DYNAMICS OF THE GALAXY In the early 20th century, as the mystery of the structure of the Galaxy was being unraveled, the
motions of stars were also being determined. Astronomers recorded the slow perceived position changes of stars (proper motion) and their motions toward or away from Earth (radial velocities); the latter are easily measured by the Doppler shift of the stars' spectral lines. In 1904, Kapteyn found that stars move in two streams flowing in opposite directions along the Milky Way, one converging in the constellation of Orion and the other converging 180¡ away, in Scutum. The Swedish astronomer Bertil Lindblad showed that this streaming motion is simply the result of the rotation of the Galaxy. Stars traveling in nearly circular orbits around the galactic center with the Sun will have larger motions relative to the Sun either toward or away from the centerÑdepending on whether they are approaching the nearest or farthest points in their elliptical orbitsÑthan in the direction of motion. Therefore, we preferentially see motions toward us or away from us in these directions. In 1927, Jan H. Oort of the Netherlands showed that the motions of stars in different parts of the Milky Way could be used to derive the properties of the rotation of the Galaxy, including the
velocity of the Sun through space. When modern values are used in Oort's equations, it is found that the Sun's velocity is approximately 250 km/sec in its orbit around the galactic center. The velocities for stars at larger distances from the center are lower than in the inner part of the Galaxy. These velocity differences cause differential rotation in the disk, and they may be the primary cause of the spiral shape of the arms (and also, incidentally, of the rotation of the bodies in the solar system, including the Earth). The dynamics of the spiral arms are still only imperfectly understood. Differential rotation will make spiral arms out of almost any structural feature in a galaxy, but the arms should only last a fraction of the age of the galaxy. It would lead to a rapid winding-up of the arms in the 50 or so rotations that have occurred since our Galaxy was formed. One possible explanation is that the arms are not constant physical entities, but are waves of high star density moving more slowly than the stars. Stars slow down and pile up temporarily in an arm because of its higher gravitational field, then pass out of the arm and proceed until they encounter the next arm. Determinations have been made of the Galaxy's movement as a whole, relative to the rest of the universe. For example, high-altitude measurements were made of the universe's background
radiation, the residual glow of the so-called "big bang" that is assumed to have occurred in the first moments of the universe (see cosmology). The measurements indicated that the Galaxy is moving, relative to the universe, in the same direction as the constellation Leo lies relative to the Earth, and with a velocity of more than 600 km/sec (373 mi/ sec). The Galaxy is also moving at about 100 km/sec (62 mi/ sec), relative to the center of mass of the local group of galaxies. The local group, in turn, is moving at a comparable velocity relative to the supercluster of galaxies to which it belongs. Some astronomers have proposed that the flow is moving toward a huge, distant region of space that has been called the Great Attractor, but others have since disputed this theoretical structure. RADIATION FROM THE GALAXY From a distance, the Galaxy could be detected by a wide variety of means, because it emits radiation at almost all wavelengths. It is optically bright, emitting the equivalent of approximately 200 billion Suns in visible radiation. It is a strong source of radio-line emission, especially from its large mass of neutral hydrogen. It is a source of radio continuum noise, both from the hot gas clouds in its arms and from its dense, hot nucleus. Its huge, dark, cool complexes of dust anemit infrared emission. It shines in the ultraviolet region, because of its very hot, recently formed stars. And it gives off X radiation from many sources.