Small distortion and amplification of background Concentration of 106 to 1012 stars, dust and gas, that are gravitationally bound. Our galaxy contains ~2 × 1011 stars. There are four main types of galaxies: • Elliptical • Lenticular • Spiral • Irregular Click on Term to Read More images by foreground mass concentrations. In the weak lensing regime, distortion of an individual image is undetectably small (unlike in Effect due to gravitational bending of light by mass concentrations which results in (1) the strong distortion of background galaxy images (arcs) behind some massive foreground galaxy clusters, and (2) multiple images of the same background quasar behind a foreground galaxy. Over 50 lensed quasi-stellar objects are known. Image source:), and the effect must be detected by searching for correlated distortions of thousands of source images. Weak lensing has been used to map the projected mass distribution of a number of galaxy clusters and used statistically to constrain the masses and sizes of dark galaxy halos.