Lauri Jetsu, an astrophysicist at the University of Helsinki, is following a tradition that may stretch back thousands of years.
For more than a decade, he’s studied the bright star Algol, one of the most unusual stars in the sky, with its brightness visibly changing every few days. His research suggests those fluctuations have been noted since at least ancient Egyptian times, and he believes observations made more than 3,000 years ago can contribute to the modern scientific understanding of why Algol behaves as it does.
Jetsu’s latest study, published last month in the Astrophysical Journal, suggests the so-called “Demon Star” could be orbited by up to five more stars, most of them too dim to see.
“It was a surprise to me,” he said. “The probability of finding just two more stars is very low. No one has found this many.”
Algol takes its name from the Arabic phrase “Raʾs al-Ghūl,” meaning “the Demon’s Head” — a name shared by a villain in “Batman.” It’s long been considered an “unlucky star,” possibly because of its changes in brightness.
Astronomers have known since the 1880s that Algol is an “eclipsing binary” star. Its changes in brightness are caused by two stars orbiting only a few million miles apart, so that they look like one star here on Earth. The bright light from the larger star is “blotted out” every few days by its dimmer and much smaller companion.
But scientists argue whether tiny variations in the light from Algol could be caused by even more stars orbiting in the system. To answer that, Jetsu applied a new mathematical method to years of recorded data of the light from Algol and revealed regular signals that suggest there may be up to five “companion” stars in the system.
One of the companions is already known to be Algol’s third star, which orbits the central pair at about twice the distance between the Earth and the sun. Many other stars have an orbiting companion star — so called “double stars” or “binaries” — and a few have evolved to have three. But the new study adds strength to the idea that the Algol system has up to four more companion stars that are yet to be seen.
Jetsu said the companions will have to be verified by observations, perhaps with an astronomical technique called interferometry to distinguish their light from the glare of the two main stars.
Jetsu and his colleagues have published four research papers on Algol, and he bears a hieroglyphic tattoo on his right hand that gives the star’s ancient Egyptian name.
“I put it there in 2018 because it took us about 10 years to publish our results, and two of my co-authors died during that time,” he said.
He suggests the ancient Egyptians had noted Algol’s changes in brightness, which can be seen with the naked eye. It becomes noticeably dimmer for about 10 hours almost every three days when the smaller star of the closely-orbiting central pair “eclipses” or passes in front of the larger star.
Jetsu’s research also suggests that changes in the duration of Algol’s eclipses recorded in ancient and modern times — he proposes they’re now almost half-an-hour shorter than 3,200 years ago — can be used to determine how fast Algol’s smaller central star is being “eaten up” by the gravity of its larger star, as astrophysical theories predict.
It’s not known for sure, but the idea that Algol was an unlucky star may have been adopted from Egypt by the ancient Greeks, who decided that it represented an eye in the head of the gorgon Medusa.
According to myth, Medusa was slain by the hero Perseus, whose constellation dominates that part of the sky. Old star maps portray Perseus clutching Medusa’s head, which had retained the power of turning the living to stone.
Algol’s common name comes from the medieval Arab tradition and seems to be based on that Greek tradition, said astronomer Ed Krupp, the director of the Griffith Observatory in Los Angeles.
“This does not reflect an older folk tradition of an odd and disturbingly variable star,” he said. “It is in fact a consequence of the transmission of Greek astronomy to the Islamic world.”
Krupp has written several books about ancient astronomy, but he was not involved in the latest research.
Algol’s visible changes in brightness have prompted modern astronomers to wonder if its unusual behavior was noticed in antiquity, but that hasn’t been scientifically established, he said. Still, the Demon Star’s evil reputation persists.
“Part of the attraction of any story about Algol is the romance associated with the star,” he said.
Astrophysicist Jason Ybarra, a visiting assistant professor at Davidson College in North Carolina who was not involved in the study, is cautious about the results.
“The probability of a multiple stellar system consisting of seven stellar companions forming is very low,” he said.
More research could be done to rule out other causes of the regular variations in the light detected from Algol, such as cycles in the magnetic fields of the main stars that could affect their brightness directly, he said.
But “Algol is a very important system to study because it is the first eclipsing binary discovered, and thus we have the longest time baseline of observation,” he said. “Jetsu’s work is important for understanding eclipsing binaries and possibly exoplanetary systems.”