AST 248 HW Chapter 11
Ryan Richards
Compared to a star of spectral type K, a star of spectral type A is generally (a) hotter, more
luminous, and more massive,
Stars of type O and B are unlikely to have planets with life because (a) they have short stellar
lives
How does the habitable zone around a star of spectral type M compare to that around a
star of spectral type G? (c) It is smaller and closer to its star.
The main sequence on an H-R diagram represents stars that are (b) fusing hydrogen into
helium in their cores.
What are multiple star systems? What are binary star systems? Briefly discuss the prospects
of finding habitable planets in multiple star systems.
The text defines a multiple star systems as a system of two or more stars that are in close orbit
around each other. Clearly, a binary star system is a special case of a multiple star system with
only two stars. Also discussed in the text, the prospects of finding habitable planets in multiple
star systems depend on the orbit’s stability. For a binary star system, there are three possible
scenarios. They are:
Stable Orbits: The planet orbits both stars. If we call the separation distance between the stars d,
and the radius of the orbit R, we have the condition that R ≫ d. In this case, the habitable zone is
in a region between the stars.
Potentially Stable Orbits: The planet orbits one of the stars. In this case d ≫ R. As for habitable
zones, this would depend only on the star the planet orbits.
Unstable Orbits: The planet’s orbit is neither much larger nor much smaller than the separation
distance (it is possibly comparable to the distance i.e. R ~ d).
Briefly summarize the life cycles of stars from birth to death. At which stage of stellar life
does it seem possible that life-bearing planets could exist? Why?
The life cycle of stars is given on the bottom of Figure 3.6 on page 60. The stars formed from
dust and gas clouds. As the star ages, it begins to fuse hydrogen into helium in its core. Before
the star is able to do this is known as a protostar. After it exhausts all its hydrogen, it grows
larger and brighter becoming a giant or supergiant star. At this stage, the supergiant’s core is hot
and begins fusing helium and heavier elements. When the star runs out of elements within its
core, it dies (it undergoes fusion for 90% of its life). Life-bearing planets are could exist when
the star is fusing hydrogen in its core.