Science.-Rise and fall of a rare star dance documented since the 19th century

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01/15/2021 Image of a 1945 photographic plate, which was digitized for Harvard’s Digital Access to a Sky Century catalog, or DASCH POLITICA INVESTIGACIÓN Y TECNOLOGÍA DASCH / HARVARD UNIVERSITY


Astronomers have cataloged 126 years of changes in HS Hydrae, a rare nearby binary star system, using data from 19th-century astrophotographic plates to NASA’s TESS mission.

This system is what is known as an eclipsing binary: from Earth, the two stars appear to pass over one another, or outshine each other, while orbiting a shared center of gravity. Eclipses cause the amount of light emitted by the binary to periodically dim.

The two stars began to eclipse each other in small numbers starting about a century ago, increasing to almost complete eclipses in the 1960s. The degree of eclipse then plummeted over the course of just half a century and will cease around February 2021.

“There is a historical record of observations of HS Hydrae that essentially spans modern astronomy, beginning with photographic plates in the late 19th century to satellite images taken in 2019. By diving into those records, we document the full rise and fall of this rare kind of eclipsing binary, “team leader James Davenport, an assistant research professor of astronomy at the University of Washington, said in a statement. He presented results at the meeting of the American Astronomical Society.


The eclipses of the two stars that make up HS Hydrae are changing because another body, most likely an unobserved third companion star, is changing the orientation of the binary relative to Earth. Systems like this, which are called evolving eclipsing binaries, are rare, with only about a dozen known to date, according to Davenport. Identifying this type of binary requires multiple observations to look for long-term changes in the degree of attenuation, which would indicate that the orientation of the binary is changing over time.

HS Hydrae has such an observation record because, at 342 light years away, it is a relatively close and bright system and the two stars orbit each other every 1.5 days. Scientists first reported that HS Hydrae was an eclipsing binary in 1965. In a 2012 article, astronomers based in Switzerland and the Czech Republic reported that the amount of attenuation of HS Hydrae decreased from 1975 to 2008, indicating that the two stars were dwarfing smaller and smaller portions of each other over time. That team also predicted that eclipses would end around 2022.

Davenport and his team recorded at HS Hydrae using observations of the system in 2019 by NASA’s Transiting Exoplanet Study Satellite, or TESS. They only saw a 0.0075 magnitude drop in light from HS Hydrae, a sign that the two stars were barely covering each other during eclipses. For comparison, the eclipses in 1975 saw a drop of more than 0.5 degrees.