ASTR 321
Test 1

Old Test

1. M type stars do not show strong hydrogren lines in the visible light portion of their spectra. Is this true because M stars do not contain hydrogen? If this is not true (M stars do in fact contain hydrogen), explain why M stars do not show strong hydrogen lines in their spectra.

2. What is Wien's Law? What is the Stefan-Boltzmann Law? To what kind of object do the Wien Law and Stefan-Boltzmann Law apply, strictly speaking?

3. Describe how astronomers determine the surface temperatures of stars using their continuous spectra OR their absorption line spectra.

4. Sketch a Hertzsprung-Russell (H-R) diagram. Label the axes and indicate the locations of the Super-giants, normal giants, Main Sequence stars, and white dwarfs. Circle the region in the H-R diagram where the bulk of the observable stars are found. Compare the radii of a G2V star, a G2I star, and a G2 white dwarf.

Stellar Properties:

What are the ranges for the properties of stars? (What is the most important property of stars mentioned in class?)

How do we determine radii, temperatures, and luminositites of stars? What is the hardest part of determining the luminosities of stars?

How could we determine the distance to Sun? What method for the determination of stellar radii has (historically) been the most useful? Roughly, how does the method work?

What is the difference between flux and luminosity? Of flux (energy flow per unit area and per unit time) and luminosity (total power). Which determines how bright an object appears to us (an observer on Earth?) The total emission from an object over all wavelengths is its bolometric luminosity. How are bolometric luminosities determined?

What information can be extracted from spectra (both from continuous and absorption line spectra)? How is information extracted from spectra?

Discuss how atoms are put together (what are electons, protons, and neutrons?), How large is an atom? How large is the nucleus of an atom? Where is most of the mass of an atom contained? What force holds an atom together? What force holds the nucleus together? What determines the type of element an atom is? What is an isotope?

Why does the line spectrum serve as the fingerprint of an element? How do absorption and emission lines arise? What are Lyman lines, Balmer lines, Paschen lines, Brackett lines, and Pfund lines?

Roughly sketch the energy level diagram for a hydrogen atom. On your diagram draw arrows to indicate an absorption process, an emission process, an ionization process, and a recombination process. Draw another energy level diagram for hydrogen and then indicate the Lyman, Balmer, Paschen, Brackett, and Pfund transitions.

Spectral Classification. What is the Morgan-Keenan (MK) spectral classification scheme? How are spectra classified in the MK scheme? (What criteria are used to classify stellar spectra?) Why do different stars have different appearing spectra? Explain why the strength of the hydrogren lines change as you go from O --> M.

Consider a star with temperature T = 10,000 K. This temperature corresponds to what energy? Give your answer in electron volts. The first excited state of hydrogen sits 10.2 eV above the ground state. For a star with temperature 10,000 K, what fraction of its particles have energy large enough to excite hydrogen atoms? what fraction of its particels have energy large enough to ionize hydrogen atoms? Use the Boltzmann distribution to find your answer.
Repeat your answer for a star with temperature 5,000 K and for a star with temperature 30,000 K.
What is meant by HI, HII, CaII, Fe XXVI? More to the point what is meant the Roman Numerals appended to the abbreviations for the elements?
How can we infer that the Sun (or any star) contains iron or other elements such as carbon, oxygen, nitrogen, and so on.
What important stellar property is carried by the spectral class of a star? List the order of the spectral classes from high to low temperature.
What are the Luminosity Classses? For what types of star do the symbols I, III, and V stand?
What is a blackbody? Give an example of a blackbody? What is the most perfect blackbody known? What characeristic of a blackbody determines its properties? What are the Wien Law and Stefan-Boltzmann Law? Which blackbody law is used (and how may it be used to determine stellar temperatures)? Why are blackbodies interesting and useful idealizations for astronomers? How does the color of a blackbody depend on its temperature? (Draw a plot to support your answer.) Which of the following blackbodies produces the most IR radiation, a 10,000 K blackbody or a 3,000 K blackobdy. Which of the two previous blackbodies appears the reddest?

Hertzsprung-Russell Diagram

What is the utility of making plots of large data sets? What do correlations in plots mean? What don't correlations in plots show?
What is the Herzsprung-Russell Diagram (H-R)? Draw an H-R diagram and carefully label the axes and the various portions of the diagram. Carefully mark the location of the Sun.
What information can we deduce about how stars evolve using how stars cluster in the HR diagram? Where do we find the largest (in diameter) stars in the HR diagram? Where do we find the smallest (in diameter) stars in the HD diagram?

Lifetime of a Main Sequence Star with mass M > 0.7 Solar Masses:

What are Main Sequence Stars, giants (Red Giants), Super-Giants, Asymptotic Giant Branch (AGB) stars, white dwarfs, planetary nebulas, neutron stars?
What is the Mass-Luminosity relation? How can it be used to infer a scaling law for stellar lifetimes? What stars have the longest lifetimes, massive orlow mass stars? What is the argument one uses to infer the relationship which describes how the lifetime of a star depends on its mass? What is the Kelvin-Helmholtz time scale? the nuclear time scalae?, the gravitational time scale?
What is the luminosity function? Which type of stars are the most numerous in our Galaxy?

Poperties of the Sun and Stars:

What are the different ways discussed in class that stars can generate energy? What is the most efficient way to generate energy of those mentioned in class? What mechanism is used by Main Sequence stars to generate energy?
Nuclear Energy Generation--the conversion of 4 hydrogen nuclei into a helium nucleus + energy + other particles. What conservation laws are important for consideration of nuclear fusion? What are leptons? What are bosons? What are quarks? What are neutrinos? Why is nuclear fusion so difficult (that is, what is the major impediment to fusion)? Using the electrostatic potential, thermal energy, strong force, and the Boltzmann distribution to explain why, classically, nucelar fusion is difficult.
What is the proton-proton cycle (pp-cycle)? What is the carbon-nitrogen-oxygen tricycle (CNO cycle)?
What are the energy transport mechanisms used by stars? Which ones are the most important for the Sun?
What important role outside of energy transport does convection play in the observable properties of the Sun?
Main Seqeunce stars are in equilibrium; both mechanical equilibrium (hydrostatic equilibrium -- stars are not changing in size very quickly) and thermal equilibrium (the temperature structures of stars are not changing very quickly -- the energy losses due to radiation and particles from stars are roughly balanced by the energy production due to nuclear fusion reactions). Show that, in the absence of pressure effects, a star like the Sun would collapse (due to gravity) in less than 1 hour. Contrast this time for low mass and high mass stars. Use dimensional arguments to justify your answer.
- Write down the equations of stellar structure.
- Why are neutrinos so much useful as probes of the interior of the Sun than are photons (the light we receive from the Sun)? What other methods are used to probe the interior of the Sun? What was the Solar Neutrino Problem? How was the Solar Neutrino Problem resolved?
Use dimensional arguments to show how the temperature of star dominated by radiation pressure scales with the mass and radius of a star.
Use dimensional arguments to show how the density of a star varies with mass.
Use dimensional arguments to show how the radius of a star varies with mass.
Use dimensional arguments to show the rough mass where radiation pressure and gas pressure play similar roles in the structure of a star
Use dimensional arguments to show how the luminosity of a Main Sequence star depends on mass for a star dominated by electron scattering, or by bound-free or free-free transitions.