Tiny newfound ‘Unicorn’ is closest known black hole to Earth
Astronomers have apparently found the closest known black hole to Earth, a weirdly tiny object dubbed ‘the Unicorn’ that lurks just 1,500 light years from us. The nickname has a double meaning: not only does the black hole candidate reside in the constellation Monoceros (the Unicorn), its incredibly low mass – about three times that of the Sun – makes it nearly one of a kind. The Unicorn has a companion – a bloated red giant star that’s nearing the end of its life. That companion has been observed by a variety of instruments over the years, including the All Sky Automated Survey and NASA’s Transiting Exoplanet Survey Satellite.
Scientists analysed the data from these instruments and noticed something interesting. The red giant’s light shifts in intensity periodically, suggesting that another object is tugging on the star and changing its shape. They determined that the object doing the tugging is likely a black hole – one harbouring a mere three solar masses – based on details of the star’s velocity and the light distortion. For perspective, the supermassive black hole at the heart of our Milky Way galaxy packs about 4.3 million solar masses.
“Just as the Moon’s gravity distorts Earth’s oceans, causing the seas to bulge towards and away from the Moon, so does the black hole distort the star into a rugby-ball shape, with one axis longer than the other,” said Todd Thompson, chair of Ohio State’s astronomy department. “The simplest explanation is that it’s a black hole – and the simplest explanation is the most likely one.”