The center of our galaxy is an extreme place. There lies a supermassive black hole of 4 million solar masses. If a binary star ventures too close, tidal forces tear the pair apart. One star gets captured into a highly elongated orbit, while the other is flung away at immense speed. That's how hypervelocity stars like S5-HVS1 are born: it's racing away from the center at 1799 km/s.
Now astronomers have found a star, S301, that fits the description of such a companion. Its orbit is a highly elongated ellipse with a length of 687 astronomical units. Why does this matter? From the law of conservation of energy, the orbit size depends on the star's mass: roughly 500 AU per solar mass. S301 weighs about 1.4 solar masses, and the calculated semi-major axis matched the observations perfectly. Such a coincidence is unlikely to be random: Bayesian inference (assessing the probability of two explanations) shows that the hypothesis of their kinship is 90 times more likely than chance. The next step is to compare the chemical composition of the stars to close the case.
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.