larger elliptical galaxy. Galaxy clusters The effect of dynamical friction explains why the brightest (more massive) galaxy tends to be found near the center of a galaxy cluster. The effect of the two body collisions slows down the galaxy, and the drag effect is greater the larger the galaxy mass. When the galaxy loses kinetic energy, it moves towards the center of the cluster. However the observed velocity dispersion of galaxies within a galaxy cluster does not depend on the mass of the galaxies. The explanation is that a galaxy cluster relaxes by violent relaxation, which sets the velocity dispersion to a value independent of the galaxy's mass. Star clusters The effect of dynamical friction explains why the most massive stars of SCs tend to be found near the center of star cluster. This concentration of more massive stars in the cluster's cores tend to favor collisions between stars, which may trigger the runaway collision mechanism to form intermediate mass black holes. Photons Fritz Zwicky proposed in 1929 that a gravitational drag effect on photons could be used to explain cosmological redshift as a form of tired light. However, his analysis had a mathematical error, and his approximation to the magnitude of the effect should actually have been zero, as pointed out in the same year by Arthur Stanley Eddington. Zwicky promptly acknowledged the correction, although he continued to hope that a full treatment would be able to show the effect. It is now known that the effect of dynamical friction on photons or other particles moving at relativistic speeds is negligible, since the magnitude of the drag is inversely proportional to the square of velocity. Cosmological redshift is conventionally understood to be a consequence of the metric expansion of space. See also Dynamic friction Notes and references External links Astrophysics Effects of gravitation Stellar dynamics