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Star Data: The Hertzsprung Russell Diagram
Background: The Hertzsprung-Russell diagram is actually a graph that illustrates the relationship
that exists between the average surface temperature of stars and their absolute magnitude, which is
how bright the star would appear to be if they were all the same distance away. Absolute
magnitude is defined as the magnitude that a star would have if you saw it from a distance of 10
parsecs (about 32 light-years). Rather than speak of the brightness of stars, astronomers use the
term “luminosity”. Luminosity is a measure of how much energy a star emits in a certain period of
time. Once you know the luminosity and temperature (or color) of a star, you can plot the star as a
point on the H-R diagram. Typically astronomers plot the luminosity on the y-axis with brighter stars
going toward the top. Since brighter stars have lower magnitudes, if you choose to plot magnitude on
the y-axis, the values will decrease as you go up! That's OK - just remember that the luminosity of the
star is increasing.
Plot temperature on the x-axis. However, since we can't know a star's real temperature, you should
instead plot color on the x-axis. Traditionally, hotter stars have been placed at the left of the chart and
cooler stars to the right.
The first H-R diagram you will try is a diagram for the brightest stars in the sky. The table below
shows the 24 brightest stars in the sky. Instead of plotting their luminosities (which are so large that it
is difficult to visualize), plot the stars' absolute magnitudes. Stars with higher luminosities put out
more light, so they are brighter - they have lower apparent magnitudes. Stars with lower
luminosities put out less light, so they are dimmer - they have higher absolute magnitudes.
Objective: Use the star table to plot the position of each star on the star chart template.
Part A: Procedures:
1. Complete the Star Chart Table, using the Star Color Chart and Spectral Class Table.
Classify as many of the stars as you can by type, such as white dwarfs, super giants and so
on. Record this in the Star Chart Table in the column for Star Color and Type include the
color of the star with the type. Record the spectral type for each star in the Star Class column.
2. Plot each star on the Star Chart Template using a dot to correspond with temperature,
brightness and color. Include the name of the star. Draw a small circle around the dots you
plotted that represent the color you selected (use colored pencils to match the color you
selected for each star). Remember the higher the star is on the diagram the larger the circle,
small stars on the bottom of the diagram.
3. When completed use the Star Chart to help you answer the Analysis and Conclusions
questions.
Star Chart Table
STAR
TEMP.
in (K)
TIMES
BRIGHTER
than the Sun
DISTANCE
in Light-Years
from the Earth
3/202
SUN
6373
Sirius A
10,973
23
9
Canopus
7,973
1200
99
Alpha
6773
1.5
4.2
Arcturus
5073
90
36
Rigel
12,373
40,000
815
Betelgeuse
3773
11,000
489
Beta
21,573
33
293
Alpha Crucis
21,573
2,700
391
Antares
4573
4,400
293
Beta Crucis
22,573
4,800
489
Procyon B
7073
0.001
11
Deneb
10,573
40,000
1402
Vega
10,673
60
25
Capella
5,873
150
42
Altair
7973
10
17
Spica
21,273
1900
262
Epsilon Indi
4173
0.13
11.8
Tau Ceti
5300
0.55
11.9
Gliese 185
3920
0.063
28
EZ Aquarii A
3100
0.0079
11.3
Pollux
4900
46
34
Polaris
6900
2200
430
Sirius B
25,200
0.056
8.8
Star Color and Type
Star
Class
Centauri A
Centauri
The Light Year is a standard of measure of distance. It is the distance light travels in one year. Since
light travels at the speed of 670 million miles per hour, one light year equals 670 million miles x 24
hours x 365 days (an earth year), or about 5,900,000,000,000 miles.
Since the earth is 93 million miles from the sun, it takes the light from the sun about 0.15 hour to
travel from the sun to the earth — about 3/20th of an hour.
Analysis and Conclusions:
1. What is the hottest star listed and what is its temperature?
2. What is the coolest star listed and what is its temperature?
3. How many times hotter is Alpha Crucis then Deneb?
4. What is the farthest star? How far is it in light years?
5. What is the closest star? How far is it in light years?
6. What is the difference between the temperature of the hottest and coldest star?
7. What is the difference between the temperature of the hottest and medium stars?
8. What is the difference between the temperature of a medium star (Sirius) and the coldest star
(Betelgeuse)?
9. How can you tell these temperatures and distances are only estimates?
10. Which are the brightest stars? Are they much brighter than our own sun? How do you know?
11. What does it mean when a star is five times brighter than the sun? (The light of five suns equals
the light of that one star.)
12. Why are some stars hotter than others but not as bright as some that are cooler?
Spectral Class Table