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A picture of the Hubble Space Telescope, which was used to make the findings reported on Wednesday in the journal Science. Photo by NASA

Einstein’s theory and ‘bent light’ reveal a way to weigh stars for first time

It’s been a busy month for Einstein’s legacy.

Coming hot off the heels of LIGO’s third detection of gravitational waves, astrophysicists have applied Einstein’s theory of general relativity to create a scale for weighing stars. But rather than expose whether celestial objects need to go on a diet, this new scale offers a chance to learn more about the life cycle of stars, including our sun.

To make this interstellar scale, the scientists pointed the Hubble Space Telescope at the nearby binary star system Stein 2051, and then relied on a phenomenon called “gravitational microlensing.”

That’s when one star passes in front of another, and gravity of closer star slightly bends the light coming from the more distant star. This warping of light is what’s known as an “Einstein ring.”

Einstein thought it would be impossible to find the right stars to feasibly use microlensing to measure a star’s mass.

“When you look at the stars, they generally look steady and not moving, but actually they do move a tiny amount,” said Kailash Sahu, an astrophysicist at the Space Telescope Science Institute in Baltimore and lead author on the study published Wednesday in Science. “So we tried to actively look for events where one star would come in front of another.”

This animation shows the motion of a white dwarf star passing in front of a distant background star. During the passage, the faraway star appears to change its position slightly, because its light path has been altered by the white dwarf’s gravity. When the white dwarf Stein 2051 B passed in front of a background star, the distant star’s light was offset by only about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68 percent of the sun’s mass.

This animation shows the motion of a white dwarf star passing in front of a distant background star. During the passage, the faraway star appears to change its position slightly, because its light path has been altered by the white dwarf’s gravity. When the white dwarf Stein 2051 B passed in front of a background star, the distant star’s light was offset by only about 2 milliarcseconds from its actual position. This deviation is so small that it is equivalent to observing an ant crawl across the surface of a quarter from 1,500 miles away. From this measurement, astronomers calculated that the white dwarf’s mass is roughly 68 percent of the sun’s mass.
Animation by NASA, ESA, and G. Bacon (STScI)

Einstein proposed in 1936 that scientists could use the warping of light to determine the mass of a star. People had observed gravitational lensing prior to Einstein’s proposal, during a solar eclipse in 1919. But Einstein could not use these measurements to determine the mass of the sun given the eclipse involved just one star. But tracking such phenomena outside our solar system — due to the longer, interstellar distances — can make things tricky for scientists.

“We are trying to see the deflection of this background star, which is much farther away, and much much fainter,” Sahu said. The team needed two or more stars in just the right position and distance from Earth to detect an Einstein ring.

Spotting huge amounts of gravity and lensing among galaxies and black holes is easy with modern telescopes, because those objects are enormous. Stars are much smaller, so the best candidates for the study had to sit in our celestial neighborhood.

An example of gravitational lensing in action. Photo by NASA

An example of gravitational lensing in action. Photo by NASA

“[This process] can’t be done for any arbitrary nearby star. That star has to pass in front of something and fortunately we can tell when that’s going to happen,” said Rosanne Di Stefano, an astronomer at Harvard University who was not involved in the study, but has been following Sahu’s work, which was also presented today at the 230th spring meeting of the American Astronomical Society in Austin, Texas.

Sahu considered our closest neighbor — the three-star system of Alpha Centauri located 4.2 light-years away — but then opted for Stein 2051 for three reasons. Stein 2051 is 18 light-years away, so still in our stellar backyard. Second, Stein 2051 was scheduled to experience a gravitational lensing event earlier than Alpha Centauri. Finally, one of the stars in Stein 2051 is a white dwarf. Astronomers know very little about white dwarfs and other objects generated by dying stars, so the opportunity to peer into Stein 2051 became too good to pass up.

Illustration of how a white dwarf star can bend light around nearby stars. Photo by NASA, ESA, and A. Feild (STScI)

Illustration of how a white dwarf star can bend light around nearby stars. Photo by NASA, ESA, and A. Feild (STScI)

When the Hubble telescope observed the Stein 2051 system in March 2014, the irregularly shaped Einstein ring was clearly visible. Using their measurements of the ring, astronomers were able to determine that the white dwarf star — one of two stars in the system — is 68 percent of our sun’s mass. Astronomers also found that the white dwarf’s core is made from carbon and oxygen, which is commonly found in similar stars.

Scientists thought the star’s orbit suggested something strange about the composition, but the findings show that the stars mass is more reflective of a standard white dwarf as opposed to a more irregular star, one with iron in its core.

Researchers are hoping to use newer and better technologies like the upcoming James Webb Space Telescope to get a better look at these curious stars.

“I think this is the future of the field,” Di Stefano said. “This is the frontier.”

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