In the Tarantula Nebula, where thousands of stars are born, one bright blue star is behaving strangely. It moves toward us and then away, as if something is tugging it on an invisible string. That 'something' completes an orbit every 17 days, but telescopes can't see it.

Astronomers observed the star 30 times with a spectrograph, which splits light into colors like a prism. By measuring the shift in the spectral lines, they calculated that the invisible partner is at least 5 times more massive than the Sun. But instead of it, there is nothing. The scientists even added artificial stars to the data to check whether an ordinary star might be hiding there. It isn't.

The blue star itself turned out to be extremely hot (about 49,000 degrees) and it is only 1.6 million years old. For a star, that's infancy. But here's the odd part: such young stars usually don't have black holes as companions. Perhaps the star once absorbed material from a dying neighbor and 'rejuvenated,' while that neighbor collapsed into a black hole.

This pair is like two dancers, one of whom is invisible. They whirl around each other, and someday, millions of years from now, they will merge in a powerful explosion that detectors of gravitational waves on Earth will feel.