The one in Sagittarius revealed itself when it passed in front of a dim background star, magnifying the star’s light and slowly shifting its position due to the black hole’s gravity.

HERE’S THE BACKGROUND — The team behind the initial discovery was originally observing a binary star system called HR 6819 when they began noticing evidence for a third, invisible object.

An artist’s impression of the companion object as a black hole. The bright star in background is its pulsar companion. Illustration: Daniëlle Futselaar (artsource.nl) A team of scientists found ...