1799 kilometers per second — that's the speed at which star S5-HVS1 is permanently leaving the galaxy. 4.8 million years ago, it was ejected from the center of the Milky Way during a close encounter with the supermassive black hole Sgr A*. Such an event leaves two fossil traces: a hypervelocity runaway star and its companion, captured into an elongated orbit.
The second trace may have been found. Star S301 orbits the same black hole on a path with eccentricity 0.983 — a huge cigar spanning 687 astronomical units. At periapsis, it comes within 11.5 AU, closer than any known S-star. That distance is comparable to Saturn's orbit.
S301's orbit is no coincidence. Law of conservation of energy and Law of Universal Gravitation link the captured star's semi-major axis to its mass: roughly 500 AU per solar mass. With S301's mass at 1.1–1.5 solar masses — as inferred from stellar evolution models — the predicted semi-major axis is exactly 687 AU. A precise match.
Bayesian analysis says the same. The probability that S301's orbit accidentally matches the predictions is 90 times lower than the hypothesis that it is related to S5-HVS1. Simulations show that over 4.8 million years, the eccentricity has barely changed, because angular momentum scattering in the Galactic center is a slow process.
Two fossil imprints of the same event must carry the same chemical pattern. S5-HVS1 and S301 were born from the same cloud, so their compositions should match. High-resolution spectroscopy with the future Extremely Large Telescope will compare the elements — and then the pair will either be proven or forever dissolved.
The periastron of S301—11.5 astronomical units — is comparable to the distance from the Sun to Saturn. At that distance, a black hole roughly 20 times larger than our star would span a disk visible to the naked eye in the sky, if one could see the event horizon through the accretion flow.