At the center of the Milky Way lies Sagittarius A*, our supermassive black hole that recently got imaged for the first time. Around it orbits a group of stars that get so close they are a fantastic testbed for Einstein's theory of general relativity. Now researchers have found the fastest known star to orbit Sagittarius A* in the record shortest time.
Star S4716 orbits the supermassive black hole in about four years, a few years shorter than the previous record-holder. It gets as close as 15 billion kilometers (almost 10 billion miles) to the black hole and reaches speeds of 8,000 kilometers (5,000 miles) per second.
The discovery is reported in The Astrophysical Journal and challenges some expectations regarding just how close stars can get in a stable orbit around a supermassive black hole.
"For a star to be in a stable orbit so close and fast in the vicinity of a supermassive black hole was completely unexpected and marks the limit that can be observed with traditional telescopes," Dr Florian Peißker from the University of Cologne said in a statement.
To find these stars, the team employed almost 20 years’ worth of data collected. Five telescopes observed the stars, with four of them acting as a single much larger telescope to provide an accurate and detailed analysis of its orbit.
"One prominent member, S2, behaves like a large person sitting in front of you in a movie theater: it blocks your view of what's important," added Peißker, the lead author of the new study. "The view into the center of our galaxy is therefore often obscured by S2. However, in brief moments we can observe the surroundings of the central black hole."
The discovery, together with the previous observations of very close stars, expands humanity’s understanding of where and how close stars can orbit a black hole. But S4716 and the several members of its group highlight new mysteries in how exactly they got there.
"The short-period, compact orbit of S4716 is quite puzzling," explained Michael Zajaček, an astrophysicist at Masaryk University in the Czech Republic who was involved in the study. "Stars cannot form so easily near the black hole. S4716 had to move inwards, for example by approaching other stars and objects in the S cluster, which caused its orbit to shrink significantly."