An Einstein ring can be seen in this image of the gravitational lens 5921+0638 from the COSMOS survey. An Einstein ring is a complete circle image of a background galaxy, which is formed when the background galaxy, a massive, foreground galaxy, and the Hubble Space Telescope are all aligned perfectly. Here, also a quadruple lens can be seen.

This is one example of the rich diversity of 67 strong gravitational lenses found in the COSMOS survey. The lenses were discovered in a recently completed, large set of observations as part of a project to survey a single 1.6-square-degree field of sky (nine times the area of the full Moon) with several space-based and Earth-based observatories.

Gravitational lenses occur when light travelling towards us from a distant galaxy is magnified and distorted as it encounters a massive object between the galaxy and us. These gravitational lenses often allow astronomers to peer much further back into the early Universe than they would normally be able to.

This is a greyscale image taken by Hubble ACS camera and colourised with ground-based data from the CFHT (Canada-France-Hawaii Telescope).

Credit: NASA, ESA, C. Faure (Zentrum für Astronomie, University of Heidelberg) and J.P. Kneib (Laboratoire d’Astrophysique de Marseille)