2005/03/15 | 暗物质[Dark Matter]
类别(¤〖天文〗) | 评论(0) | 阅读(58) | 发表于 13:42
An object can be detected in one of two ways: either by observing it directly, or observing the effects it has on other objects. For example, self-luminous objects like stars and gas clouds emitting radiation at radio wavelengths can be observed directly, while a cool brown dwarf star could not.

Astronomers studying the rotation curves of galaxies, i.e., the speed with which objects in a galaxy orbit the center as a function of distance away from the center, noticed some time ago that the shape of the curve at large distances was not what would be expected based on the total mass of observed stars, but rather seems to indicate the presence of a significant amount of additional unseen mass.

At the same time, a similar effect was being seen in clusters of galaxies (which are loose associations of galaxies held together by gravity). In the 1930s, astronomers Fritz Zwicky and Smith studied the velocities of the individual galaxies in the Coma and Virgo clusters. They did this because the velocities of the galaxies indicate the total mass of the cluster in two different ways. Firstly, the greater the mass in the cluster, the greater the forces acting on each galaxy, accelerating the galaxies to higher velocities. Secondly, if the velocity of a given galaxy is too large, the galaxy will be able to break free of the gravitational pull of the cluster and escape. As a result, by knowing that all of the galaxies have velocities of less than the escape velocity, the total mass can be estimated. What Zwicky and Smith found was that these velocities were a factor of ten to 100 larger than expected, indicating more mass than expected.

The mass which seems to be necessary to explain the dynamical properties of astronomy objects but which has not yet been observed is sometimes called missing mass.
0

评论Comments

日志分类
首页[1408]
∑〖数学〗[349]
Ω〖物理〗[357]
¤〖天文〗[343]
℃〖化学〗[359]