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Hubble's 'Einstein Cross' Supernova Strikes Back

An ancient supernova that was serendipitously captured in four Hubble space telescope images thanks to a naturally occurring cosmic magnifying lens has reappeared, as astronomers predicted.

An ancient supernova that was serendipitously captured in four Hubble space telescope images thanks to a naturally occurring cosmic magnifying lens has reappeared, as astronomers predicted.

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The exploded star, known as Refsdal in honor of Norwegian astronomer Sjur Refsdal, first appeared in November 2014. Scientists were stunned to find four images of the supernova around a galaxy, a configuration known as an "Einstein Cross."

The multiple images were caused by light from the supernova, which exploded about 10 billion years ago, taking different paths around a gravitationally warped region of space, relative to Hubble's line of sight.

These warped regions are due to massive galaxy clusters bending space and time, a phenomenon predicted by Albert Einstein 100 years ago.

PHOTOS: The Warped Beauty of Gravitational Lenses

Astronomers have been taking advantage of so-called "gravitational lensing" to boost Hubble's imaging powers.

The four supernova images captured by Hubble appeared within a few weeks of each other in November 2014. Scientists predicted the lensed region of space would cause additional images of the supernova to appear sometime in the next five years. They found it on Dec. 11.

"We used seven different models of the cluster to calculate when and where the supernova was going to appear in the future. Remarkably, all seven models predicted approximately the same time frame." physicist Tommaso Treu, with the University of California at Los Angeles, said in a statement.

Frontier Fields: Hubble Lens to Get Superboost

Having nailed Refsdal's reappearance, scientists can now refine their estimates of the lensing cluster's mass, particularly its non-light emitting dark matter.

This Hubble image shows the supernova's signal appearing in the Einstein Cross as predicted by astronomers.

In November 1915, Albert Einstein published his famous theory of general relativity, a theory that began a revolution in physics and transformed our view on the entire universe. A key component of general relativity is that a massive object like a planet, star, galaxy or cluster of galaxies can have a dramatic warping effect on the "fabric" of the universe -- known as "spacetime." As light travels in straight lines through spacetime, should a mass cause an otherwise "flat" spacetime to curve, the path of light also becomes curved. Therefore, by this theoretical reasoning, we should be able to see the warped light of distant galaxies as that light travels past other galaxies on its way to being observed at Earth. And sure enough, there are countless examples in the cosmos of this warped light caused by a mechanism known as "gravitational lensing" -- like artifacts etched in ancient light, stunning arcs, misshapen orbs and even near-perfect circles have been observed in star fields. These artifacts are the lensed light from distant galaxies and these observations have been used to superboost some of our most powerful telescopes.

Hubble at 25: Space Telescope's Top Science Discoveries

In this dramatic observation by the Hubble Space Telescope and NASA's Chandra X-ray Observatory

, a cartoon "Cheshire Cat" seems to be looking back at us. In fact, this is a collection of galaxies over 4 billion light-years away in the constellation Ursa Major -- some of the galaxies' light has become warped and deformed on its way through the universe to our telescopes, creating what looks like a Cheshire Cat smile.

This diagram

provides a good description as to how gravitational lensing works. Light from a distant galaxy travels through spacetime as it curves around a cluster of galaxies in the foreground. Interestingly, the mass of the foreground cluster has a similar effect on this distant light as a glass lens would have if placed in front of a candle flame. Should the positioning be just right, the gravitational lens can amplify the distant light, creating a natural lens in space, magnifying the light from distant galaxies that would have otherwise remained too faint to be seen. It is this effect of gravitational lenses that is being leveraged by Hubble astronomers who have embarked on a project called "

Frontier Fields

" that is on the lookout for cosmic lenses to superboost Hubble's observing power.

Hubble at 25: What's Next for the Space Telescope?

Of course, the alignment isn't always perfect between Earth, gravitational lens and distant galaxy. Also, the foreground object creating the lens is usually not regularly shaped. These factors cause most lensed galaxies to appear as broken arcs. Multiple instances of the same galaxy can also be projected as the distant starlight becomes warped and fragmented.

In this Hubble observation of the galaxy cluster Abell 370

, many galaxies are present, but several prominent arcs of galactic light can be seen. Often, for well-defined examples, these arcs can be reconstructed to reveal what that galaxy looks like without being warped.

This is another massive galaxy cluster called Abell 2218 filled with some stunning examples of gravitationally-lensed galaxies. These arcs are thought to be light from galaxies located 5 to 10 times further away from Earth than the galactic cluster. The cluster is credited with amplifying the weak light from galaxies that existed over 13 billion years ago, less than a billion years after the Big Bang. These arcs truly are artifacts from the beginning of time.

ANALYSIS: Hunting Black Holes Through a Gravitational Lens

Should the alignment be just right, and the lensing object be less complex than a cluster of galaxies, it's possible to see near-perfect circles of light or "horseshoe" shaped lenses where the light of a distant galaxy has been almost perfectly warped 360 degrees around the lensing object. The passage of an isolated massive black hole, for example, in front of a distant galaxy could create such a dramatic scene.

ANALYSIS: Monstrous Star-Forming Regions Seen in Ancient Galaxy

As shown in this striking Atacama Large Millimeter/submillimeter Array (ALMA) observation

, the light of a distant galaxy has formed a complete circle aptly known as an "Einstein Ring." The light originated from an ancient "starburst" galaxy called SDP.81 and is the finest example of an Einstein Ring found to date.

MORE: ALMA Captures Ancient Galaxy's Near-Perfect Einstein Ring

Occasionally multiple images of the same object can be seen around gravitational lenses. In this spectacular example, an ancient supernova in a distant galaxy has been magnified by the masses of galaxies contained within the MACS J1149.6+2223 cluster, located 5 billion light-years away. The supernova, located another 4 billion light-years behind the cluster, has been multiplied 4 times as the light from the same supernova took different paths around the lens. As this was a transient event, the different supernova images were detected at different times by Hubble. Such a configuration of lensed images is known as an "Einstein Cross."

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