11.The lower edge of the Main Sequence band known as : 1777237
11.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.
A.the Coulomb barrier
C.the birth line
D.the zero-age main sequence
E.the evolutionary track
12.____ is the thermonuclear fusion of hydrogen to form helium operating in the cores of massive stars on the main sequence.
A.The CNO cycle
B.The proton-proton chain
D.The neutrino process
E.none of the above
13.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?
14.In the proton-proton chain
A.no neutrinos are produced.
B.energy is released because a hydrogen nucleus has a greater mass than a helium nucleus.
C.no photons are produced.
D.carbon serves as a catalyst for the nuclear reaction.
E.energy is produced in the form of gamma rays and the velocity of the created nuclei.
15.The carbon-nitrogen-oxygen cycle
A.operates at a slightly lower temperature than the proton-proton chain.
B.is most efficient in star less massive than the sun.
C.occurs when carbon and oxygen combine to form nitrogen, which produces energy.
D.produces the energy responsible for bipolar flows.
E.combines four hydrogen nuclei to form one helium nucleus, which produces energy.
16.What causes the outward gas pressure that balances the inward pull of gravity in a main sequence star?
A.The rapid outward flow of gas.
B.The rapid inward flow of the gas.
C.The high temperature of the gas.
D.The high density of the gas.
E.c and d
17.The free-fall contraction of a molecular cloud
A.can be initiated by shock waves from supernovae.
B.can be initiated by nearby spectral type G stars.
C.can be initiated by the rotation of the cloud.
D.causes the cloud to become transparent to ultraviolet radiation.
E.causes the particles in the cloud to decrease the speed with which they move.
18.Convection is important in stars because it
A.increases the temperature of the star.
B.mixes the gases of the star.
C.transports energy outward in the star.
D.carries the neutrinos to the surface of the star where they can escape.
E.b and c
19.____ 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.
A.An emission nebula
C.The proton-proton chain
D.The thermonuclear fusion of hydrogen
E.A bipolar outflow
20.Which of the following is NOT evidence of the existence of an interstellar medium?
A.extinction of light from distant stars
B.very narrow calcium absorption lines in the spectra of O and B stars
C.reddening of more distant stars
D.dark cloud silhouetted against background stars
E.molecular absorption lines in the spectra of cool stars