Most stars in our galaxy are in pairs, and if the alignment is right, some will periodically eclipse each other. Historical observations that span 126 years show that one such pair of eclipsing binaries won’t be doing it anymore after next month.
The system in question is called HS Hydra. The eclipses started about a century ago, becoming very prominent in the 60s. HS Hydra was first reported as an eclipsing binary in 1965, a period when this subject matter was particularly en vogue for astronomical study.
Analysis in 2012 showed that the system was a lot more dynamic than previously thought. From the mid-70s to 2008, the eclipsed portion of each star got smaller and smaller. The researchers of that study, based in Switzerland and the Czech Republic, estimated that the pair would stop eclipsing by 2022.
Now, a team of astronomers from the University of Washington, Western Washington University, and the University of California, Irvine has added data from before the 60s and from after 2008. What was particularly important was the addition of data from NASA's Transiting Exoplanet Survey Satellite (TESS) in 2019. They estimate that the two stars will stop eclipsing around February 2021.
"There is a historical record of observations of HS Hydra that essentially spans modern astronomy – starting with photographic plates in the late 19th century up through satellite images taken in 2019. By diving into those records, we documented the complete rise and fall of this rare type of eclipsing binary," team leader James Davenport, from the University of Washington, said in a statement.
The two stars continue to orbit each other every 36 hours or so. What is changing is the orientation of their orbit with respect to our line of sight, and the cause of that is probably a third star in the system, quite distant from the eclipsing pair. The object, reported in the 2012 paper, is slowly but surely tugging at the dancing duo, changing the inclination of their orbit.
If the pull of this star stays the same as it has been for the last century, we will be able to start seeing the system eclipsing again from the year 2195. However, this might not be the case, as we don't know enough about the dynamics of the system.
"We won't know for sure unless we keep looking," concluded Davenport. "The best we can say right now is that HS Hydra has been changing constantly over the course of modern astronomy."
The findings were presented at the 237th American Astronomical Society Meeting.
