HR Diagram
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School
Eastern Michigan University *
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Course
105
Subject
Astronomy
Date
Dec 6, 2023
Type
docx
Pages
4
Uploaded by diontaelt
Hertzsprung-Russell Diagram
Open the
Hertzsprung-Russell Diagram Explorer
. The red x represents the location of the sun on the HR diagram.
*
A scale is considered linear if the value changes by the same amount between equally-spaced tick marks on the
graph. For example, if four equally-spaced tick marks in a row have values of 2, 4, 6, 8, then that scale is linear. A
scale is considered logarithmic if it changes by a factor of ten between equally-spaced tick marks. For example, if
four equally-spaced tick marks in a row have values of 0.1, 1, 10, 100, then that scale is logarithmic.
1. The red line represents the location of the stars that are considered main sequence. Click the
‘show luminosity
classes’
button in the simulation and unclick the
‘isoradius lines’
button. The green region shows the bandwidth for
the main sequence.
a. Is the sun located on the main sequence?
b. Look in the bottom-left panel of the simulation. What is the approximate temperature of the sun?
Don’t
forget your units!
2. Change the x-axis scale to
‘spectral type’
.
a. Did the position of the sun on the HR diagram change?
b. Is the spectral type scale linear?
c. What spectral type (O, B, A, F, G, K, or M) is the sun?
3. Change the x-axis scale back to
‘temperature’
.
a. What is the luminosity of the sun in terms of solar luminosity (L
☉
)?
b. Is the luminosity scale linear?
It may be worth writing down the correct values for the sun so you can reset them in the simulation later.
The luminosity, in terms of solar luminosity, is a measure of the power, or energy per time (Watts) a star emits as
measured from the top of Earth’s atmosphere if the star were located in the same position as our sun relative to
the Earth.
4. Change the y-axis scale to
‘magnitude’
to display the absolute magnitude on the vertical axis.
a. Did the position of the sun change on the HR diagram?
b. Is the absolute magnitude scale linear?
c. What is the approximate absolute magnitude of the sun?
d. Which indicates a brighter object in the sky, a negative magnitude or positive magnitude?
5. In the bottom-left panel, change the luminosity to 2.0 (2 times as luminous as the sun).
a. What approximate absolute magnitude does a star with this characteristic have?
b. Is this value double the absolute magnitude of the sun?
Let’s get to know what the position of a star on the HR diagram can tell us about that star. Change the vertical scale
back to luminosity. Notice that the HR diagram tells us about the brightness of a star (luminosity or absolute
magnitude), temperature of a star, and the size of a star. The size of the star based on its location is shown
compared to the size of our own sun in the panel in the left window.
6. Temperature and color of a star
a. Is the temperature higher on the left side of the diagram or the right side of the diagram?
b. Where are blue stars and red stars located on the diagram?
c. Which color indicates a higher temperature, red or blue?
7. Luminosity or brightness of a star
a. Is the luminosity (brightness) greater on the top side of the diagram or the bottom side of the diagram?
b. Are brighter stars located toward the top of the diagram or the bottom of the diagram?
8. The relationship between temperature, brightness, and size of a star
a. For a particular temperature (i.e. focusing on a single temperature value), how does the radius of a star
change as the luminosity increases?
b. For a particular luminosity (i.e. focusing on a single luminosity value), how does the radius of a star
change as temperature increases?
c. Click the
‘show isoradius lines’
button. Is the isoradius scale linear?
d. How does the luminosity of a star with a particular radius (lies along an isoradius line) change as the
temperature of the star increases?
Let’s learn about nearby stars. Click the
‘nearest stars’
option.
9. What type of star is the typical nearby star?
10. Brightness of nearby stars
a. What is the luminosity of the brightest nearby star on the diagram in terms of solar luminosity
(
L
☉
)
?
b. What is the luminosity of the dimmest nearby star on the diagram in terms of solar luminosity
(
L
☉
)
?
11. Temperature of nearby stars
a. What is the temperature of the hottest nearby star on the diagram?
b. What is the temperature of the coolest nearby star on the diagram?
12. Size of nearby stars. Turn on the
‘isoradius lines
’ if you turned them off earlier.
a. What is the radius of the smallest nearby star on the diagram in solar radii
(
R
☉
)
?
b. What is the radius of the largest nearby star on the diagram in solar radii
(
R
☉
)
?
13. Describe, in terms of brightness, temperature, and radius, how most nearby stars compare to the sun.
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Let’s learn about the brightest stars. Click the
‘brightest stars’
option.
14. What type(s) of star is the typical bright star on the diagram?
15. Brightness of brightest stars
a. What is the luminosity of the brightest bright star on the diagram in terms of solar luminosity
(
L
☉
)
?
b. What is the luminosity of the dimmest bright star on the diagram in terms of solar luminosity
(
L
☉
)
?
16. Temperature of brightest stars
a. What is the temperature of the hottest brightest star on the diagram?
b. What is the temperature of the coolest brightest star on the diagram?
17. Size of brightest stars. Turn on the
‘isoradius lines
’ if you turned them off earlier.
a. What is the radius of the smallest bright star on the diagram in solar radii
(
R
☉
)
?
b. What is the radius of the largest bright star on the diagram in solar radii
(
R
☉
)
?
18. Describe, in terms of brightness, temperature, and radius, how most bright stars compare to the sun.
19. Click the
‘both nearest and brightest stars’
option. Where do a majority of all of the stars fall on the diagram?
Hint:
90% of all stars fall in this region.
20. Choose the correct star in each of the situations described below.
a. Two stars have the same luminosities, but one star is hotter than the other. Which star is larger?
b. Two stars are the same size, but one is more luminous than the other. Which star is hotter?
21. Are there many giant stars that are hotter than the sun? If not, why are those stars so much brighter than the
sun?
Hint: Pay attention to the isoradius lines.