A phenomenon produced by a special kind of gravitational lens. Light from a distant galaxy or quasar is distorted by a foreground lensing object (usually a cluster of galaxies) into an arc shape. From the geometry of such arcs, the mass of the cluster can be estimated.
luminosity function: The luminosity function for stars within 20 parsecs of the Sun from Hipparcos data. The number of stars is plotted against their absolute magnitude. The flattening of the slope around 6th–7th magnitude is known as the Wielen dip.
Subjects: Astronomy and Astrophysics.