The night sky has long attracted and impresses a person with many stars. An amateur telescope can see a much greater variety of deep space objects - an abundance of clusters, spherical and scattered, nebulae and nearby galaxies. But there are extremely spectacular and interesting phenomena that only powerful astronomical instruments can detect. Among such treasures of the universe are the events of gravitational lensing, and among them are the so-called Einstein's crosses. What it is, we will find out in this article.
Space lenses
A gravitational lens is created by a powerful gravitational field of an object with a significant mass (for example, a large galaxy), accidentally caught between the observer and some distant light source - a quasar, another galaxy or a bright supernova.
Einstein's theory of gravity considers gravitational fields as deformations of the space-time continuum. Accordingly, the lines along which light rays propagate in the shortest time intervals (geodesic lines) are alsoare bent. As a result, the observer sees the image of the light source distorted in a certain way.
What is the "Einstein cross"?
The nature of the distortion depends on the configuration of the gravitational lens and on its position relative to the line of sight connecting the source and the observer. If the lens is strictly symmetrical on the focal line, the deformed image turns out to be ring-shaped, if the center of symmetry is shifted relative to the line, then such an Einstein ring can be divided into arcs.
With a sufficiently strong shift, when the distances covered by the light differ significantly, lensing forms multiple dot images. Einstein's cross, in honor of the author of the general theory of relativity, within the framework of which phenomena of this kind were predicted, is called the quadruple picture of the lensed source.
Quasar in four faces
One of the most "photogenic" quadruple objects is the quasar QSO 2237+0305 belonging to the constellation Pegasus. It is very far away: the light emitted by this quasar traveled more than 8 billion years before it hit the camera lenses of ground-based and space telescopes. It should be borne in mind in relation to this Einstein Cross that this is a proper name, although unofficial, and is written with a capital letter.
At the top of the photo is the Einstein Cross. The central spot is the nucleus of the lensing galaxy. The picture was taken by spacethe Hubble telescope.
The galaxy ZW 2237+030, acting as a lens, is 20 times closer than the quasar itself. Interestingly, due to the additional lensing effect produced by individual stars, and possibly star clusters or massive gas and dust clouds in its composition, the brightness of each of the four components undergoes gradual changes, and uneven ones.
Variety of shapes
Perhaps no less beautiful is the cross-lensed quasar HE 0435-1223, almost the same distance as QSO 2237+0305. The gravitational lens, due to a completely random set of circumstances, occupies such a position here that all four images of the quasar are located almost evenly, forming an almost regular cross. This extraordinarily spectacular object is located in the constellation of Eridani.
And finally a special occasion. Astronomers were lucky enough to capture in a photograph how a powerful lens - a galaxy in a huge cluster in the foreground - visually enlarged not a quasar, but a supernova explosion. The uniqueness of this event is that a supernova, unlike a quasar, is a short-term phenomenon. The explosion, dubbed the Refsdal supernova, occurred in a distant galaxy over 9 billion years ago.
Some time later, to the Einstein cross, which strengthened and multiplied the ancient stellar explosion, a little further away, another fifth image was added, belated due to the peculiarities of the lens structure and, by the way, predictedin advance.
In the picture below you can see the "portrait" of the supernova Refsdal, multiplied by gravity.
Scientific significance of the phenomenon
Of course, such a phenomenon as the Einstein cross plays not only an aesthetic role. The existence of objects of this kind is a necessary consequence of the general theory of relativity, and their direct observation is one of the most obvious confirmations of its validity.
Along with other effects of gravitational lensing, they attract the close attention of scientists. Einstein's crosses and rings make it possible to study not only such distant light sources that could not be seen in the absence of lenses, but also the structure of the lenses themselves - for example, the distribution of dark matter in galaxy clusters.
The study of unevenly stacked lensed images of quasars (including cruciform ones) can also help to refine other important cosmological parameters, such as the Hubble constant. These irregularly shaped Einsteinian rings and crosses are formed by rays that have traveled different distances in different times. Therefore, comparing their geometry with brightness fluctuations makes it possible to achieve great accuracy in determining the Hubble constant, and hence the dynamics of the Universe.
In a word, the amazing phenomena created by gravitational lenses are not only pleasing to the eye, but also play a serious role in modern space sciences.
