The S stars
A decade before EHT released its image of Sgr A*, researchers put together a time-lapse “movie” of the stars in the galactic center, a grouping called S stars. The dance of stars around the position of Sgr A over a 10-year period (from 1992 to 2002) reveals a nightmarish, relativistic wonderland. Under the influence of the black hole, several of the S stars are moving in tight, highly elliptical orbits. They can reach breakneck speeds as they make their closest approach the black hole, up to a few percent the speed of light.
Recently, a University of Cologne team led by Florian Peissker announced the discovery of five new S stars that orbit the black hole even more closely than any previously seen. They are S4711, S4712, S4713, S4714, and S4715. The fastest may be S4714, traveling 15,000 miles/second (24,000 km/second).
We’re likely not done finding new S stars, Peissker tells Astronomy. He believes some 40 to 140 additional stars could be discovered orbiting Sgr A* as part of the 130-million-light-year-wide S star cluster, just based on the current limits of modern-day telescopes.
More stars, more mysteries
The tightly packed gang of S stars is not homogeneous, but rather comes in an assortment of types, Peissker says. Some are aging, evolved stars, but some are quite young.
The variety and nature of S stars present a mystery — particularly the presence of young stars, Peissker says. “We know for a fact that these stars must have formed close by. This is strange since star formation is not expected that close to Sgr A*.” That’s because stars cannot form in the classical way within regions hosting the extreme tidal forces found around Sgr A. Star formation through the coalescence of cold material should not be possible in those wild gravitational conditions; objects tend to be torn apart rather than glomming together.
Astronomers have several theories, however, as to how such young stars might find themselves in the vicinity of Sgr A*. Among them is the idea that some stars may have formed normally in binary pairs at a greater distance and then been pulled inward. As they approach the black hole, one stellar member could be captured in a roughly circular orbit, while the second is given an orbital kick that sends it completely out of the galaxy. Another model suggests that both stars could be captured into eccentric, elliptical orbits, much as those we see in the recently discovered S stars.
More research is needed, Peissker concludes, to determine how the S stars were formed and got into their current orbits.
The wild trajectories of S stars at the heart of the galaxy illustrate both the raw power and quantum complexity of black holes. Certainly in years to come, astronomers hope to begin unlocking more of the mysteries surrounding the Milky Way’s massive monster and the speedy stars that circle it.