1. Protostars are difficult to observe because
the protostar stage is very short.
they are surrounded by cocoons of gas and dust.
they radiate mainly in the infrared.
all of the above.
they are all so far away that the light hasn't reached us yet.

2. Interstellar gas clouds may collapse to form stars if they
have very high temperatures.
encounter a shock wave.
rotate rapidly.
are located near main sequence spectral type K and M stars.
all of the above.

3. Opacity is
the balance between the pressure and force of gravity inside a star.
the force that binds protons and neutrons together to form a nucleus.
the force that binds an electron to the nucleus in an atom.
a measure of the resistance to the flow of radiation (photons) through a gas.
the temperature and density at which a gas will undergo thermonuclear fusion.

4. The visual pink glow of the Great Nebula in Orion
is a Herbig-Haro object.
is a reflection nebula of dust.
is an emission nebula of thin gas.
contains only young low mass stars.
is believed to be about 5 billion years old.

5. Emission nebulae are also called ____ because they are composed of ionized hydrogen.
HI regions
HII regions
Bok globules
HeI regions
reflection nebulae

6. ____ are star-like objects that contain less than 0
____ are star-like objects that contain less than 0.08 solar masses an will never raise their core temperatures high enough that the proton-proton chain can begin. Other minor fusion reactions do occur in these objects. They fall in a gap between the low-mass M dwarf stars and the massive planets in which nuclear fusion never occurs.
Brown dwarfs
Herbig-Haro objects
Bok globules
T-Tauri star
Main sequence stars

7. The average star spends ____ of its lifetime on the main sequence.1% 2%
10%
20%
90%

8. The lower edge of the Main Sequence band is known as ____ and represents the location in the HR-diagram at which stars begin their lives as main sequence stars.
the Coulomb barrier
hydrostatic equilibrium
the birth line
the zero-age main sequence
the evolutionary track

9. ____ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.
The CNO cycle
The proton-proton chain
Hydrostatic equilibrium
The neutrino process
none of the above

10. The diagram below is an HR diagram
The diagram below is an HR diagram. The line indicates the location of the main sequence. Which of the five labeled locations on the HR diagram indicates a luminosity and temperature similar to that of a T Tauri star? 1 2 3 4 5

11. In the proton-proton chain
no neutrinos are produced.
energy is released because a hydrogen nucleus has a greater mass than a helium nucleus.
no photons are produced.
carbon serves as a catalyst for the nuclear reaction.
energy is produced in the form of gamma rays and the velocity of the created nuclei.

12. The carbon-nitrogen-oxygen cycle
operates at a slightly lower temperature than the proton-proton chain.
is most efficient in star less massive than the sun.
occurs when carbon and oxygen combine to form nitrogen, which produces energy.
produces the energy responsible for bipolar flows.
combines four hydrogen nuclei to form one helium nucleus, which produces energy.

13. What causes the outward gas pressure that balances the inward pull of gravity in a main sequence star?
The rapid outward flow of gas.
The rapid inward flow of the gas.
The high temperature of the gas.
The high density of the gas.
C and D.

14. Convection is important in stars because it
increases the temperature of the star.
mixes the gases of the star.
transports energy outward in the star.
carries the neutrinos to the surface of the star where they can escape.
B and C.

15. ____ occurs when most of the material collapsing to form a protostar has fallen into a disk around the star and a strong wind from the warm protostar ejects material from its poles.
An emission nebula
Hydrostatic equilibrium
The proton-proton chain
The thermonuclear fusion of hydrogen
A bipolar outflow

16. Which of the following is NOT evidence of the existence of an interstellar medium?
extinction of light from distant stars
very narrow calcium absorption lines in the spectra of O and B stars
reddening of more distant stars
dark cloud silhouetted against background stars
molecular absorption lines in the spectra of cool stars

17. Molecular clouds can be observed
using infrared telescopes to detect ionized gas in the clouds.
using x-ray telescope to observe the x-ray radiation from the molecules in the cloud.
using radio telescopes to observe the CO emission from the clouds.
by looking for blue wispy regions near star clusters.
by looking for the 21-cm radiation from hydrogen.

18. The HR diagram main sequence of stars has a limit at the lower mass or energy output end because
low mass stars form from the interstellar medium very rarely.
low mass objects are composed primarily of solids, not gases.
pressure does not depend on temperature in degenerate matter.
the lower limit represents when the radius of the star would be zero.
there is a minimum temperature for hydrogen nuclear fusion (the definition of a star).

19. Due to the dust in the interstellar medium, a distant star will appear to an observer on Earth to be
brighter and cooler than it really is.
brighter and hotter than it really is.
fainter and cooler than it really is.
fainter and hotter than it really is.
unchanged in brightness or apparent color.

20. The lowest mass object that can initiate thermonuclear fusion of hydrogen has a mass of about
1 solar masses
80 solar masses
8 solar masses
0.08 solar masses
0.001 solar masses

21. Stars with masses less than ____ produce most of their energy via the proton-proton chain.
0.01 solar masses
0.1 solar masses
1.1 solar masses
11 solar masses
110 solar masses

22. The creation of ____ requires that a young hot star (T=25,000 K) be relatively nearby.
Emission nebulae
HI regions
Molecular clouds
The cool gas of the interstellar medium
21 cm radiation

23. The extinction of starlight due to the interstellar medium I
The extinction of starlight due to the interstellar medium I. is the greatest in the ultraviolet. II. is the greatest in the infrared. III. is caused by ionized hydrogen. IV. is caused by dust particles.
I & III
II & III
I & IV
II & IV
only IV

24. ____ are small dark nebulae about 1 light-year in diameter that contain 10 to 1,000 solar masses.
HI regions
HII regions
Emission nebulae
Bok globules
Reflection nebulae

25. Absorption lines due to the interstellar medium indicate that some components of the interstellar medium are cold and of a very low density because
the lines are blue shifted.
the lines are red shifted.
the lines are extremely broad.
the lines are extremely narrow.
the lines are much darker than the stellar lines.

26. Stars are born in
reflection nebulae.
dense molecular clouds.
HII regions.
the intercloud medium.
the local bubble.

27. Absorption lines due to interstellar gas
are wider than the lines from stars because the gas is hotter than most stars.
are narrower than the lines from stars because the gas has a lower pressure than stars.
indicate that the interstellar medium contains dust.
indicate that the interstellar medium is expanding away from the sun.
none of the above.

28. What is the lifetime of a 10 solar mass star on the main sequence?
3.2×107 years
320 years
3.2×1012 years
1×109 years
1×1011 years

29. What is the lifetime on the main sequence of a 2 solar mass star compared to the approximately 10 billion year life of our Sun? Pick the best.
1.8×1011 years
1.8×1010 years
1.8×109 years
1.8×106 years
1.8×103 years

30. The H-R diagram of a young star cluster shows
that high mass stars have not yet reached the main sequence yet.
that low mass stars have not yet reached the main sequence yet.
mainly giant stars.
no stars since none have reached the main sequence yet.

31. As a star begin to form the initial energy source is from
nuclear fusion.
nuclear fission.
gravitational potential energy.
magnetic fields.

32. While on the main sequence a star's primary energy source comes from
nuclear fusion.
nuclear fission.
gravitational potential energy.
magnetic fields.

33. What must occur for an object to be considered a main sequence star?
Hydrostatic equilibrium
Nuclear fusion reaction in the core
Protostar life begins
Both A and B

34. What force(s) are responsible for the collapse of an interstellar cloud?
nuclear
gravity
electric
magnetic
both C and D

35. Star clusters are important to the study of stellar evolution because stars in a given cluster have the same
temperature.
mass.
age.
luminosity.

36. Why do higher mass stars live shorter lives on the main sequence than lower mass stars?
Higher mass stars burn through their nuclear fuel faster.
Lower mass stars don't get their energy from that same nuclear fusion source as higher mass stars.
Higher mass stars have less hydrogen fuel to burn.
Lower mass stars spend a longer time evolving to the main-sequence.
All of the above are false.

37. Why do nuclear fusion reactions only take place in the interior of a star?
The magnetic fields are strongest there.
The temperature is the hottest in the center.
The density of material is very high in the center.
Both B and C.
FALSE; Nuclear fusion reactions can take place in the core and corona where temperatures are high enough.

38. What characteristic of a star primarily determines its location on the main sequence?
age
distance from the galactic center
mass
space motion
radius

39. Some young star clusters contain large numbers of protostars.
True
False

40. Ninety percent of all stars fuse helium to form carbon and lie on the main sequence.
True
False

41. The sun has a core in which energy travels outward primarily by radiation.
True
False

42. The sun makes most of its energy by the CNO cycle.
True
False

43. The sun makes most of its energy by the proton-proton cycle.
True
False

44. Energy flows by radiation or convection inside stars but almost never by conduction.
True
False

45. Hydrostatic equilibrium refers to the balance between weight and pressure.
True
False

46. The Orion region contains young main sequence stars and an emission nebula, but the original molecular cloud they formed out of has been dispersed.
True
False

47. T Tauri stars are believed to be young, high mass main sequence stars.
True
False

48. The dust in the interstellar medium can make distant stars look redder than they really are.
True
False

49. Molecular clouds are mapped using CO instead of hydrogen because CO is much more abundant than hydrogen in molecular clouds.
True
False

50. HII regions are found near stars cooler than 25,000 K because large amounts of ultraviolet photons would totally destroy the hydrogen atoms in the gas.
True
False

51. The pressure of a gas depends on the temperature and density of the gas.
True
False

52. Some massive stars live only a few million years.
True
False

53. Pre-main sequence stars are above and to the right of the main sequence.
True
False

54. High mass stars evolve more slowly to the main sequence than lower mass stars.
True
False

55. Main sequence stars many times the mass of the sun produce their energy by the set of nuclear reactions called the CNO cycle.
True
False