In the summer of 2016, astronomers saw a star in the Cygnus constellation 2,500 light-years away flash to life, as though prepared to explode in a fiery supernova. The star dimmed back to normal the next day, with no fuss or kaboom. The strange pattern repeated itself within a few weeks: the star abruptly brightened, then dimmed within a day. The cycle repeated itself again over the next year, five times in 500 days.
A binary pair of red dwarf stars could be warping space-time and distorting our perception of a far farther distant star. (Image credit: M. Rębisz)
“This was a very unusual behavior,” Łukasz Wyrzykowski, an astronomer who studied the strange star at the Astronomical Observatory of the University of Warsaw, Poland, said in a statement. “Hardly any type of supernova or other star does this.”
According to a study published in the journal Astronomy and Astrophysics, the strange star, named Gaia16aye, wasn’t doing anything unusual at all. Rather, it appears like a swarm of interfering binary stars (two stars that orbit around a common gravitational core) is warping space-time in front of Gaia16aye, effectively generating a field of cosmic magnifying glasses, according to the study’s authors. These lenses amplify the star’s light every time it passes behind them. And those stars were effectively invisible from Earth.
This stellar magnifying effect, wherein massive objects seem to bend space-time around them, is known as gravitational lensing and was predicted by Albert Einstein’s theory of general relativity. Scientists have utilized the phenomena to obtain a closer look at some of the universe’s earliest stars, galaxies, and objects, but it can also disclose the properties of much closer, dimmer items.
Take the binary pair that is creating trouble for Gaia16aye. While the pair would seem to us to be completely invisible, the strength and frequency of their gravitational lensing allowed the researchers to work backward and determine “basically everything” about them, according to study co-author Przemek Mróz, a postdoctoral scholar at the California Institute of Technology.
The team found that the binary pair must be creating many pockets of magnification in order to create the regular, daylong brightening of Gaia16aye (also known as gravitational microlensing). According to the study’s authors, these stars are most likely a pair of tiny, red dwarfs around 0.57 and 0.36 times the mass of the Earth’s sun separated by about twice the Earth-sun distance.
If microlensing events like this one can reveal invisible stars, they may also be able to reveal even more unusual and enigmatic cosmic phenomena. Hopefully, that will include black holes, which are generally only detectable when they are sucking in nearby matter and burping up jets of gassy light. According to the researchers, the Milky Way could be filled with millions of stand-alone black holes that are too far away from any surrounding stars to put on such a light show, and gravitational lensing could be the key to locating them.
Astronomers can work backwards to determine the true nature of an undetectable black hole if it causes a lensing effect that distorts the light behind it.