A sparkling ring around a black hole
Radio astronomers get new images with the Event Horizon Telescope. Images from the heart of the active galaxy Messier 87 in the constellation Virgo reveal a sparkling ring around the central extremely massive black hole.
There was a sensation when astronomers published the first image of a black hole in April 2019 with the Event Horizon Telescope (EHT). One of the most massive black holes ever was recorded, which with 6.5 billion solar masses is located in the center of the galaxy Messier 87 (M87) in the middle of the Virgo galaxy cluster in the constellation Virgo (see "Active galaxy core"). However, the EHT team's radio image was just a snapshot of the black hole in M87. EHT astronomers have now evaluated radio data from 2009 to 2013 that show the black hole in action: the ring of matter around the hole sparkles! This was published by the team in the September issue of The Astrophysical Journal.
It has been known for a long time that black holes create an absolutely black spot around them. The reason: Black holes are so compact that beyond a certain distance - the event horizon - not even light, other electromagnetic radiation or gravitational waves can escape. The gravitational pull of the hole is so strong at this critical limit that all waveforms are infinitely redshifted. Nothing can escape from a black hole's event horizon. Black holes are extremely compact, meaning they pack a lot of mass into a very small space. Turning our sun into a black hole would require squeezing the 1.4 million kilometer diameter ball of gas down to just 6 kilometers across!
Astronomers discovered two main mass ranges for black hole candidates: Stellar (star-like) black holes have about 3 to 100 times the mass of the Sun and usually form from the collapse of massive stars. Extremely massive (English: supermassive) black holes are hidden in the centers of galaxies and weigh millions to billions of solar masses. In the center of our galaxy is such a black hole called Sagittarius A with around four million solar masses. Intermediate-mass black holes are suspected at the centers of globular clusters and in ultra-bright X-ray sources. In the gravitational-wave event GW190521, measured by the LIGO and Virgo detectors, a middle-class black hole with 142 solar masses was detected.
The difficult thing about photographing black holes is that their compactness and great distance make them appear extremely small in the sky. Stellar black holes are well beyond the resolution limit of telescopes. However, the event horizon grows linearly with the hole mass. Therefore, only the extremely massive black holes are suitable targets for the astronomical paparazzi, because their black spot in the sky reaches angular diameters of about ten millionths of an arc second. That's how big a two-euro coin appears on the moon – viewed from Earth.
