Answers

ConceptChecks

ConceptCheck 18-1: No. Toaster filaments glow red because they are hot blackbodies. The red light from an H II region comes from a single red emission line. The gas in an H II region is too thin to emit as a blackbody.

ConceptCheck 18-2: No. The blue gas is simply reflecting starlight, but with more reflection at blue wavelengths.

ConceptCheck 18-3: Yes. The evolutionary track for a 1 M_⊙ solar star like our Sun in Figure 18-10 indicates that such a protostar is almost 100 times more luminous when it forms then when it reaches the main sequence.

ConceptCheck 18-4: The red dashed lines indicate how long it takes for protostars to cross those lines on their way to the main sequence. A 2 M_⊙ star reaches the main sequence after about 10⁷ (10 ten million) years.

ConceptCheck 18-5: No. The energy source of a protostar is gravitational contraction compressing and heating the protostar’s interior. The energy source for all main-sequence stars are nuclear reactions converting hydrogen to helium in the star’s core.

ConceptCheck 18-6: For a main-sequence star that ends up with less than 3 M_⊙ it was more massive as a protostar; these protostars lose more mass than they gain. The masses shown in Figure 18-10 are for the final main-sequence stars.

ConceptCheck 18-7: No. The pillars are eroded away by hot luminous stars outside of the pillars that emit ultraviolet radiation.

ConceptCheck 18-8: Ultraviolet light from hot O and B stars creates an H II region that expands into the surrounding molecular cloud. This compresses the cloud, causing star formation.

ConceptCheck 18-9: Protostars begin to form when gravity overwhelms a gas cloud’s internal pressure. This is initiated when an external process compresses the cloud.