A key feature of scientific theories is that they make successful predictions, which we can use to determine whether they're likely to be right. For general relativity, one of its key predictions had to wait for an eclipse of the Sun. Relativity predicted that the mass of the Sun should distort space enough to bend the light arriving from distant stars. We needed an eclipse in order to be able to see the stars, rather than ending up blinded by pointing telescopes toward the Sun.

That historic eclipse was front page news nearly a century ago. Now, researchers have managed to repeat the experiment, but they upped the degree of difficulty considerably by tracking the distortion of space produced by a white dwarf more than 18 light years away. Not only was relativity confirmed yet again, but the result cleared up a potentially awkward possibility, namely that the star was older than the Universe itself.

Twisted space

An optical lens works because light takes different paths through a material. According to relativity, space itself is distorted by the presence of large masses, and these distortions can also cause light to take different paths between their origin and Earth. This gravitational lensing has turned into a powerful tool in astronomy. Researchers have used it to magnify distant objects and to identify concentrations of dark matter by the distortions they produce. The temporary brightening gravitational lensing produces has even been used to identify a handful of planets.

Despite all these uses, however, researchers had never managed to repeat the original eclipse experiment—in which an intervening mass makes a known object look like it's somewhere else—with any star other than our Sun. This seemed to be another case where Einstein was annoyingly right, as when he first predicted it was technically possible but despaired that "there is no hope of observing this phenomenon directly."

So an international team of scientists decided to show that relativity was right, but Einstein's prediction about its observation was wrong. To do that, they had to find the right star, one that essentially moved in front of a more distant star from Earth's perspective. This involved looking through a catalog of 5,000 stars that have large apparent motions from Earth. Their future travels were projected forward in time, and the scientists compared those tracks to a list of stars that are distant and stable enough to act as guide stars for telescopes and spacecraft.

This analysis identified a case in which a nearby star would be passing nearly directly in front of a more distant one. That closer star happened to be a white dwarf named Stein 2051 B, and it came with its own somewhat awkward history.

Stein’s strangeness

Stein 2051 B happens to be one of the closest white dwarf stars. These objects are the end point in the evolution of most lower-mass stars once fusion has ended (over 90 percent of stars will end up as a white dwarf eventually). They're thought to be largely composed of carbon and oxygen, with just a bit of un-burned hydrogen and helium in their outer envelopes.

Because of their simple structure, the properties of white dwarfs should all be related—if you know the mass and temperature, you can predict the radius, and so on. Plus, since white dwarfs are no longer engaged in fusion, all of these properties will be related to their age.

But testing whether those relationships hold has been challenging because we need to measure the mass of a white dwarf. That, in turn, means we need to find a system where the white dwarf is orbiting with another star and infer its mass from the orbit. But this is remarkably hard to do. In 10 of the cases, the stars were close enough that they'd probably shared material, which would confuse any analysis. That leaves just three stars where we've tested this relationship.

And then there's Stein 2051 B. Measures of its orbit suggested it was unexpectedly light. For its radius, however, that means that it has to be dense, which suggests an iron core. But stars that form white dwarfs aren't big enough to undergo iron-generating fusion. So, that's a problem. The other issue is that iron fusion would imply a very high starting temperature for the white dwarf, which means it would have taken much longer to cool off to its current temperature. So long, in fact, that it makes the age of Stein 2051 B "uncomfortably close to the age of the Universe."

So Stein 2051 B is a bit of a problem, which makes it an even better candidate for detailed study.

Bend it like a dwarf

While ground-based telescopes wouldn't cut it for tracking the background star, the Hubble Space Telescope was more than up to the task. Over a two-year period, the team managed to obtain eight sets of observations as Stein 2051 B slid in front of a background star and then exited the scene. Some of these observations had to be thrown out because the dwarf was too close to the background star and it swamped the detectors. By chance, in another set of images, the background star ended up right in the middle of a diffraction artifact. But they still managed to get four good sets of images.

The images on their own aren't enough, however, as both the Earth and all the stars in between them will move, even if only slightly. So the team used a set of 26 stars in each of the images to create a single, undistorted reference frame before correcting every image to this reference frame.

With that, the researchers were finally ready to track the apparent position of the distant star. And they found that, instead of showing up in its actual position, it was shifted to different locations based on the precise position of Stein 2051 B. And, in each of the four sets of images, the new position was within the error of the estimated position calculated using general relativity.

This also gave the scientists the mass of Stein 2051 B. Based on its other properties, a carbon-oxygen white dwarf like this should have a mass of about 0.67 times that of the Sun; the new measurement placed it at 0.675 times. And with that, all the problems with Stein 2051 B went away. The authors helpfully note that "our measurement does not conflict with the age of the Universe."

All in all, a pretty impressive bit of work.

Science, 2017. DOI: 10.1126/science.aal2879 (About DOIs).