Researchers from the Harvard-Smithsonian Center for Astrophysics have discovered a new category of binary stars, made up of one fully-formed and one undeveloped star. While most stars come in pairs of almost identical masses, this class of stars is a subset of extreme mass-ratio binaries, a commonly known exception to that rule.  

Maxwell C. Moe and his colleague Rosanne Di Stefano, both CfA researchers, sifted through data from a project known as OGLE, the Optical Gravitational Lensing Experiment.

“They stared, or OGLEd at the Large Magellanic Cloud satellite galaxy for seven years, just monitoring the brightness fluctuations of 35 billion stars,” Moe said of the researchers at OGLE.

Moe and Di Stefano worked to catalog extreme mass-ratio binaries from this data set for about a year before discovering 18 binaries that Moe initially classified as “weird outliers.”

“I almost threw them away from our dataset because we really didn’t understand them but after banging my head against the desk for a little while we finally figured it out,” Moe said.

These “outliers” proved to be a previously undocumented subset of extreme mass twin stars where one star is at least five times larger than its partner and one star is still in its pre-main sequence.  A star that is “pre-main sequence” is still contracting and compressing, Di Stefano said.

The two stars can create an eclipsing system, in which the less bright pre-main sequence star blocks the light of the main sequence star. This eclipse effect helped Moe and De Stefano locate extreme mass-ratio binaries.