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Astronomers Discover the Universe's Longest Known Stellar Eclipse

3.5 years, but hardly dark or cold.
Artist's impression of eclipsing binary. Image: ESO/L. Calçada

On July 16, 2186, Earth will experience its longest total solar eclipse in 12,000 years. Some locales will see over seven minutes of darkness. The wolves feasting on the bones of human civilization might think that this abrupt sunrise and sunset is weird, or they might not care at all. Seems worth a howl, at least.

Seven minutes is nothing. A team of astronomers from Vanderbilt and Harvard has discovered what they argue is the longest known stellar eclipse, universe-wide. It takes place once every 69 years in the binary star system TYC 2505-672-1 and lasts for approximately three and a half Earth-years.

The super-eclipse discovery is described in a paper accepted for publication in the Astronomical Journal—but currently accessible at the arXiv pre-print server—and is based on observations made with the help of the American Association of Variable Star Observers (AAVSO) network and the Digital Access to a Sky Century @ Harvard (DASCH) program. Each project offered the astronomers hundreds of photographic plates to examine, some of which trace all the way back to 1890. In was in the process of digitizing these old plates that Harvard astronomer Sumin Tang took notice of TYC 2505-672-1.

Tang and soon others wound up with more than 10,000 plates to look at. Combined with an additional sampling from the system's most recent eclipse event, they were able to put together a reasonable picture of the system. For one thing, they observed that the objects are incredibly distant from each other at 20 astronomical units, or roughly the distance from the Sun to Uranus. This accounts for the 69 years between eclipses.

A three and a half year solar eclipse would be enough to handily wipe out life on Earth, wolves and all. It would just get too cold and everything would die. The TYC 2505-672-1 eclipse is a bit of a different story, as it's a stellar eclipse, e.g. an eclipse that occurs on a star itself. This is possible in a binary star system, in which two stars orbit each other.

"The evolutionary status of this hot companion is unclear," the paper notes, "but may be a rare example of a low-mass, recently 'stripped red giant' destined to become a Helium white dwarf." That is, the affected star will thus be just fine when its partner goes dark.