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| Star Death |
1. Because the central star is very hot, most of its light is not in the visible spectrum but instead is in the UV. Thus, much of the energy being emitted to power the nebula is not directly visible when emitted from the central star, but becomes visible when the nebula absorbs the UV and re-emits much of the energy in the visible spectrum.
3. None, for two reasons. First, the brightest magnitudes are 8 or larger, but the naked eye seeing limit is 6-7. Second, even if the magnitudes were larger, the known white dwarfs are often companions to brighter stars and thus difficult to see in the glare of the other star.
5. Although the cores of these stars end up as white dwarfs, there are many fundamental differences. For one thing, the composition of the white dwarf will be different. For another, the fate of their envelopes will be completely different (since the more massive stars eject much of their mass before becoming white dwarfs but in the less massive stars there is probably little ejection of mass; the entire star becomes the white dwarf). This implies very different roles in modifying the interstellar medium for the next generation of stars. Note also that because low mass stars have such long main sequence lifetimes, none of them have had time yet to evolve to the white dwarf stage, but for the intermediate mass stars there has been time for multiple generations of stars to become white dwarfs.
9. There are no stable mass 5 and 8 isotopes of any element. This makes it very difficult to make elements above mass 4 in either the Big Bang or in stars until in the hot cores of red giants the triple-alpha reaction can produce carbon and this can be used to produce heavier elements. Thus, the elements between helium and carbon occur with low abundance in nature.
11. A type II supernova is associated with the death of a massive young star, so it occurs in a star-forming region, which is more likely in a spiral arm of the disk than the halo.
13. The half-life is the time for the original population to decrease by one half. Thus, every month half the existing population of iron-59 would beta decay to cobalt-59. In twelve months, the total reduction would be one half raised to the twelfth power, which is 0.000244, the inverse of which is a little over 4000. So iron-59 formed by the s-process in a typical red giant is several thousand times more likely to beta decay than to capture another neutron if the average capture rate is one per year.
15. Most stars late in their lives will expand to become giant or supergiant stars because of events that take place in their interiors when then begin to exhaust their nuclear fuel. In a close binary, this can cause the star to overflow its Roche lobe, leading to accretion onto the companion.
17. Take the radius of the Solar System to be 40 AU ~ 40 x 150,000,000 km. The time to expand by that radius is (40 x 150,000,000 km) / (3000 km/s) ~ 2,000,000 seconds ~ 23 days.
19. No, they are completely unrelated. The blue color of Uranus and Neptune is an absorption feature: the methane in their atmospheres absorbs red and IR wavelengths very strongly, leaving the light that we see with a blue tinge. The colors of planetary nebulae are an emission feature, resulting from emission of photons by ions that have been excited by UV radiation from the hot central star.