ConceptCheck 12-1: The helium would turn into silicon by first fusing into carbon, which would then fuse into neon, which would finally fuse into silicon.

ConceptCheck 12-2: Sunlike stars are far too small for heavy metal fusion to occur at all.

ConceptCheck 12-3: Both. A core-collapse supernova occurs because its central core is no longer able to support its outer layers. These unsupported outer layers first fall inward then bounce back explosively in the outward direction.

ConceptCheck 12-4: A ring of dust might cause part of the expanding envelope to move slower, so if it was not surrounded by a retarding ring of dust, the expansion should have been nearly circular.

ConceptCheck 12-5: Virtually none, if there is no surrounding gas or dust with which to collide. Expanding supernova remnants are most easily seen when they expand and collide with interstellar gas and dust, causing the dust to glow.

ConceptCheck 12-6: Neutrons, which compose a neutron star, can be packed together much more closely than carbon atoms in a white dwarf.

ConceptCheck 12-7: A pulsar can be most easily described as a quickly, rotating neutron star that appears to flash on and off quickly as its brightest portions are alternatively observed and not observed.

ConceptCheck 12-8: The pulsar that is spinning slowest, and thus having longer periods between pulsations, is the oldest, because isolated pulsars tend to slow down over time after their formation.

ConceptCheck 12-9: Nothing would change. A black hole made of the same mass as our Sun would have the same effect on the planets as our Sun currently does because it would have the same gravitational properties at planetary distances.

ConceptCheck 12-10: A nonrotating black hole has infinite density and a zero diameter singularity, independent of its mass.

ConceptCheck 12-11: The more massive of the two stars would have evolved faster, depleted its core fuel supply first, and would have become a black hole, while the second, less massive star would have been evolving more slowly.

ConceptCheck 12-12: Gas near the center of galaxies is observed to be moving at tremendously high speeds, which could only be moved so quickly due to the tremendous gravitational effects of a supermassive black hole.

ConceptCheck 12-13: A white dwarf can only become a Type Ia supernova if it is part of a very close binary star system and it can gravitationally draw matter for fuel from its nearby companion star.

ConceptCheck 12-14: Type Ia supernovae are brighter.

ConceptCheck 12-15: Type II supernovae have a slower brightness decay rate.

ConceptCheck 12-16: The luminosity of a nova occurs as material from a companion is ignited only on the white dwarf’s surface for a short time, whereas the luminosity of a Type Ia supernova occurs when the white dwarf completely blows itself apart, leaving nothing behind.

ConceptCheck 12-17: Novae occur when explosive hydrogen fusion takes place on a white dwarf, which lasts for weeks, whereas explosive helium fusion on a neutron star produces an X-ray burster that lasts for seconds.