PURPOSE: to learn how astronomers classify different stars and quantify their birth, life, and death

MATERIALS: calculator, graph paper

INSTRUCTIONS: complete your prelab, print out these pages, and complete the activities below

2.1. (1 pt) The Universe is composed of _____ hydrogen, _____ helium, and _____ heavier elements.

 a)   98%; 1%; 1%
 b)   74%; 25%; 1%
 c)   25%; 74%; 1%
 d)   50%; 49%; 1%

2.2. (1 pt) A giant molecular cloud is a:

 a)   large, cloud of hydrogen that supplies the fuel for stars to shine
 b)   a large region of interstellar matter where stars die
 c)   cold, dense region of interstellar matter from which protostars can form
 d)   the results of a massive explosion ending a star's life

2.3. (3 pts) A cloud of interstellar matter has a density of ρ = 3 x 10^-17 kg/m³. If all the necessary conditions were met, how long (in years) would it take for this cloud to collapse into a protostar?

2.4. (1 pt) What temperature must a protostar reach in order for nuclear fusion to begin and the star to be born (i.e., begin main sequence phase)?

 a)   10⁶ K
 b)   10³ K
 c)   10⁹ K
 d)   10⁷ K

2.5. (1 pt) Main sequence stars have masses between _____ and _____ M_SUN.

 a)   1; 1000
 b)   0.08; 100
 c)   10; 100
 d)   0.01; 1

2.6. (3 pts) One of the brightest stars in the sky, Sirius, has a mass of M = 2.35M_SUN. If we take α = 4, what is the luminosity of Sirius (in units of L_SUN)?

Wow, that is definitely bright! This luminosity actually corresponds to 10²⁶ times the average light bulb. And similar to our own Sun, Sirius will continue to burn hydrogen and shine for a long time still to come.

2.7. (1 pt) The overall process of generating heavier elements from lighter elements via nuclear fusion in stars is called:

 a)   proton-proton chain
 b)   triple alpha process
 c)   CNO cycle
 d)   nucleosynthesis

2.8. (1 pt) A supernova is an:

 a)   immense explosion that ends the evolution of all stars
 b)   exceptionally bright object
 c)   immense explosion that marks a star's birth
 d)   immense explosion that ends the evolution of a massive star

2.9. (1 pt) Which of the following is NOT a possible fate for a star when it ends its life?

 a)   neutron star
 b)   black hole
 c)   brown dwarf
 d)   black dwarf

2.10. (3 pts) Again, the mass of the star Sirius is M = 2.35M_SUN. If we assume that β = -3, what is the approximate lifetime for Sirius?

Open this image of the standard Harvard spectra.

3.1. (1 pt) In the standard Harvard classification scheme (OBAFGKM), which has the highest temperature? Which has the lowest temperature?

3.2. (1 pt) In the standard MK luminosity classification scheme, which class is most luminous? Which is least luminous?

3.3.   Now examine the various spectra (O6.5 thru M5...ignore the bottom three) more closely.

a) (1 pt) Which class(es) has/have the least spectral lines? Which has/have the most spectral lines?

b) (1 pt) What do you notice about how the pattern(s) of spectral lines changes (with respect to color and width of lines) as you move from type O down to type M5?

3.4. (1 pt) Examine the 'Spectra of Various Elements' worksheet at this website. Carefully
compare the unknown stellar spectrum to the elemental spectra. Of the 10 elements shown, which compose this star?

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**EXPERIMENT**
Let's examine stellar spectra to better understand how stars are classified using the "Spectral Types of Stars" applet, © Jodrell Bank Observatory.

Open the applet page and read the instructional information on how to use it. It essentially allows you to try and match blackbody curves to the spectra to stars of different Harvard types.

3.5.   Experiment with the applet and different stellar spectral types.

a) (1 pt) Recall what you learned in "The Nature of Light" Lab. What type of spectrum (continuous, absorption, or emission) do these stars exhibit?

b) (2 pts) For the 7 different possible stars (O through M), try to match a blackbody curve to each as close as possible and estimate its peak temperature. (For best results, be sure to try different fiducial wavelengths until you achieve the best fit). [HINT: just focus on fittin the blackbody curve to the long wavelength end of the spectrum if you can't get a good match] Record your results:

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**EXPERIMENT**
Let's find out the numbers of stars of different types.

3.6.   Below is a table of the 26 nearest stars. Data is from Introductory Astronomy & Astrophysics by Zeilik, Gregory, and Smith © 1992.

STAR	TYPE		STAR	TYPE
Sun	G2		Proxima Centauri	M5
Alpha Centauri A	G2		Alpha Centauri B	K5
Alpha Centauri C	M5		Barnard's Star	M5
Wolf 359	M6		Lalande 21185	M2
Luyten 726-8A	M6		Luyten 726-8B	M6
Sirius A	A1		Ross 154	M5
Ross 248	M6		ε Eridani	K2
Luyten 789-6	M6		Ross 128	M5
61 Cygnus A	K5		61 Cygnus B	K7
ε Indi	K5		Procyon A	F5
BD +59 1915 A	M4		BD +59 1915 B	M5
BD +43 44 A	M3		BD +43 44 B	M4
Lacaille 9352	M2		τ Ceti	G8

a) (4 pts) Construct a bar graph (i.e., histogram) on the graph paper provided showing how many stars of each
spectral type are listed here. (Note: Do not consider the numerical subclass when making your graph).

b) (1 pt) Based on your graph, which Harvard spectral type is most common in the local Universe? Which type is least common in the local Universe?

c) (2 pts) Old stars which have cooled off constitute most of the total mass of the Galaxy. Does this data support
this fact? Does this data lead us to conclude that the Galaxy formed earlier or later in the formation of the Universe
as a whole?

4.1. (1 pt) A Hertzsprung-Russell (HR) diagram is a graph of __________ versus __________.

 a)   temperature/color; luminosity/magnitude
 b)   luminosity/magnitude; mass/radius
 c)   luminosity/magnitude; temperature/color
 d)   temperature/color; radius/luminosity

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**EXPERIMENT**
Look at this HR diagram. Notice that the Sun is marked as an open circle. Print out this page, answer the questions
below, and be sure to turn it in with the rest of your lab activities.

4.2.   Use the HR diagram to find out about the star Sirius.

a) (1 pt) The star Sirius A is spectral type A1. Which of the 4 evolution stages shown here could this star be in?



b) (1 pt) Sirius is in fact type A1 V. Plot it on the diagram.

c) (2 pts) From its position on this HR diagram, what is is Sirius’ approximate luminosity? What is its approximate temperature?

4.3. Use the same diagram to find out about another star.

a) (1 pt) The star ζ Cep has a temperature of T ~ 3700 K. What Harvard type is it?


b) (1 pt) This star has a luminosity greater than the Sun. Which of the 4 evolution stages could this star be?


c) (1 pt) The star ζ Cep is in fact type K2 Ib. Plot it on the diagram.

4.4. (1 pt) How do these two stars - Sirius A and ζ Cep - compare to our Sun in terms of their evolution (i.e., are they older or younger, more or less evolved, etc.)?

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**EXPERIMENT**
Go to this "HR Diagram Exercise" page. Here there is a HR diagram with 22 numbered stars on it.

4.5. (25 pts) For each description, enter the number of the star they are referring to in the empty box. Note that a few of the numbered stars will be used more than once. When finished, print out this page and be sure to turn it in with the rest of your lab activities.

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**EXPERIMENT**
Now make your own HR diagram for a star cluster called the Pleiades using the "Color Magnitude Diagram of the
Pleiades" applet, © M. Briley.

Now open this webpage. Once loaded, click on the "worksheet" link at the top of the page and print out both the data table and the special graph paper.

Next, click on the link at the bottom of the page that says '...here to continue the
exercise...'. After a moment, the introduction page entitled 'An HR Diagram for the Pleiades' should open. Notice at
the bottom of the page there is a black arrow in a circle that says 'Next'. Read the following several pages of info for
review until you reach the directions page (entitled 'The Pleiades'). Read these two pages CAREFULLY and then click 'Next'
again to get to the applet page (it has a black square at the bottom instead of an arrow). [Hint: you will click 'Next' 9 times from the starting page to the applet page you'll use.] Before you begin, here
is some additional info to keep in mind:

- The Pleiades cluster is about 415 light-years from here and is young at just under 100 million years old. It is
considered to be an open cluster.
- The first image was taken at redder wavelengths with the V filter, while the second photo was taken in the blue
wavelength region with the B filter.
- Once you determine the V and B magnitudes for a star, the blue highlighting circle will become a circle with an 'X'
through it so you do not record data for the same star twice.
- We will generate a type of Hertzsprung-Russell diagram called a color-magnitude diagram. It is essentially the same
thing, except that, the latter plots change in magnitude (i.e., color) versus magnitude. In other words we will be plotting
B-V color versus blue magnitude V.

4.6. (3 pts) Find the V and B magnitudes for 30 different stars in this cluster according to the directions given in the applet's introduction pages. Record your results in data table accordingly. When you are finished, you may close the applet if you like.

4.7. (3 pts) Now, subtract the magnitude data to find the B-V color for each star. Record your results in the data table as well.

4.8. (5 pts) Using the specialized graph paper provided, make a plot of the V magnitude (x-axis) versus the B-V color (y-axis). Do NOT try to connect the data points in any way. Be sure to label the axes appropriately.

4.9. (2 pts) Now label the axes again as either luminosity or temperature. Indicate with an arrow in which direction each of these quantities increases.

4.10. (1 pt) Examine your HR diagram plot. Label the main sequence, red giant, and white dwarf regions.

4.11. (1 pt) Why do we not find many red giants or white dwarfs in this star cluster?

4.12. (3 pts) List at least 3 things we can infer about a star by contructing and analyzing an HR Diagram.

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