科研之友
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摘要
The rotational speeds of stars in the disc of a spiral galaxy are virtually independent of the distances of the stars from the centre of the galaxy. In common parlance, the stellar speed versus distance plot known as a galactic rotation curve is by observation typically nearly flat. This observation provides strong evidence that most galactic matter is dark, i.e. undetectable through luminosity measurements. This article is intended to serve the pedagogical purpose of using the dark matter problem as the basis of exercises accessible to university students. Gauss's law is first used to explain why luminosity measurements lead to the prediction of incorrect speed distributions for disc stars in spiral galaxies. The question of how mass can be distributed within a galaxy to enable flat rotation curves is then considered. Gauss's law concisely answers this question for an idealized galaxy with spherical symmetry. For an idealized planar galaxy with circular symmetry, constructing and evaluating an integral for the gravitational field as a function of distance from the centre of the galaxy provides the answer to the question.
- 出版日期2010-5
