CONCLUSION: Why are the nearest stars to our solar system so faint as we see them,
but the brightest stars in the sky are so far away?
TABLE 1
25 of the 100 Brightest Stars
Star
m
M
Distance Spectral
(pc)
Class
Polaris
2.0
-4.5
200
F8
Mira
2.0
-1.0
40
M6
Algol
2.1
-0.5
31
B8
Aldebaran
0.8
-0.8
21
K5
Capella
0.1
-0.6
14
G8
Rigel
0.1
-7.0
270
B8
Bellatrix
1.6
-4.1
140
B2
Mintaka
2.2
-6.0
450
B0
Betelguese
0.4
-5.9
180
M2
Sirius
-1.4
1.4
2.7
A1
Castor
1.6
0.8
14
A1
Procyon
0.4
2.7
3.5
F5
Pollux
1.2
1.0
10.7
K0
Regulus
1.3
-0.8
26
B7
Merak
2.4
0.6
23
A1
Dubhe
1.8
-0.6
30
G9
Denebola
2.1
1.6
13
A3
Mizar
2.1
0.0
26
A2
Spica
1.0
-3.1
65
B1
Arcturus
-0.1
-0.2
11
K1
Antares
0.9
-4.7
180
M1
Vega
0.0
0.5
8.1
A0
Altair
0.8
2.2
5.0
A7
Deneb
1.2
-7.3
500
A2
Markab
2.5
0.0
32
B9
TABLE 2
25 of the 100 Nearest Stars
Star
m
M
Distance
(pc)
15 1915 A
8.9
11.2
3.5
15 1915 B
9.7
12.0
3.5
L347-14
13.7
14.8
5.9
αDra
4.7
5.9
5.7
Altair
0.8
2.2
5.0
δPav
3.6
4.7
5.8
HR 7703 A
5.3
6.5
5.8
HR 7703 B
11.5
12.7
5.8
-45 13677
8.0
9.0
6.3
61 Cyg
5.2
7.5
3.4
-39 14192
6.7
8.7
3.9
-49 13515
8.9
10.6
4.6
ε Ind
4.7
7.0
3.5
DO Cep A
9.8
11.8
4.0
DO Cep B
11.4
13.4
4.0
L789-6
12.6
14.9
3.4
-21 6267 A
9.3
11.0
4.6
-21 6267 B
11.0
12.7
4.6
43 4305
10.0
11.5
5.0
Ross 780
10.2
11.7
4.9
-36 15693
7.4
9.6
3.7
56 2966
5.6
6.5
6.6
Ross 248
12.2
14.8
3.2
1 4774
8.9
10.0
6.1
γCeti
3.5
5.7
3.6
Spectral
Class
M4
M5
M7
K0
A7
G7
K4
M5
M0
K5
M0
M3
K5
M3
M4
M6
M2
M3
M5
M5
M2
K3
M6
M2
G8