Download (HR) Diagrams

ASTR 101
Name:
Lab: Analyzing Star Populations on a Hertzprung-Russell (H-R) Diagram
What are most stars like? Why are they like that? To take a first step toward finding
out, plot the nearest stars and the brightest stars on the same H-R diagram.

Use a different symbol for each type of data; for instance, use dots for the nearest
stars and the + symbol for the brightest stars. Or, if you have different colors to
use, you could use blue for the nearest and red for the brightest.
Plot the 33 nearest stars (including our Sun) and the 20 apparently brightest stars from the
appendices at the back of your textbook.
What gets plotted on a H-R diagram is spectral class (O1 through M9, which also
corresponds to higher-to-lower temperatures) vs. absolute magnitude (absolute visual
magnitude, MV).
Note that Sirius B has an A2 spectral class and Procyon B has an A4
classification.
Absolute visual magnitude relates to how bright the star really is, not just how
bright it appears to be as seen from Earth.
Remember that the SMALLER the MV number, the MORE LUMINOUS the star is, in
absolute terms. A star with MV = –5.0 is MUCH more luminous than an MV = 1.0 star.
To put it another way, a star with MV = –5.0 appears SPECTACULARLY more bright
– IF SEEN FROM THE SAME DISTANCE – than a star with MV = 1.0.
Five units of visual magnitude correspond to a hundred-fold difference in brightness.
For example, a star with MV = 1.0 is 100 times brighter than one with MV = 6.0, as seen
from the same distance.
When plotting stellar class, keep track of the 1-9 number. For instance, a
G0 star plots directly above the G mark on the horizontal axis of the graph,
but a G9 star plots above a point 9/10 of the way toward K along the
horizontal axis.
Also note that the spectral classes of stars correspond to a variation in temperature. O
stars have the hottest photosphere and atmosphere, and M stars the coldest. According to
the laws of physics for the behavior of blackbodies (Wien’s law and the StefanBoltzmann law), it is these temperatures that account for why O and B stars have a
bluish tint to their color and M stars have a reddish tint.
O stars, by the way, are rare. Few stars so massive form in the first place, and when they
do, they “live fast and die young,” having a very short stellar lifespan. Therefore, at a
given time in one sector of our galaxies spiral arms, you might not find any O stars.
When you are done plotting the nearest and brightest stars on the H-R diagram, please
answer the following questions, USING COMPLETE SENTENCES:
1. What is meant by the “intrinsic brightness” of a star?
2. What are two different units that can be used to express the intrinsic brightness of
a star, and how is each of those units defined?
3. Why is it important to compare stars in terms of their intrinsic brightness?
4. Compared to the brightest stars, do the nearest stars tend to plot toward one side
of the H-R diagram in terms of O through M? If so, which side?
5. Compared to the brightest stars (brightest as seen from Earth), do the nearest stars
tend to plot toward the top or bottom of the H-R diagram?
a. Are the nearest stars more luminous or less luminous than the brightest stars,
in terms of their true, intrinsic luminosity?
6. Compared to the nearest stars, do the brightest stars (brightest as seen from Earth)
tend to plot toward one side of the H-R diagram in terms of O through M? If so,
which side? Or are the brightest stars spread across the temperature spectrum
from O/B to M?
7. Are the nearest starts intrinsically bright or intrinsically dim, on average?
a. Why are the nearest stars that way?
(This may be the most important question in this part of the lab.)
8. Are the brightest-looking starts intrinsically bright or intrinsically dim, on
average?
a. Why do the brightest-looking stars tend to be that way? Is it because of
their distance from Earth, or...what?
(This may be the second most important question in this part of the lab.
It’s not an easy answer to express clearly, but it can be done.)
Hertzprung-Russell Diagram
-10.0
( more
luminous )
-5.0
0.0
Mv
+5.0
+10.0
+15.0
( less
luminous )
+20.0
B
(  hotter )
A
F
G
Spectral Class
K
M
( colder  )