General Caption: Illustrations of gravitational lensing.

1. Image 1 Caption: An illustration of gravitational lensing which is the bending of light beams from a distant astronomical object (a spiral galaxy) by a massive foreground astronomical object (a galaxy cluster) which is the gravitational lens itself. The white arrows give the light beam paths from the spiral galaxy.

   Features:

   1. Image 1 is a bit tricky in that it is showing the 3-dimensional structure of the Earth, the gravitational lens galaxy cluster surrounded by a blue blue, and the gravitational lensed spiral galaxy.

      But superimposed is 2-dimensional sky image of the of the galaxy cluster with orange arrows coming from the gravitational lens image which is a set of arcs.

   2. If the source and gravitational lens are circularly symmetric and exactly aligned and there are NO other perturbing masses, the arcs will form a complete perfect ring called an Einstein ring---called that since Albert Einstein (1879--1955) himself predicted the existence of such images.

   3. The focusing effect of gravitational lensing allows one to find astronomical objects too faint to found without gravitational lensing. For example, very remote and therefore very early galaxies have been found using gravitational lensing (e.g., galaxy EGSY8p7 in constellation Bootes). However, the record distant galaxy as of 2022, galaxy HD1, was NOT found using gravitational lensing (see Harikane et al. 2202).

   
2. Image 2 Caption: An animation illustrating gravitational microlensing.

   Features:

   1. When a smallish astronomical object (e.g., star, brown dwarf, planet, rogue planet, white dwarf, neutron star (NS), or stellar mass black hole) passes nearly in front of a background star, the focusing effect of gravitational lensing causes a transient brightning of the background star illustrated in the sub-image light curve.

   2. NEITHER the background star NOR the gravitational lens is resolved.

   3. Gravitational lensing involving smallish, unresolved astronomical objects is called gravitational microlensing.

   4. Gravitational microlensing is probably the only way of detecting isolated stellar mass black holes which are otherwise virtually invisible---unless you were so close you could resolve their black hole shadows (which are a bit larger than their event horizons). However, detecting possible isolated stellar mass black holes and verifying that they are indeed isolated stellar mass black holes are different questions. In 2022, the first, and until circa 2025 at least, the only certain stellar mass black hole detected by gravitational microlensing was reported (see Wikipedia: Gravitational microlensing: History). See also Wikipedia: List of black holes for NO further information on stellar mass black holes detected by gravitational microlensing.

Image Credits:

1. Credit/Permission: NASA, circa or before 2000 / Public domain.
   Image link: Wikipedia: File:Gravitational lens-full.jpg.
   
2. Credit/Permission: , NASA Ames, Jet Propulsion Laboratory (JPL)-Caltech, Tim Pyle (1969--) (see also JPL-Caltech: Tim Pyle), 2016 (uploaded to Wikimedia Commons by User:User:Hadron137, 2020) / Public domain.
   Image link: Wikimedia Commons: File:Gravitational lens.gif.
   

Local file: local link: gravitational_lensing.html.
File: Relativity file: gravitational_lensing.html.

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