Gravitational lensing is when a massive object bends light from a distant light source due to the deformation of space-time. It can cause quasars to appear in different places and produce multiple images. There are three types of gravitational lenses: strong, weak, and microlensing.
Gravitational lensing is an astronomical phenomenon observed when a massive object, such as a galaxy cluster or black hole, bends light from a very distant light source, such as a quasar (bright young galaxy). This occurs due to the gravitational deformation of space-time first described by Albert Einstein in his general theory of relativity. It is called gravitational lens by analogy with a conventional lens. Both can bend light, but in this case the mechanism is very different: instead of bending light using curved matter, in this case space-time itself is curved.
Gravitational lensing can cause distant quasars to appear in places where they don’t actually exist. Because light is bent in the gravitational well of a massive object, the apparent position deviates from the actual position. Gravitational lensing can also cause multiple images of a quasar to appear in the sky: light is bent around the massive object in both directions, thus producing the multiple image. The “Twin Quasar” Q0597+561, also known as Old Faithful, is the first confirmed object to appear twice in the sky due to gravitational lensing. Each of the quasar images is separated in the sky by 6 degrees. Although Fritz Zwicky had postulated that galaxy clusters could act as gravitational lenses in 1937, it was not until 1979 that the effect was confirmed by observation.
There are three types of gravitational lenses. There is a strong lens, where easily visible distortions are visible, such as Einstein rings, multiple images or arcs. These are the rarest gravitational lenses. Then there is a weak lens, which can only be discovered by a large statistical analysis of the fields of stars and galaxies. This lens shape reveals itself as a slight stretch towards the center of the lens. The last one is microlensing, which is quite rare, but has proven to be the most useful for astronomical study. Microlensing manifests itself as subtle changes in the brightness of nearby objects (within our galaxy) caused by the lenses of stars. Distinguishing true microlensing from changes in star brightness due to other reasons (variable stars, novae, etc.) can be difficult.
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