For the first time ever, astronomers have seen a distant star warp the light of another star, making it seem as though the object changed its position in the sky. It’s a huge discovery — since even Albert Einstein didn’t think such a thing would be possible to observe. Now that we’ve proved Einstein wrong (but also right), astronomers hope to spot even more events just like this, as well as use these occurrences to learn more about the stars in our Universe.
Einstein first predicted gravitational microlensing in his theory of general relativity. It works kind of like how it sounds: it’s gravity acting like a lens that can manipulate light. Basically, supermassive objects — such as stars and black holes — warp space and time around them. This warped space-time can then act like a magnifying glass, changing the path that light takes through the Universe. Specifically, microlensing can occur when one star — the “source” — passes directly behind another star — the “lens” — along our line of sight from Earth. The gravity from the lens warps the light from the source, making it appear brighter and slightly distorted.
These events require stars that are very far apart to line up perfectly. That’s why Einstein once wrote that "there is no hope of observing this phenomenon directly." Our telescope technology has become far more sophisticated than in Einstein’s day — which is what allowed us to observe something he thought we’d never see. In 2014, a group of astronomers using NASA’s Hubble Space Telescope spotted a rare type of microlensing, when a dense white dwarf star passed in front of another star thousands of light-years away. The stars weren’t exactly aligned, but they were close enough that the white dwarf made it seem like the background star performed a small loop in the sky. “It looks like the white dwarf pushed it out of the way,” Terry Oswalt, an astronomer at Embry-Riddle Aeronautical University who was not involved in this discovery but wrote a perspective piece in Science, tells The Verge. “That’s not what happened, of course. It just looks like that.”
The astronomers also used the apparent movement of the background star to measure the mass of the passing white dwarf, a novel technique detailed in a paper published today in Science. And they say this isn’t the last time they’ll make measurements like this either. Now that they’ve figured out how to spot these kinds of lensing events, they’re hoping to find even more with new ground- and space-based telescopes that are coming online soon. “This opens up a new field,” Kailash Sahu, the astronomer at the Space Telescope Science Institute who led this discovery, tells The Verge. “Nobody had tried this before, so it’s a new technique. And it gives us a very unique and direct metric for measuring the mass of a star.”
Microlensing is just one of many types of effects that occur when a huge object passes in front of a bright one. Whole galaxies have been spotted warping the light from other galaxies before. And sometimes, a foreground object can even break apart the light from a background object, creating four different images in what’s known as an “Einstein cross.”
Microlensing is a special type of this phenomenon. In this case, a star passing behind another object is focused and amplified into something that looks much brighter than it actually is. It’s become popular in the last 20 years in the search for exoplanets and dark matter, since it can temporarily brighten distant objects that would otherwise seem dim.
In fact, Einstein predicted that if two stars were to perfectly align, the background star would appear as a bright ring around the star in the front. We haven’t seen this perfect ring from two stars outside our Solar System yet. But this discovery is the closest thing that we’ve ever seen — the stars weren’t perfectly aligned, so we saw this weird shift in position instead.
We’ve only ever seen this type of movement before with our own Sun, during an eclipse. The Sun has enough gravity to bend the light of background stars, says Oswalt. “The catch, of course, is the Sun is so bright you can’t see stars next to it in the sky, even though they’re there.” So during the 1919 total solar eclipse, astronomers measured the position of stars next to the Sun when it was covered up by the Moon, and compared the eclipse measurements to the stars’ positions at night. Sure enough, the stars seemed to have changed their locations, indicating they had been warped when the Sun was present.
Nearly 100 years later, no one has seen a star change the position of another star quite like that before. But for many years now, Sahu and his team have been searching for stars in the sky that may possibly align and create this effect outside our Solar System. To do this, he identified 5,000 stars that make the largest sweeping movements through the night sky. “If they have a large space motion, then the chance they will come close to another background star is high,” says Sahu.
After making enough projections, Sahu and his team identified around three candidates that may causing a microlensing event. One of those was a white dwarf named Stein 2051 B, which was expected to pass in front of another star in March 2014. With Hubble, the astronomers observed this white dwarf multiple times before and after the crossing, capturing the relative movement of the background star. And sure enough, the more distant star shifted its position ever so slightly as the white dwarf passed by. “It was like trying to measure this little firefly moving across a quarter next to a bright light bulb,” says Sahu.
But those tiny relative movements were enough to tell the team exactly how massive Stein 2051 B is. Until now, figuring out the mass of a white dwarf has been difficult. These objects, thought to be the leftover remnants of dead stars, are very small, making them hard to detect and analyze. Before this paper, only three white dwarfs had their masses precisely measured. Astronomers even tried to estimate the mass of Stein 2051 B, but they thought the white dwarf’s mass was extremely low, making it fairly exotic for this kind of object. The microlensing event paints a different story: Stein 2051 B’s mass seems to be fairly normal.
Measuring the masses of stars this way could be much more precise than other techniques, since it involves observing the effect that one star has on another, Sahu says. There could be more measurements like this headed our way soon. Another one of the candidates Sahu identified was Proxima Centauri, the closest star to our Sun. The team watched as the star passed in front of a background star, and the results of those observations could be released soon.
But there are even more telescopes coming online that could be used to spot this phenomena. The Large Synoptic Survey Telescope being built in Chile will eventually provide super detailed images of the Universe, and NASA’s James Webb Space Telescope will be able to peer deeper into space than ever before. All of this equipment and more could be used to predict when another microlensing event will occur.
“These surveys will be measuring a billion or more stars’ motions,” says Oswalt. “So these kinds of events can be predicted now. The search is on.”