Download IND 6 - 1 Stars and Stellar Evolution In order to better understand

Name: ________________________
Partner First Name: _____________
Stars and Stellar Evolution
In order to better understand the stars in our Universe, we have to know how they live and how
they die. In this lab, we will examine how we know what we know about stars.
Part 1: Our Closest Star
We are fortunate to have our favorite star for many reasons , but one important feature is that
we Earthlings orbit this star. Because we orbit the Sun, we may use Newton’s Version of
Kepler’s Third Law to determine mass. We saw/will see this equation in the “Exoplanets” lab.
Newton’s version of Kepler’s Third Law
P2 =
4p 2
a3
G ( M star + M planet )
P = orbital period
a = semi-major axis
G = the gravitational constant
Mstar = mass of the parent star
Mplanet = mass of the planet
It says that the orbital period (squared) is: a) proportional to the semi-major
axis (cubed) and b) inversely proportional to the total mass of the objects
(stars and planets) in the system.
A note about numbers in this lab.
If the number in your final answer is less than 106 (i.e. one million), do not use scientific
notation and have no numbers after the decimal. If you do obtain a value that is higher
than 106, use scientific notation but use no more than 3 numbers after the decimal. You
should also state the number using words (i.e. 4.6 x 109 is 4.6 billion).
1. We know how long it takes Earth to go around the Sun (the period) and we know far Earth
is from the Sun (the semi-major axis). Using Newton’s version of Kepler’s Third Law,
calculate the mass of the Sun. Show your work. Put the answer in words too! (see above)
Important numbers: distance Sun -> Earth: 1.496 x 1011 m
G = 6.67 x 10-11 m3/(kg s2) (note this unit! Times must be in
seconds, distances in meters, mass in kilograms)
TA Check _____
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2. Now that we know the mass of the Sun (this was first calculated around 1700), we can start
looking at how the Sun is powered (i.e., why the Sun shines). From experiments, we know
the amount of energy the Sun puts out (the solar flux) is 3.9 x 1026 Joules/second .
a. Calculate how long (in years) the Sun would shine if it were powered by burning wood.
Wood has an energy density of 16 million Joules/kilogram. Show your work and be
careful with units!
(hint: you calculated the mass in Q#1 and you’ve been given Joules/second and Joules/kg)
b. Calculate how long (in years) the Sun would shine if it were powered by burning
gasoline. Gasoline has an energy density of 46 million Joules/kilogram. Show your
work.
c. Please state at least two things you find interesting about these numbers.
Scientists in 1700 already knew that Earth was hundreds of millions of years old (fossil records
easily indicate this) so the Sun cannot be powered by anything that chemically burns (like
wood or gasoline). Therefore a new mechanism had to be found and for 200 years, scientists
didn’t know! We now know that mechanism is thermonuclear fusion (usually just called
“nuclear fusion” in astronomy).
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Now you get to use Einstein’s famous equation E = m c2 !
E is Energy (in Joules), m is mass (in kilograms), and c is the speed of light (in meters/sec)
3. During nuclear fusion, hydrogen fuses into helium but about 0.7% of the combined mass
gets converted to energy. Therefore our equation becomes: E = 0.007 msun c2
But only about one-tenth of the Sun will ever get hot enough and dense enough for fusion
to occur, so now our equation becomes: E = 0.007 x (0.1 msun) c2
Calculate how much energy (in Joules) the Sun will give off (over its whole hydrogen-fusing
lifetime). Show your work.
4. Now that you know how much energy the Sun will give off over it’s whole lifetime, use the
current solar flux (from Q#2) to calculate the Sun’s hydrogen-fusing lifetime (in years).
Show your work.
5. From radioactive dating of Solar System objects (especially meteorites – rocks from space),
we know that the Sun has been around for 4.6 billion (4.6 x 109) years. How much longer
will it be before the Sun becomes a red giant (and is fusing helium)?
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Part 2: Stellar Classification
Now we will look at how astronomers classify stars. You will be given spectra of several stars
(like you saw in the “Light and Spectroscopy” lab). Note that these are absorption spectra
which are a continuous background (the blackbody curve) with overlaid absorption features
from specific elements (remember that each element has a specific fingerprint). On the screen
you’ll see the comparison of the emission and absorption spectra for hydrogen.
6. In the comparison spectra on the screen, what do you notice that is the same and what do
you notice that is different between these two hydrogen spectra?
Go to the website: http://astro.phy.vanderbilt.edu/~conroyk/test/spectra.html
Or go to: http://tinyurl.com/VandySpectra
You will find the spectral of several stars. Note that they are absorption spectra.
7. Using the 13 stellar spectra on the screen, put them in order in a scheme that you think
makes sense. Then record the order you put them below.
1. __________________________
8. __________________________
2. __________________________
9. __________________________
3. __________________________
10. __________________________
4. __________________________
11. __________________________
5. __________________________
12. __________________________
6. __________________________
13. __________________________
7. __________________________
8. Why does the scheme you used above make sense to you? That is, what characteristics are
you ordering by?
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The first stellar classifiers (women at Harvard lead by Annie Jump Cannon) classified by the
strength of the hydrogen spectral lines – shown on the screen – and here are absorbed as dark
lines of red, blue-green, and violet and best seen in HD 116608.
Strongest were “A” stars and next were “B” stars and on down the alphabet to “O” stars.
 We now know that these hydrogen lines can get weaker with increasing temperature
because the atoms get ionized (the electron escapes so cannot be moving energy levels)
and also can get weaker when the temperature decreases (the electron is less likely to move
to high energy levels in the first place). Therefore, the ordering of the classes had to
change in order to reflect the actual temperatures of the stars.
 Also, the classifiers realized that many classes could be combined so now we have the
OBAFGKM classification scheme of today.
Putting the stars you were working with in order by temperature (hottest to coolest):
Spectral
Spectral
Star Name
Star Name
Type
Type
1
8
HD 12993
O 6.5
HD 28099
G0
2 HD 158659
9
B0
HD 70178
G5
3 HD 30584
10 HD 23534
B6
K0
4 HD 116608
11 SAO 76803
A1
K5
5 HD 9547
12 HD 260655
A5
M0
6 HD 10032
13 Yale 1755
F0
M5
7
BD 61 0367
F5
9. Place the spectra in the temperature order outlined above.
_____ checkmark
10. Notice that the strength of the Hydrogen lines (they’re violet, blue-green, and red) changes
as one goes from the hottest to the coolest stars (“strength” is basically the darkness – the
stronger the absorption, the darker the line). The lines are best seen in the spectrum of
HD 116608. Please describe the change in hydrogen line strength (i.e. darkness) you
observe as one goes from the hottest to the coolest stars (note that there are both increases
and decreases!!).
11. What other lines do you notice having a trend from hottest to coolest and what is that
trend? You should find at least two.
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12. We can use the background continuum to tell us
about the blackbody curve of the star (and thus the
temperature of the star. As a refresher, to the right
you will see two blackbody curves you saw in the
“Photometry and Color-Magnitude Diagrams” lab.
a. Which star (E or D) is the hotter star?
b. For star E, describe how the intensity of the light
changes going from violet to red.
13. How do the blackbody curves above help you understand why/how the background
rainbow intensity changes in the strips of spectra you’ve just placed in temperature order?
Recall in previous labs (Photometry and Color-Magnitude Diagrams, Star Clusters), we learned
about the ages of stars and how they relate to temperature and luminosity on the main
sequence.
14. Please complete the following table with the information you know about stars ON THE
MAIN SEQUENCE.
Mass of Star
Color of star
(redder or bluer)
Temperature of star
(hotter or cooler)
Lifetime on Main Seq
(longer or shorter)
More massive
Less massive
(make sure to check correctness by talking to others and then the TA)
In Part 4, you will examine these lifetimes more completely and also learn about different
evolutionary stages.
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In order to understand more about what kinds of stars there ACTUALLY are out there, we need
to know how many of which classification and the masses of those stars. This table from
Wikipedia (in the “Stellar Classification” page) will help:
Stellar classification of Milky Way MAIN SEQUENCE STARS (from Wikipedia)
Effective
Class
temperature
O
B
A
F
G
≥ 30,000 K
10,000–
30,000 K
7,500–
10,000 K
6,000–
7,500 K
5,200–
6,000 K
K
3,700–
5,200 K
M
2,400–
3,700 K
Fraction of
Conventional Actual
Mass Radius
mainLuminosity
color
apparent (solar (solar
sequence
description
color masses) radii)
stars
≥ 6.6
≥ 16 M☉
≥ 30,000 L☉ ~0.00003%
blue
blue
R☉
deep blue 2.1 –
1.8 –
25– 30,000
blue white
0.13%
16 M☉ 6.6 R☉ L☉
white
1.4 –
1.4 –
5–25 L☉
white
blue white
0.6%
2.1 M☉ 1.8 R☉
1.04 –
1.15 –
1.5–5 L☉
yellow white white
3%
1.4 M☉ 1.4 R☉
yellowish 0.8 –
0.96 –
0.6–1.5 L☉ 7.6%
yellow
1.04 M☉ 1.15 R☉
white
pale
0.45 –
0.7 –
0.08–0.6 L☉ 12.1%
orange
yellow
0.8 M☉ 0.96 R☉
orange
light
0.08 –
≤ 0.7
≤ 0.08 L☉
red
orange
76.45%
0.45 M☉ R☉
red
15. What spectral type (i.e., Class) are the most dominant main sequence stars in the Milky
Way? Why?
16. If the Milky Way has about 400 billion stars (the current best estimate), estimate how many
Sun-like stars exist (show your work).
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Part 3: Basic Stellar Evolution
You will fill out a short flow chart / concept map to help you with basic stellar evolution and the
different evolutionary paths. There are words you will need in the description following:
As we learned in the “Star Clusters” lab, MASS is the Great Determinator. Therefore, when a
star runs out of hydrogen to fuse in its core and starts turning into a red giant, the mass of the
star determines the path it follows.
 A low mass star (less than 8 times the mass of our Sun ( < 8 Msun)) eventually ejects its outer
layers to produce a planetary nebula. The now naked stellar core remaining is called a
white dwarf (because it is very hot but dim).
 In contrast, a high-mass star, more than 8 times the mass of our Sun ( > 8 Msun), will
eventually explode as a massive star supernova (often known as a “Type II” supernova).
Depending on the original mass of the star, this supernova will leave behind either a
neutron star or, if the original star was extremely massive (at least 25 Msun), a black hole.
Note that all material thrown off as planetary nebulae and from supernova explosions will just
go out into the galaxy, collect into new interstellar gas clouds, and eventually form new stars!
It is the ultimate recycling!
17. Using the table on the previous page, estimate what percentage of main sequence stars
fall into these categories:
a. massive enough to produce a black hole remnant
________
b. massive enough to produce a neutron star remnant (but not a black hole) ________
c. produces a planetary nebula + white dwarf ________
You obviously couldn’t get exact percentages for the massive stars. Describe how you
chose your percentages for the massive stars.
18. Use the information above and the word list on the next page to fill in the ovals on the next
page. Be sure to look at the arrows and words between the ovals to make sure these links
between ovals make sense. Check your work with another group and then your TA.
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Word list: neutron star
black hole
planetary nebula
white dwarf
massive star supernova (Type II)
99.9% of all MS stars
0.1 % of all MS stars
“small” mass (less than 8 Msun)
“large” mass (more than 8 Msun)
more than 25 Msun
make sure to put
these masses in
the ovals too!
Red Giant
with a
with a
which are
less than
which are
more than
produces a
leaving
behind a
produces a
leaving
behind a
which if
from a star
leaving
behind a
19. Using your linked ovals, record two sentences, each describing one of the paths of stellar
evolution (note you have three choices).
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Part 4: The Digital Demo Room Simulations
Now that you have explored the basics of stellar evolution, you will use computer simulations
to help you get some numbers so you may do some comparisons and experiments.
Please go to the Digital Demo Room: Stars and Stellar Evolution simulators website:
http://rainman.astro.illinois.edu/ddr/stellar/index.html
First, we will look at movies that have been made already then you can make your own with
this simulator website. Note that the colors of the stars shown is as accurate as they could
make them. Also note that the timescales aren’t always the same because stars have WILDLY
varying lifetimes (O-stars less than a few million years while M-stars are more than 100 billion).
20. Choose the “Beginner” page. Choose “Sun (Evolution Tracks - ON)” movie. NOTE that the
movie is a little boring at first but that’s because a Sun-like star takes a while to die…
What features did you notice from the ovals on p. 9?
Does the flow-chart (on p. 9) make sense with what you saw in the movie? Explain.
21. Back on the “Beginner” page, choose the movie “10,000 Stars”. This simulates a star
cluster.
a. Comment on the relative amount of time it takes high-mass stars to die in this movie.
b. Comment on the relative amount of time it takes the lower mass stars to move to the
Giant Branch (the upper right)
c. How does this movie show you how the “turn-off” point evolves over time?
22. Back on the “Beginner” page, choose the movie “High Mass Star Death – Supernova.”
Describe at least three interesting features you noticed (could chose color, rotation, size,
events, etc.)
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Open the “Tutorial” page in a new tab or new window so you can refer to it later. You can find
definitions for terms you don’t know within.
Go to the “Intermediate” page now – you’re going to make your own simulations and movies
and obtain data about various stages.
First, we have to look at one of the boxes  “Metallicity”
 In astronomy, anything that is not Hydrogen or Helium is a “metal” – only H and He were
formed in abundance in the Big Bang and EVERY OTHER ELEMENT had to be formed
inside a star! IRON is used as a proxy for all “metals” (because it’s abundant) and metallicity
is measured as the logarithm of the amount of iron relative to the amount of hydrogen.
 One interesting thing we can determine from metallicity is a rough age. The more “metalrich” the star is, the more recycled material from previously exploded stars are contained
within (so low metallicity stars are from MANY stellar generations ago).
23. The range of metallicity values that one can use for the Digital Demo room simulations is
0.0001 to 0.03. The Sun’s value is 0.02 and it is a relatively metal-rich star which means
there is a bunch of recycled material within it (part of a rather recent stellar generation).
What should the metallicity be if a star was born in the Milky Way:
a. around the same time as our Sun? (circle one)
closer to 0.0001
around 0.02
closer to 0.03
b. billions of years before our Sun turned on? (circle one)
closer to 0.0001
around 0.02
closer to 0.03
c. right now? (circle one) closer to 0.0001
around 0.02
closer to 0.03
24. In the history of the Universe, which kinds of stars provide more material to be cosmically
recycled, O-stars or M-stars? Explain the reasoning behind your choice. (hint: think of M-star
lifetimes (Star Clusters lab))
25. Choose a star of 1 solar mass, a metallicity of 0.02, and turn on the evolution tracks. Click
“Submit” and the simulation will start. You will get numbers and a movie is generated.
Record your data on the next page.
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Stage
Time
(Myrs = millions of years)
Mass
(solar masses)
Main sequence Star
Hertzsprung Gap
Giant Branch
Core Helium Burning
First AGB
Second AGB
Carbon/Oxygen WD
Carbon/Oxygen WD
this row represents
the stage at the end
of the simulation so
IGNORE
26. Let’s interpret. The main sequence lifetime of this star is the time from the start until the
“Hertzsprung Gap.” For this star, how long, using BILLIONS of years, is this?
How long from the start of the Hertzsprung Gap until the start of “Carbon/Oxygen WD”?
This is basically how long the Sun will take to “die” – what percentage is that of the
Main Sequence lifetime?
27. Now for some definitions - go to the “Tutorial” page and then “Stellar Evolution.”
a. Why is it difficult to observe a star in the “Hertzprung Gap” stage?
b. What is the star fusing in the “Asymptotic Giant Branch” (AGB) stage?
28. Looking at your data table, what do you notice about the mass of the star as it is dying?
Note that the outer layers are getting ejected. What are those outer layers about to
become? (hint: it’s one of the words in the ovals on p. 9)
How much mass gets ejected into this feature?
Explain why this feature is important to making new stars.
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Phase II – Does the Evidence Match the Conclusion?
Let us consider the two research questions:
- How does stellar mass affect the end states of stars with solar metallicity?
- What masses of stars with solar metallicity become black holes at the end of their lives?
For each star mass, record the TIME in the appropriate places in the table below. Note that
you could fill out the mass of the star at each stage but we’re not asking for that at this time.
 not every box will be filled for every star mass chosen
 use solar metallicity (though it does affect when a star can become a black hole ;) )
 you and your partner could divide and conquer ;)
 you’ve already gotten the 1 Msun data!
Star Mass:
What spectral type?
(use previous tables)
Main Sequence Star
Hertzprung Gap
First Giant Branch
Core Helium Burning
First AGB
Second AGB
Main Sequence
Naked Helium Star
Hertzsprung Gap
Naked Helium Star
Giant Branch
Naked Helium Star
Helium
White Dwarf
Carbon/Oxygen
White Dwarf
Oxygen/Neon
White Dwarf
Neutron Star
Black Hole
1 Msun
5 Msun
8 Msun
10 Msun
22 Msun
35 Msun
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29. Describe at least three things you noticed with your data (at least two trends needed, maybe
something just interesting – a “trend” is something like “As _____ increases, so does
________).
30. Pick one term from the data table that you didn’t know and write a one- to two-sentence
definition of that term. The “Tutorial” page is a good source…
31. If a student proposed a generalization that “All stars above 8 solar masses will eventually
become black holes,” would you agree or disagree with any or all of the generalization
based on the evidence you collected? Explain your reasoning and provide evidence either
from the above questions or from evidence you yourself generate using the simulator.
32. Describe a useful graph you could make using the data you collected on p. 13. You should
discuss axis labels, what trends it could show, linear or logarithmic, things like that.
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DO NOT USE THE COMPUTER FOR THIS PHASE!!
Phase III – What Conclusions Can You Draw From the Evidence?
A student collected data to provide evidence for the following research question using the
steps below and that data is shown in the table.
Question: How does metallicity affect the main sequence lifetime of a Sun-like star?
1. Go to the Digital Demo Room Stellar Structure and Evolution simulator at
http://rainman.astro.illinois.edu/ddr/stellar/index.html
2. Choose the “Intermediate” page
3. Choose six metallicities in the range of 0.0001 and 0.03 making sure to have a variety.
4. Input “1” for the mass of the star and the chosen metallicity in the boxes.
5. Record the time of the “Hertzprung Gap” as that is the first step in star death; all time
before the “HG” was on the main sequence.
6. Record any other data that seems relevant or could be interesting later.
METALLICITY
0.0001
0.0005
0.001
0.005
0.01
0.03
M-S LIFETIME
5919 MYr
6004 MYr
6178 MYr
7788 Myr
9084 Myr
12346 Myr
LIFETIME (in words)
5.9 billion yrs
6.0 billion yrs
6.2 billion yrs
7.8 billion yrs
9.1 billion yrs
12.3 billion yrs
Mass when becomes WD
0.606
0.583
0.578
0.570
0.536
0.514
33. What conclusions and generalizations can you make from the evidence gathered in order
to answer the above research question? (careful! what is the question asking!) You must
explain your reasoning and provide evidence from the table to support your reasoning.
Evidence-based Conclusion:
34. What conclusions can you draw from other included data?
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Phase V – Formulate a Question, Pursue Evidence, & Justify Your Conclusion
Your task is a) design an answerable research question about the evolution of stars using the
Digital Demo Room simulator (hint: comparisons are good), b) propose a plan to pursue
evidence, c) collect data using the Digital Demo Room website and d) create an evidencebased conclusion about stars and stellar evolution. If you make a graph, you must print it or
must show your TA after making a sketch.
Research Report:
Specific Research Question (must be checked off by TA _____ before collecting data in earnest):
Step-by-Step Procedure, with Sketches if Needed, to Collect Evidence:
Data Table / Graph and/or Results (not just a summary, need actual DATA):
(must be checked off by TA _____)
tip: extra data table
on p. 18
Evidence-based Conclusion Statement:
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Phase VI – Summary
Create a 50- to 65-word summary, in your own words, that describes general stellar
evolution.
You should cite specific evidence YOU have collected in your description, not describe what
you have learned in class or elsewhere. Feel free to create and label sketches to illustrate your
response.
Topics could include: mass, fuel sources, metallicity, types of stars, lifetimes of stars, death of
stars, comparisons…
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Appendix: Extra Data Table
Star Mass:
What spectral type?
(use previous tables)
Main Sequence Star
Hertzprung Gap
First Giant Branch
Core Helium Burning
First AGB
Second AGB
Main Sequence
Naked Helium Star
Hertzsprung Gap
Naked Helium Star
Giant Branch
Naked Helium Star
Helium
White Dwarf
Carbon/Oxygen
White Dwarf
Oxygen/Neon
White Dwarf
Neutron Star
Black Hole