This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Active black holes are easy to detect. Their superheated accretion disks emit bright X-rays, and the jets streaming from their poles are easily seen at visible and radio wavelengths.
But most stellar-mass black holes are inactive. They either drift through the galaxy alone or orbit a companion star without consuming its material. We can't observe inactive black holes directly.
We can only observe their effects on nearby objects. We currently know of three such black holes thanks to the Gaia spacecraft. The main mission of Gaia is to map the positions and motions of more than a billion stars in our region of the Milky Way.
To achieve its goal, its motion measurements must be precise enough to detect small wobbles in a star's motion. Usually, this means the star has a planet, and the planet's gravitational tug causes the star to wobble. But in at least three cases, the gravitational tug is so large that the companion must have a stellar mass.
If it were a star, Gaia would see it. Since Gaia doesn't, the companion must be a black hole. Each of the quiet black holes we've discovered this way has a small stellar companion.
This means that in the past, these were asymmetrical binaries, with one star much larger than the other. The remnants of the larger stars became black holes, forming the systems we see today. In one of these systems, Gaia BH3, the binary is fairly wide, just as we would expect.
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