The science behind dwarf stars has become a bit clearer thanks to the discovery of a certain pair of stars obscuring each other.

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In this artist conception of the unique binary star NLTT 11748, the larger but less massive helium white dwarf star is partially eclipsed by the smaller but more massive normal white dwarf, which is about the size of the earth.

Astrophysicists at UCSB have discovered two white dwarf stars in an eclipsing binary system, allowing for the radius of these rare types of stars to be measured for the first time. The term “eclipsing binary system” describes two stars that rotate around and eclipse each other at times.

The discovery was initially made by UCSB physics graduate student Justin Steinfadt, who has been working with physics professor Lars Bildsten, while looking for “pulsating helium-core white dwarf stars.”

Steinfadt had been observing the star NLTT 11748 for pulsations, or ringing, which is when the stars increase and decrease in brightness over a couple minutes. Steinfadt and Bildsten had found out about this star from a digital sky survey, identifying it as a low-mass white dwarf with a helium core.

A series of rapid pictures of the star found a few groups of images where the star was slightly less bright. This observation led to closer investigation.

“We didn’t find what we were looking for,” Bildsten said. “We were looking for this star that was ringing, but it actually had another star in front of it. We had some expectations that it could be ringing, but we had no clue that there was going to be any variation in the object.”

Information gathered from three-minute eclipses of NLTT 11748 suggested that there was another star caught in orbit with NLTT 11748, which was less bright, and passed in front of the line of sight from Earth.

The implications of the discovery were of great enough magnitude to warrant the use of the Keck Observatory’s 10 meter telescope in Mauna Kea, Hawaii. The team contacted David Kaplan, a post-doctoral fellow at UCSB’s Kavli Institute for Theoretical Physics, who had been already performing research there in collaboration with Bildsten by studying the remains of an explosion that had happened a long time ago.

Geoff Marcy, a professor from UC Berkeley, and one of his students had been scheduled to use the telescope at the time for finding planets around stars.

“I gave them the location for the target and as soon as they got it, I started analyzing it,” Kaplan said.

The rest of the team sat in on the observation from a remote observing room.

The team was then able to observe as the helium-filled white dwarf passed in front of its smaller, more massive companion, measuring the Doppler shift produced by its relative approach and retreat.

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David Kaplan, Justin Steinfadt, Avi Shporer, Lars Bildsten

This particular star, however, is one of a group of rare helium-core white dwarfs, which are 10 to 20 percent of the mass of the sun.

“The core is more than 99 percent of the total mass,” Bildsten said. “The hydrogen is only on the surface, which is where all the nuclear burning is occurring.”

These helium-core stars had been predicted to be much larger than a normal white dwarf, but astrophysicists have not been able to measure their size. That is, up until now, thanks to the observations of NLTT 11748.

Though the helium-core star observed in the system is rare, binary stars are extremely common and play a large part in astronomy.

“Fifty percent of stars exist in a binary system,” Steinfadt said. “This new information on this specific binary system can help us better understand everything from stellar evolution to supernovae.”

David Kaplan has created a video simulation of the movement of the stars, which can be viewed here.