What Holds Galaxies Together?
There is also another way of estimating the distribution of matter, by assuming that its average density at any location is proportional to the brightness of the galaxy there. With that assumption, the rate at which brightness falls off with distance from the center should also give the distribution of mass, except in the dark lanes (see picture) where dust blocks out starlight. Obviously, it is of interest to compare the results of both methods. The observation of the Doppler effect in M31 is not easy, and becomes even harder in more distant galaxies. As already noted, M31 is a dim object, and observing it in detail requires long time exposures. Furthermore, to observe the Doppler effect, only a small part of the light can be used, just one “spectral line” isolated by a sensitive spectrograph, and taken from just one small part of the galaxy, whose motion we want to detect.
• View of Earth and Moon from Mars • Appearance of the Moon (1) • Appearance of the Moon (2): Does it “roll around”? • Altitude of the tail of the Big Dipper • Sudden decompression, 5 miles up Thanks for the “Math Refresher” in Spanish • The Pressure of Sunliht • How is the instant the seasons change determined? • Operation of Ion Rockets • Physical Librations of the Moon • The De-Laval Nozzle • Why does the space shuttle rotate at take-off? • Cold Fusion • What if a Neutron Star hit the Sun?Why did the Moon appear Red? • Centrifuge for Whirling Astronauts • What Holds Galaxies Together? • View of Earth and Moon from Mars • Appearance of the Moon (1) • Appearance of the Moon (2): Does it “roll around”? • Altitude of the tail of the Big Dipper • Sudden decompression, 5 miles up • Do Negative Ions make you Feel Good? • Shape of the Earth’s Orbit • Questions about the Solar Corona: (1) Why don’t its particles separate by weight? (2) What accelerates the solar wind? • Why does the rising S
