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.”
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.
“[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.