Phys110, Exam 1 Key

1. What spectral class is found between O and A?

a.	G
b.	K
c.	M
d.	F
e.	B

2. A positron is a particle that is also the result of a proton-proton chain in the core of a main sequence star like our Sun. What happens to this positron?

a.	It quickly annihilates an electron
b.	It quickly combines with a neutron forming a proton
c.	It quickly escapes the interior of the star
d.	It quickly decays into a proton and neutron
e.	It quickly decays into two gamma rays

3. Hydrostatic equilibrium describes the balance between

a.	Temperature and density
b.	Mass and luminosity
c.	Pressure and gravity
d.	Radius and luminosity
e.	Magnitude and spectral type

4. The measurement of the parallax of a star allows us to determine directly the star’s:

a.	Temperature
b.	Color
c.	Mass
d.	Age
e.	Distance

5. A helium flash is:

a.	The nearly explosive beginning of helium fusion in the dense degenerate core of a red giant star
b.	A flash of light as the star encounters a cloud of helium
c.	A bright X-ray flash from a helium accretion disk surrounding a black hole
d.	An infrared burst from helium-rich proto-stars
e.	A flash of light as the star explodes as supernova type I

6. Which of the following is a key difference between the pressure in a normal gas and in a degenerate gas?

a.	There is no difference at all.
b.	In a degenerate gas, pressure varies rapidly with time.
c.	Degenerate pressure exists whether matter is present or not.
d.	In a degenerate gas, pressure does not depend on density.
e.	In a degenerate gas, pressure does not depend on temperature.

7. What process is responsible for the formation of a new star?

a.	The chemical collapse of part of a cloud of interstellar medium
b.	The nuclear collapse of part of a cloud of interstellar medium
c.	The thermal collapse of part of a cloud of interstellar medium
d.	The gravitational collapse of part of a cloud of interstellar medium
e.	The magnetic collapse of part of a cloud of interstellar medium

8. What kind of stars are found scattered across the top of the H-R diagram?

a.	White dwarfs
b.	Supergiants
c.	Main sequence stars
d.	Stars made of helium
e.	Protostars

9. Two stars have the same color, but differ by 5 magnitudes in absolute magnitude. What stellar property must be different in the two objects?

a.	Distance
b.	Radius
c.	Mass
d.	Temperature
e.	Composition

10. When a star is just evolving off the main sequence, which element is most common in the core?

a.	Carbon
b.	Iron
c.	Helium
d.	Oxygen
e.	Silicon

11. The spectral class of a star is a measure of the star’s:

a.	Mass
b.	Temperature
c.	Chemical composition
d.	Luminosity
e.	Distance

12. As soon as its core hydrogen is exhausted, a star like the Sun begins to evolve:

a.	Toward the upper right in the H-R diagram
b.	Toward the lower right in the H-R diagram
c.	Toward the upper left in the H-R diagram
d.	Toward the lower left in the H-R diagram
e.	Up the main sequence

13. As you move up and along the main sequence on the H-R diagram:

a.	Stellar temperature increases
b.	Stellar mass increases
c.	Stellar radius increases
d.	Stellar luminosity increases
e.	All of the above

14. An emission nebula, or HII region, is hot enough to emit its own light. What heats the gas in the emission nebula?

a.	Young and massive stars embedded in the gas
b.	Shock waves from nearby supernova explosions
c.	Radioactive decay of elements such as uranium and plutonium in the gas
d.	Collisions between distinct clouds of gas
e.	Fusion of hydrogen in the nebula

15. Star cluster X is older than star cluster Y, thus (what must be true?):

a.	Cluster X has fewer stars on the main sequence than does cluster Y.
b.	The turnoff point for star cluster X is at lower luminosity and temperature than that for cluster Y.
c.	The turnoff point for cluster X is at higher luminosity and temperature than that for cluster Y.
d.	Cluster X has a smaller radius than does cluster Y.
e.	Cluster X is spherical, whereas cluster Y is not.

16. The H-R diagram of a cluster shows that the least massive stars lie above the main sequence (i.e., they are more luminous than main sequence stars). What can we conclude about this cluster?

a.	It is quite old
b.	It is quite young
c.	It has an unusual chemical composition
d.	Its stars have large magnetic fields
e.	Its least massive stars are more massive than the stars near the Sun

17. A star on the horizontal branch is best described as:

a.	A giant that is converting carbon to helium in a shell surrounding the core
b.	A normal star that is burning hydrogen to helium in its core
c.	A giant that is burning helium to carbon in its core
d.	A dwarf star that has exhausted all nuclear energy sources
e.	A pulsating star that experiences regular helium flashes

18. What type of stars obey the period-luminosity relationship?

a.	RR Lyrae stars
b.	White dwarfs
c.	T Tauri stars
d.	Main sequence G-type stars
e.	Cepheid variables

19. Which star has the highest surface temperature?

a.	M4V
b.	G2I
c.	A6V
d.	G7III
e.	K3IV

20. The main sequence life of an O-type star compared with that of an M-type star is:

a.	Much longer because there is more fuel available
b.	Much shorter because the O star burns fuel at a much higher rate
c.	About the same because all stars live for about 10 billion years
d.	Much shorter because there is less hydrogen in an O star
e.	Much longer because an O star has many more types of fuel available