For at least five years, a distant star has been calm and constant. Then, it suddenly went dim. Over the course of two days, it lost 70 per cent of its brightness. By two more days later, it was back to normal. Astronomers haven鈥檛 a clue why.
The All-Sky Automated Survey for Supernovae (ASAS-SN) project uses 24 telescopes around the world to take pictures of the entire sky every night, searching for supernovae and other objects, like stars being devoured by black holes, that suddenly brighten.
On 1 June, a star begin to dim. The star, called ASASSN-V J213939.3-702817.4, is about 3.4 times the diameter of the sun and 3590 light years away from Earth. By 3 June, the star had dimmed by 70 per cent.
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鈥淚t鈥檚 been quiescent for so long and then suddenly decreased in brightness by a huge amount,鈥� says ASAS-SN team member Tharindu Jayasinghe. 鈥淲hy that happened, we don鈥檛 know yet.鈥� After 3 June, the star began to brighten again and was back at its normal brightness by 5 June.
A dip this significant couldn鈥檛 be caused by something within the star itself, Jayasinghe says. There are several potential explanations, including a planet or dust cloud passing in front of the star, but none of them exactly fit, he says. 鈥淭hat鈥檚 what makes this star really weird, we can鈥檛 immediately put it into one neat class of object.鈥�
Tabetha Boyajian at Louisiana State University聽thinks this dimming may be the effect of what appeared to be a single star actually being a pair. She previously discovered a star whose periodic dimming and brightening has yet to be definitively explained, and has even been dubbed聽 the 鈥渁lien megastructure star鈥�, after a suggestion that extraterrestrials are constructing something in orbit.
鈥淎nything that blocks out 70 per cent of the star鈥檚 light would have to be bigger than a planet,鈥� says Boyajian. 鈥淥nce you get much bigger than Jupiter, you get a star.鈥�
If there was a companion star and one eclipsed the other, the light from two stars would suddenly become the light from only one. But Jayasinghe says that this dip in the star鈥檚 light lasted longer than those sorts of eclipses usually do.
One thing is for certain: despite a that this might be a construct called a Dyson sphere built by extraterrestrials around their star, Jayasinghe and Boyajian agree there鈥檚 no reason to suspect aliens. More observations of the star will likely tell us what it is.
