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Week 10 Homework Solutions
Chap11: 1, 2, 3, 15, 19, 21
Chapter 11
1. Stars are composed principally of H and He, 75% and 25%, respectively. Stars
also are composed of heavier elements produced by stars and disseminated by
their explosive deaths. These heavier elements are important to prospect of
planets because we believe the planets begin forming with the condensation of
solid “seeds” of metal, rock, or ice, all of which are compose of elements besides
H and He.
2. Following the work of Annie Jump Cannon, we divide stars into the seven
major spectral types, O, B, A, F, G, K, M. The differences in the spectral type
correspond to differences in temperature, with the hottest stars of type O and
the coolest type M. The differences in temperature of the stars depends almost
entirely on the mass of the star, with the temperature of the star increasing
with mass. Thus mass determines the star’s spectral type.
More massive stars are more luminous than less massive stars. Even though
the more massive stars have more H (fuel) to burn, they are so luminous that
they burn through it quickl yand have shorter lifetimes.
3.Quoting the book:
A. The star (and the gas cloud form which it formed, which would have the
same chemical composition as the resulting star) should contain enough elements
besides H and He to have allowed for the formation of habitable worlds.
B. The star should last long enough for life to take hold and evolve.
C. For surface life, at least, the star should shine with steady light and should
have a habitable zone in which such life could potentially arise.
D. The star or star system should allow worlds to have stable, nearly circular
orbits. For binary stars, there can be stable orbits in which the planet orbits
around one member in a much closer orbit than the other star or around both
members in an orbit that is much larger than the orbit of the stars.
15. The standard way of calculating the size of the habitable zone assumes
rocky planets like Venus, Earth, and and Mars. Other types of planets, superEarths and water worlds can be more massive than the terrestrial planets of our
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solar system, and thus might retain more atmosphere. With more atmosphere,
the planets might have a stronger greenhouse effect so that the habitable zone
would extend farther out.
19. The Hertzsprung-Russel diagram is a way of classifying stars by plotting
the luminosity vs. temperature of each. Temperature (spectral type) is on the
horizontal axis (increasing to left) and luminosity is on the vertical axis. Accordingly, a star in the upper left is hotter and brighter than a star in the lower
right.
Note: When stars are plotted in this way, they fall into groups, with most
being in a swath running diagonally across from lower right (cool and dim) to
upper left (hot and bright) known as the main sequence.
21. A headline noting that astronomers conclude Earth-sized planets don’t
exist is utter rubbish because we would be reading it on one.
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