Answers

ConceptChecks

ConceptCheck 20-1: The energy released from core helium fusion expands the star’s core, and this slows down reactions for both the core helium fusion and the hydrogen-fusing shell. With a decreased rate of energy released in and around the star’s core, the luminosity of the star decreases. These stars are called horizontal-branch stars.

ConceptCheck 20-2: Most of the carbon in your body came from stellar winds around carbon stars.

ConceptCheck 20-3: The visible colors of a planetary nebula do not come from the temperature of the nebula’s gas. Rather, a planetary nebula’s emission is powered by the ultraviolet light emitted by the central star.

ConceptCheck 20-4: For any atom with electrons, the electrons orbit very far from the nucleus, compared to the size of the nucleus itself. Thus, any atom on the periodic table of the elements consists mostly of empty space.

ConceptCheck 20-5: Yes. Electron degeneracy pressure can support white dwarf masses up to the Chandrasekhar limit of 1.4 M_⊙. There can be no white dwarfs beyond this limit. As shown in Figure 20-10, a 1.2- M_⊙ white dwarf is smaller than Earth.

ConceptCheck 20-6: No. The Sun’s size can be read in the Cosmic Connections figure from the diagonal dashed lines. The Sun expands to about 100 R_⊙ as a red giant, but this phase ends when the Sun is about 12.23 billion years old (Stage 3 in the figure). At 12.365 billion years, the Sun is even larger (Stage 5). The Sun is also more luminous by this time, and remains so until nuclear reactions come to an end.

ConceptCheck 20-7: It is the loss of the star’s mass going into the nebula that relieves pressure on the core. The pressure becomes too low for further nuclear reactions to occur (Stages 6 and 7 in the Cosmic Connections figure).

ConceptCheck 20-8: Yes. However, these reactions do not release energy; energy must be added for these reactions to take place.

ConceptCheck 20-9: No. In order for the iron core to collapse, it must overcome degenerate electron pressure, which requires over 1.4 M_⊙ (this is the Chandrasekhar limit).

ConceptCheck 20-10: For stars that had main-sequence masses over 8 M_⊙, iron cores larger than the Chandrasekhar limit can develop and collapse. During collapse, iron is converted into neutrons, which then exert a degenerate-neutron pressure that can halt further collapse.

ConceptCheck 20-11: Neutrinos deliver energy to these bubbles. These neutrinos were produced in the collapse when electrons combined with protons to produce neutrons and neutrinos.

ConceptCheck 20-12: Atoms heavier than iron such as uranium are produced during a supernova. Nuclear reactions producing uranium require an input of energy, and there is plenty of gravitational energy in the collapse that ultimately ends up producing atomic elements heavier than iron. Uranium does not release energy when produced.

ConceptCheck 20-13: The typical progenitor star for a core-collapse supernova is a red supergiant. The blue supergiant progenitor star that led to SN 1987A was still large in mass but smaller in size than a red supergiant. That means some of the supernova energy that would have normally gone into light was used just to push the smaller star’s matter outward against its own gravity.

ConceptCheck 20-14: When a high-energy neutrino hits a proton in a big water tank, it can cause a positron to be emitted. The positron can travel faster than light travels within that same water, and this produces a shock wave of light that can be detected.

ConceptCheck 20-15: By comparison with the spectra in Figure 20-18, the lack of hydrogen, helium, or silicon lines matches the core-collapse of a Type Ic supernova.

ConceptCheck 20-16: By observing the expansion rate of the remnant (its speed), and how far it has expanded (its approximate radius), the time since the explosion can be estimated.

ConceptCheck 20-17: Just as a white dwarf is held up by degenerate electron pressure, a neutron star is held up by degenerate neutron pressure.

ConceptCheck 20-18: By shrinking during formation, neutron stars spin rapidly and have very large magnetic fields. Charged particles in these magnetic fields emit the light (often radio waves) observed at Earth. The light arrives in pulses the way a lighthouse beacon repeatedly sweeps by an observer.

ConceptCheck 20-19: In a supernova, the entire white dwarf undergoes nuclear reactions, whereas in a nova, only a surface layer undergoes nuclear fusion.

ConceptCheck 20-20: A type Ia supernova blows the object apart. For a nova or X-ray burster, only an outer layer of transferred mass undergoes nuclear reactions. When enough mass is again transferred from the binary partner, the detonation repeats itself.