Explainer: What is the Einstein Cross visible in space?

In the vast expanse of space, celestial bodies often take on peculiar shapes, leaving us awestruck by their mysterious and seemingly otherworldly formations. Some of these cosmic events occur repeatedly, like clockwork, while others grace us with their presence only once in centuries or even millions of years. However, there's one particular phenomenon that made its debut on the celestial stage just a few decades ago and has recently made a dazzling return appearance through the lens of the European Southern Observatory's Very Large Telescope: the Einstein Cross.

Imagine gazing at a cosmic tableau where one of the galaxies appears to be encircled by a delicate halo of light, resembling the petals of a celestial flower. That is what we call the "Einstein Cross."

Now, let's simplify the four petal-like shapes you see in this picture. They are not mere decorative patterns but rather a distant galaxy concealed behind the prominent orange galaxy at the center of the frame. This captivating display owes its existence to a fascinating phenomenon known as gravitational lensing, which has the power to transform ordinary cosmic sights into extraordinary ones.

But what exactly are Einstein Crosses, and how do they come to be? Well, it's a phenomenon where the light emanating from a far-off galaxy becomes divided as it traverses the foreground, resulting in a magnified and divided appearance. This optical magic, first predicted by none other than Albert Einstein in his groundbreaking general theory of relativity in 1915, was initially observed in 1985 by a team of astronomers led by Dennis Walsh.

Interestingly, the Einstein Cross isn't exclusively formed by galaxies lurking behind other cosmic entities. In fact, the inaugural Einstein Cross discovery in 1985 wasn't a galaxy at all, but a quasar, specifically named Q2237+0305, situated in the Pegasus constellation. At the time of its discovery, this enigmatic quasar held the title of being the most distant one known to humanity.

But why the name "Einstein Cross"? Well, it was christened so because during that discovery, four brilliant images radiated around the quasar, seemingly magnified by the gravitational lensing phenomenon. Einstein himself had also introduced the concept of gravitational lensing, and the name was chosen to reflect the cross-like shape and its association with Einstein's theory.

Let's now explore the scientific principles underlying this fascinating exhibition. Massive objects warp spacetime, according to Einstein's general theory of relativity. Gravity is a result of matter and energy bending the space around them, not a result of a force. When the light from a quasar passes through this highly curved spacetime near a massive galaxy, it bends, resulting in the magnified appearance that we see. This intriguing process is aptly named gravitational lensing.

The galaxy that seemingly steals the spotlight in this cosmic dance, the orange one at the forefront, is responsible for bending the light originating from a distant, far-off galaxy. This bending gives rise to the illusion of distorted and enlarged distant galaxies, culminating in the striking four-petal appearance. It's not just a visual spectacle; it's also a valuable source of information about the objects concealed behind these colossal galaxies.

Interestingly, back in 1985, there was a belief that we might never witness such a spectacle again. However, since that inaugural sighting, we've been treated to this cosmic marvel on approximately a dozen occasions. This time around, scientists observed the four-petal flower-like formation through the Multi Unit Spectroscopic Explorer (MUSE) of the European Southern Observatory's Very Large Telescope. What makes this observation even more captivating is the realization that stars are sprouting into existence at a rapid pace within this distant galaxy. These remarkable findings have been detailed in The Astronomical Journal Letters, reminding us that the universe is an ever-changing canvas of wonder and discovery.

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