"We could find no plausible astrophysical scenario that can explain the observed orbit of the system that doesn't involve at least one black hole." "Our Gemini follow-up observations confirmed beyond reasonable doubt that the binary contains a normal star and at least one dormant black hole," El-Badry said. The unseen object contains the mass of 10 suns, they determined, and orbits the system's center of mass about once every 186 Earth days. And it must be a black hole. These follow-up observations, combined with the Gaia data, allowed the team to take the system's measure in detail. So they studied the star with a number of ground-based instruments, including the Gemini North and Keck 1 telescopes in Hawaii and the Magellan Clay and MPG/ESO telescopes in Chile. The Gaia measurements suggested that a black hole could be that tugger, but the scientists needed more data to know for sure. Its motion displays tiny irregularities - an indication that something massive and unseen is tugging on it gravitationally. ![]() One of those stars is the companion to Gaia BH1. They pored over data gathered by the European Space Agency's (ESA) Gaia spacecraft, which is precisely mapping the positions, speeds and trajectories of about 2 billion Milky Way stars. The researchers employed one such alternate technique in the new study. Finding these dormant objects is even more difficult and requires different strategies. Not all stellar-mass black holes that inhabit binary systems are actively feeding, however. This fast-orbiting dust and gas emits X-rays, high-energy light that some powerful telescopes can observe. ( Gravitational-wave detectors have had more success recently, finding evidence of mergers involving these objects.) And the ones that scientists do see tend to be "X-ray binaries," black holes that pull material from a companion star into an accretion disk around themselves. Their small size makes these bodies relatively hard to detect, however, especially by telescope. Astronomers think that our Milky Way galaxy harbors about 100 million stellar-mass black holes, light-gobbling objects that are five to 100 times more massive than the sun.
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