1. The Jeans instability occurs when
A the pressure in a gascloud is less than its gravitational force.
B the pressure in a gascloud is greater than its gravitational force.
C the temperature in acollapsing cloud is hot enough to fuse Hydrogen.
D the temperature in acollapsing cloud is hot enough to fuse Helium.
2. At what stage in its life does a star pass through the protostarphase?
A After condensationbut before nuclear reactions begin in its core.
B While it is convertinghydrogen into helium in its core.
C After nuclear reactionsend in its core, but before the red giant phase.
D When it is expanding insize as a red giant or supergiant.
3. The characteristics of an open cluster of stars are
A a few hundredmembers, often very young and still embedded in the gas and dustfrom which they were formed.
B many thousand members, ofdifferent ages.
C a few dozen members, theremnant of a globular cluster of stars from which most of themembers have escaped.
D hundreds of thousands ofmembers, all very old, and no or very little interstellar gas anddust.
4. What is a protostar?
A A star near the endof its life, before it explodes as a supernova.
B A sphere of gas aftercollapse from an interstellar cloud but before nuclear reactionshave begun.
C A small interstellarcloud, before it collapses to become a star.
D A shell of gas left behindfrom the explosion of a star as a supernova.
5. What is an H II region?
A A region of hot,ionized hydrogen around one or more O and B stars.
B A region of molecularhydrogen inside a giant molecular cloud.
C A region of neutral,atomic hydrogen in interstellar space.
D A region of gas and dustformed by the explosion of a massive star.
6. The total time that the Sun will spend as a main sequence staris
A about 4.5 millionyears.
B at least 200 billion yearsyears.
C about 1 millionyears.
D about 10 billionyears.
7. How is a star's lifetime related to its mass?
A The lifetimes ofstars are too long to measure, so it is not known how (or if) theirlifetimes depend on mass.
B A star's lifetime does notdepend on its mass.
C Lower-mass stars runthrough their lives faster and have shorter lifetimes.
D Higher-mass stars runthrough their lives faster and have shorter lifetimes.
8. In the Hertzsprung-Russell diagram, how does the position of atypical star change while it is at the main sequence phase of itsevolution?
A A star's position onthe main sequence is determined only by its mass and not its age,and so, stars do not move along the main sequence duringevolution.
B Massive stars (4 solarmasses) move toward the upper left as their luminosity increases,while lower-mass stars move toward the lower right as theirtemperature decreases.
C Stars move from upperright to lower left while they are on the main sequence.
D Stars move from upper leftto lower right while they are on the main sequence.
9. Why does the core of the Sun contain more helium and lesshydrogen than does the surface of the Sun?
A Helium condenses moreeasily so, when the Sun was forming the core became helium-rich;vast quantities of hydrogen were added only after the core becamemassive enough.
B The hydrogen has beenlifted out of the core by the Sun's magnetic field.
C Helium is heavier thanhydrogen, and has sunk toward the center in a process of chemicaldifferentiation.
D Thermonuclear reactionshave converted much of the original hydrogen in the core intohelium.
10. In terms of nuclear reactions, what is the next stage of astar's life after the end of hydrogen burning in the core?
A Hydrogen burning ina thin shell around the core.
B Helium burning in thecore.
C Carbon burning.
D Death (it becomes eithera supernova or a white dwarf).