Download Physics 127 Descriptive Astronomy Homework #19 Key

Physics 127 Descriptive Astronomy
Homework #19 Key (Chapter 13)
Winter 2015
13-5. Most interstellar hydrogen atoms emit only radio waves at a wavelength of 21 cm, but some
hydrogen clouds emit profuse amounts of visible light (see, for example, Figure 11-1). What
causes this difference?
To emit visible light, hydrogen must be at least partially ionized. This happens readily in the vicinity of O- and B-type
stars which radiate copious numbers of ultraviolet photons, many of which possess plenty of energy that, when
absorbed by neutral hydrogen atoms, cause those atoms to become ionized. As ionized hydrogen atoms recombine,
i.e., formerly free protons and electrons bind together to again form neutral hydrogen atoms, those protons and
electrons yield up some of their former energy. That energy is conserved and appears as photons or electromagnetic
energy. Many of these emitted photons are in the visible light portion of the electromagnetic spectrum.
13-6. The Milky Way map taken at radio wavelengths, shown back in Figure 2-26b, has a large gap on
the side of the Galaxy opposite to ours. Why is this? [Bad question. The referenced figure
should have been 13-14. Be liberal in grading this one.]
The radio map (figure 13-14) shows the location of large clouds of neutral hydrogen which emit 21-cm radio waves.
Astronomers have no method of determining the distances of such clouds which are detected from earth in the general
direction of or opposite to the galactic center. Distances of hydrogen clouds are determined by using the measured
radial velocities of clouds fitted to a model of galactic rotation. Clouds towards or opposite the galactic center are
moving across, rather than along, the line of sight and therefore have no radial velocities. Hence even though we “see”
7
13-9. How do astronomers conclude that vast quantities of dark matter surround our Galaxy? How is
this dark matter distributed in space?
There are several pieces of evidence that lead us to conclude that the galaxy is embedded in a gigantic halo of dark
matter. Perhaps the most compelling is the rather flat rotational curve of the galaxy, i.e., the speeds at which stars and
clouds orbit about our galactic center change little with increasing distance from our galactic center all the way out to
the most remote objects we can detect. The necessary gravitational force to explain this curve requires a huge amount
of unseen, hence dark, matter. Its spatial distribution is that of a large spherical halo, centered on the visible galactic
center, which extends far beyond the visible matter in the galaxy, but does decrease in density as the distance from the
galactic center increases.
13-13. What is the evidence that there is a supermassive black hole at the center of our Galaxy? How is
it possible to determine the mass of this black hole?
The strongest evidences are the observed speeds at which stars, close to the galactic center orbit about that center. In
2000 a research group observed at star within 45 AU of the galactic center orbiting at a speed of 12000 km/s. The mass
of the central object, necessary to produce such a high orbital speed of revolution is 3.7106 M. An object of that great
a mass of radius less than 45 AU must be a black hole. The mass was calculated by applying Kepler’s third law to the
motion of the orbiting star. Evidence for this black hole is also seen in the highly variable x-ray radiation coming from
the same region.