Brightening Star Offers Astronomers New Way To Study Binary Systems

Brett Smith for redOrbit.com – Your Universe Online

Researchers searching for planets orbiting around a distant star look for a signature dimming of that star’s light, but what if the distant star suddenly gets a bit brighter?

Astronomers from the University of Washington observed just such a phenomenon, but instead of concluding they had found an ‘upside-down’ planet, the scientists realized they had found a new way to study binary star systems, according to their report published in the journal Science.

A binary star system is the term used to describe two stars orbiting around each other in a kind of cosmic waltz. The new study confirmed the existence of a “self-lensing” binary system – a system in which the light from one star is magnified by the gravity of the other star.

Like many scientific discoveries, the self-lensing system, called KOI-3278, was accidentally discovered by the study team while they were combing through data from NASA’s Kepler Space Telescope.

“I found what essentially looked like an upside-down planet,” said study author Ethan Kruse, a graduate student at the University of Washington. “What you normally expect is this dip in brightness, but what you see in this system is basically the exact opposite — it looks like an anti-transit.”

The system is around 2,600 light years away and the two stars orbit each other every 88.2 days. At a distance of around 43 million miles apart, the two stars are about the distance Mercury is from the sun. One star is a white dwarf, considered to be in the last stage of life, around Earth’s size but 200,000 times heavier. The other star appears to resemble our own Sun.

What the researchers observed was the white dwarf bending and amplifying light coming from its more faraway neighbor through gravitational lensing, like the optical effect produced by a magnifying glass.

“The basic idea is fairly simple,” said study author Eric Agol, an associate professor of astronomy at UW. “Gravity warps space and time and as light travels toward us it actually gets bent, changes direction. So, any gravitational object — anything with mass — acts as a magnifying glass.”

“You really need large distances for it to be effective,” Agol continued. “The cool thing, in this case, is that the lensing effect is so strong, we are able to use that to measure the mass of the closer, white dwarf star. And instead of getting a dip now you get a brightening through the gravitational magnification.”

Agol noted that a 2013 study from the California Institute of Technology recognized a comparable self-lensing effect without the brightening of the light as the two stars being examined were closer together.

“The effect in this system is much stronger,” Agol said. “The larger the distance, the more the effect.”

Gravitational lensing of one star by another star is seen occasionally as two stars randomly pass each other in the galaxy.

“The chance is really improbable,” said Agol. “As those two stars go through the galaxy they’ll never come back again, so you see that microlensing effect once and it never repeats. In this case, though, because the stars are orbiting each other, it repeats every 88 days.”