* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Physics 127 Descriptive Astronomy Homework #19 Key
Survey
Document related concepts
Astrophysical X-ray source wikipedia , lookup
Outer space wikipedia , lookup
Energetic neutral atom wikipedia , lookup
Hawking radiation wikipedia , lookup
Dark matter wikipedia , lookup
Main sequence wikipedia , lookup
Gravitational lens wikipedia , lookup
Weakly-interacting massive particles wikipedia , lookup
Cosmic distance ladder wikipedia , lookup
Stellar evolution wikipedia , lookup
First observation of gravitational waves wikipedia , lookup
Microplasma wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Transcript
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 3.7106 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.