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We have said repeatedly that blue light undergoes more extinction than red light, which is true for visible and shorter wavelengths. Is the same true for X-rays? Look at Figure 20.19. The most dust is in the galactic plane in the middle of the image, and the red color in the image corresponds to the reddest (lowestenergy) light. Based on what you see in the galactic plane, are X-rays experiencing more extinction at redder or bluer colors? You might consider comparing Figure 20.19 to Figure 20.14.
Figure 20.14 Barnard 68 in Infrared. In this image, we see Barnard 68, the same object shown in Figure 20.9. The difference is that, in the previous image, the blue, green, and red channels showed light in the visible (or very nearly visible) part of the spectrum. In this image, the red color shows
Figure 20.19 Sky in X-Rays. This image, made by the ROSAT satellite, shows the whole sky in X-rays as seen from Earth. Different colors indicate different X-ray energies: red is 0.25 kiloelectron volts, green is 0.75 kiloelectron volts, and blue is 1.5 kiloelectron volts. The image is oriented so the plane of the Galaxy runs across the middle of the image. The ubiquitous red color, which does not disappear completely even in the galactic plane, is evidence for a source of X-rays all around the Sun. (credit: modification of work by NASA)
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Chapter 20 Solutions
Astronomy
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- A two-arm spiral density wave is moving through the Galactic disk. At the 8-kpc radius of the Sun's orbit around the Galactic center, the wave's speed is 120 km/s, and the Galactic rotation speed is 220 km/s. How many times has the Sun passed through a spiral arm since the Sun formed 4.6 billion years ago?arrow_forwardImagine that you are observing the light from a distant star that is located in a galaxy 100 million lightyears away from you. By analysis of the starlight received, you are able to tell that the image we see is of a 10- million-year-old star. You are also able to predict that the star will have a total lifetime of 50 million years, at which point it will end in a catastrophic supernova. a) How old does the star appear to be to us here on Earth now? b) How long will it be before we receive the light from the supernova event? c) Has the supernova already occurred? If so, when did it occur?arrow_forwardSuppose the Hubble constant were not 22 but 12.1 km/s per million light-years. Then what would the critical density (in kg/m³) be? (For H = 22 km/s per million light-years, the critical density is 9.6 x 10-27 kg/m³.) kg/m³1 I Ih SAMSUNGarrow_forward
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