At the very heart of the Milky Way, stars whirl around an invisible supermassive black hole. One of them, S301, completes an orbit in just 8.7 years. Its path is a highly squashed ellipse: sometimes it grazes the black hole, sometimes it shoots far away. At closest approach, the distance to the black hole is tiny by cosmic standards.
Right there, at the brink of the abyss, space and time are so warped that the star's spin axis begins to slowly turn, like a spinning top that gets a nudge. This effect is called geodetic precession. The authors calculated how much the absorption line in the star's spectrum would shift due to this turn. If a new generation of spectrographs detects this shift, we'll have another confirmation that the curved spacetime around the black hole behaves exactly as Einstein predicted. The star itself becomes a flying gyroscope whose drift we can measure from Earth.
S301's orbital period around the black hole is 8.7 years — shorter than Jupiter's 12-year orbit around the Sun, even though the black hole is millions of times more massive.