A galaxy can hide itself perfectly behind another galaxy and then paradoxically it becomes much more visible. A massive object, a galaxy for example, which is between an observer and a distant light source, another galaxy, prints a high curvature in space-time. This has the effect of deflecting all the light rays passing near the object, thereby deforming the images received by the observer. This illusion that astronomers know well is called gravitational lensing or gravitational mirage.
When the galaxies are perfectly aligned with respect to the observer, the hubble telescope for example, there is then a particular case of gravitational mirage called the Einstein ring. If a third galaxy is also aligned and at a particular distance with the first two, then we are witnessing the formation of a second Einstein rings, one larger than the first concentric ring.
This amplification of the brightness of a distant celestial object, by a massive star in front, was predicted by the theory of general relativity in 1917. Massive objects alter the geometry of space and time in their neighborhood. Light on it always takes the shortest path, but in a curved space modified by the presence of a huge mass, the shortest path is not a straight line. The light path is bent in the vicinity of massive stars, the spokes may be in the form of a light ring surrounding the near object. In this image the galaxy in the foreground appears in yellow and behind the observed galaxy appears blue. This spectacular photo of the LGR 3-757 object was discovered in 2007 on the images of SDSS (Sloan Digital Sky Survey) program.
Massive objects that can act as a gravitational lens or deflector are galaxies, black holes and galaxy clusters. A star, although having a much lower mass than that of a galaxy, can also act as a gravitational lens on an object located to a precise distance behind it. The effect is obviously much less powerful, then we are talking about micro gravitational lensing.
The Einstein radius is the radius of a ring Einstein.
nota: The first observation of an Einstein ring in 1987, it was conducted by scientists from MIT, led by Jacqueline Hewitt (Professor of Physics). It was the object MG1131 + 0456. Article was published in Nature 333, 9 June 1988. The team talks about their discovery thus: we report the discovery of a rare structure in the radio source MG1131 + 0456. In a radio map the object appears as an elliptical ring of emission, accompanied by a pair of sources more compact, almost diametrically opposed and offset ~ 0.3 arc southwest of the ring. The morphology of this radio source suggests that it may be an example of Einstein ring, a very particular case of gravitational lensing in which the source is imaged into a ring.
Image: This particular case of gravitational mirage shows an almost complete ring stretched strange blue objects, which extend in a circle but are only multiple views of a single galaxy. The gravity of the bright red galaxy (LRG 3-757) has gravitationally distorted the light from a much more distant blue galaxy behind. This is due to the almost perfect alignment of the galaxy background and foreground galaxy, that hubble telescope was able to photograph a circle. These rings are called Einstein rings. Credit: Public Domain.
Another particular form of gravitational mirage, is the Einstein Cross. Einstein Cross corresponds to 4 images of the same object which appear around a foreground galaxy, thanks to a powerful gravitational lens. In reality, this structure formed by five points, the center point and the other four points are the images of a galaxy that breaks the light of a single distant object (here a quasar) behind by four points in the form of cross. The Einstein Cross is one of the best examples of gravitational lensing that visually confirms the theory of general relativity.
Image: remarkable image of the galaxy known as the UZC J224030.2 032131 taken by the Hubble Space Telescope (NASA / ESA). The core of the galaxy is weak and diffuse object in the center of the cross-shaped structure. The powerful gravity of the galaxy acts as a lens that bends and amplifies the light from the quasar behind it, producing a quadruple image of the distant object. The quasar is 11 billion light years from us in the direction of the constellation of Pegasus, it is seen as it was there about 11 billion years. The galaxy is about 1 billion years. The alignment between the two objects is remarkable (0.05 arc sec), which explains in part why an such structure can be visually observed. This image is probably the sharpest image ever made of the Einstein Cross, it was produced by the camera wide field Hubble.
Sometimes gravitational lenses exhibit an light fire fireworks which are only multiple projections of the same objects. The analysis of this figure shows that there are at least 33 light projections which correspond to only 11 real galaxies.
Image: This gravitational lens shows strange objects elongated blue. These cosmic objects that are spread in a circle on this picture, are that multiple views of a single ring galaxy. The singular form of the blue galaxy background (center of mass), allowed deduced that it is the same galaxy that we see on this picture to 4 hours, 10 hours, 11 hours and 12 hours. This spectacular photo of the galactic cluster CL0024+1654 was taken by the Hubble Space Telescope in November 2004.
Image: Einstein ring.
Image: Einstein cross.
Image: gravitational lens.