The night sky has long attracted and impresses a person with many stars. In an amateur telescope, you can see a much greater variety of deep space objects - an abundance of clusters, globular and open, 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 events of gravitational lensing, and among them are the so-called Einstein crosses. What is it, we will find out in this article.
Space lenses
The gravitational lens creates a powerful gravitational field of an object with a significant mass (for example, a large galaxy), accidentally caught between an 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 for the shortest periods of time (geodesic lines) are also curved. As a result, the observer sees the image of the light source distorted in a certain way.
What is this - “Einstein's Cross”?
The nature of the distortion depends on what the configuration of the gravitational lens is 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 is ring-shaped, if the center of symmetry is displaced relative to the line, then such an Einstein ring is broken into arcs.
With a sufficiently strong displacement, when the distances traveled by the light differ significantly, lensing forms multiple point images. The cross of Einstein, in honor of the author of the general theory of relativity, in 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” quaternary 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 with reference to this particular Einstein Cross that this is a proper name, although unofficial, and is capitalized.
Above in the photo is Einstein's Cross. The central spot is the core of the lensing galaxy. The image was taken by the Hubble Space Telescope.
The galaxy ZW 2237 + 030, acting as a lens, is located 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, moreover, uneven.
Variety of shapes
Perhaps the cruciform lensed quasar HE 0435-1223, which is almost the same distance as QSO 2237 + 0305, is no less beautiful. Thanks to a completely random set of circumstances, the gravitational lens occupies such a position here that all four images of the quasar are located almost evenly, forming an almost regular cross. This unusually spectacular object is located in the constellation Eridanus.
And finally, a special case. Astronomers were lucky to capture in a photograph how a powerful lens - a galaxy in a huge cluster in the foreground - visually increased not a quasar, but a supernova explosion. The uniqueness of this event is that a supernova, in contrast to a quasar, is a short-term phenomenon. The outbreak, known as the Refsdahl supernova, occurred in a distant galaxy more than 9 billion years ago.
Some time later, to the cross of Einstein, which strengthened and multiplied the ancient stellar explosion, a little further was added another - the fifth - image, which was late due to the peculiarities of the lens structure and, incidentally, predicted in advance.
In the picture below you can see the "portrait" of the Refsdal supernova propagated by gravity.
The 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 evidence of its validity.
Along with other effects of gravitational lensing, they attract the 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 clusters of galaxies.
The study of other important cosmological parameters, such as the Hubble constant, can also be helped by studying unevenly folded lensed images of quasars (including cruciform ones). These Einstein rings and crosses of irregular shape are formed by rays that have passed different distances at different times. Therefore, a comparison of their geometry with brightness fluctuations allows us to achieve great accuracy in determining the Hubble constant, and hence the dynamics of the universe.
In a word, amazing phenomena created by gravitational lenses not only delight the eye, but also play a serious role in modern space sciences.