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| The Milky Way |
1. The energy of ionization is 13.6 eV. Using the conversion factor between eV and ergs, this corresponds to 2.18 x 10-11 ergs.
2. The energy is given in the solution to the preceding exercise. The corresponding wavelength is obtained by taking the product of the speed of light and Planck's constant and dividing that by the energy and is 9.1 x 10-6 cm, which lies in the UV part of the spectrum. Thus, hydrogen is easily ionized in the presence of UV radiation.
3. Nebulae has densities of 100-1000 atoms per cubic centimeter, which is comparable to the best vacuums possible on Earth. The solar wind has a density of several particles per cubic centimeter.
4. There are hot blue stars, but not hot enough to produce enough UV to cause appreciable ionization of hydrogen. The hottest star in the cluster is spectral class B3. As a rule of thumb, one needs a B1 or hotter star to produce enough UV to ionize and emission nebula.
5. This is a trick question, to some degree. It is not that the metal content causes the elongated orbit. It is that the same thing that causes the low metal content is responsible for the elongated orbit. In this case, it is that Pop II stars formed in such a way that they have both elongated orbits out of the galactic plane and low metal content (because they are so old). This is a specific example of a general fallacy that one must guard against in scientific reasoning: If A and B are both caused by C, it may appear that A causes B when in reality they are unrelated except through the root cause C.
6. They are all very young objects on a galactic timescale. Roughly, they must be young enough to have not had enough time to move by the width of a spiral arm since their birth to be reliable tracers of the spiral structure.
7. Assuming the Sun to be 5 billion years old and the time for the Sun to move once around the galaxy is about 250 million years, the number of times it has been around is about (5 billion / 0.250 billion) ~ 20 times.
8. A gas can exert high pressure either by being very dense or by being very hot (or both). The coronal gas is very low density (0.001 - 0.0001 atoms per cubic centimeter) but it is very hot (100,000 - 1,000,000 K). Because it is hot, its pressure is high and this causes it to expand and fill large volumes with low-density hot gas.
9. Dust both lowers the apparent magnitude (extinction) and reddens its color (reddening) because it preferentially scatters blue light.
10. The average extinction is two magnitudes per thousand parsecs, so the total extinction will be about 16 magnitudes. The intensity decreases by a factor
This is enormous, so we have little chance to see any detail at that distance in visible light.
11. The gas density is much lower in the ISM than in the star's photosphere, so collisional broading should be much less in the ISM. Thus, lines produced by absorption in the intervening interstellar medium would be much sharper than those produced in the star.
12. The average extinction in the Sun's part of the galaxy is two magnitudes per thousand parsecs. From the magnitude formula, two magnitudes are a factor of
13. Radio frequency observations can penetrate the dust. They show that there are galaxies in the part of the sky called the "zone of avoidance" just like in every other direction.
14. The spectrum of the star confirms that it is a luminous blue supergiant of spectral and luminosity class O9.7Iab. It appears orange, though, because of reddening by interstellar dust. It is in Cygnus, which is near the plane of the Milky Way, so there is lots of dust around.
15. Extinction near the Sun is about two magnitudes for each thousand parsecs; from the distance-magnitude formula, the distance modulus is m - M = 10 for a distance of 1000 pc. But the light will appear to be two magnitudes dimmer, so the distance modulus will seem to be 12, which corresponds to a distance of 2512 pc. So we would estimate a distance of 2512 pc, but it is really 1000 pc when corrected for dust extinction.
16. One idea is that the initial ball of gas and dust that contracted to form the galaxy itself formed by accretion from smaller clouds that had already undergone at least one generation of star formation, thus enriching the gas of the protogalaxy with some metal content.