Invisible giant in space is bigger than expected


Black hole in the Milky Way Invisible giant in space is bigger than expected

In our galactic neighborhood, the black hole Cygnus X-1 is whirling around its companion star at breakneck speed. And it’s bigger than previously thought.  The  latest studies with the participation of the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) show that the mass of the light-devouring object had previously been underestimated by 50%.

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The black hole Cygnus X-1 and its companion star;  artist's impression
© The  black hole Cygnus X-1 and its companion star; artist’s impression (Image: NASA / CXC / M.Weiss)

Erlangen – How do you research something that swallows all light and thus all information? Black holes represent a special challenge for astrophysicists to this day. ©

 The y can only be observed indirectly through their gravitational effect on neighboring stars, or through the way in which they deflect light in their vicinity. Correctly evaluating all these indications of black holes requires a lot of computing power and patience.

An international team of researchers including the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) has now gained new knowledge about the black hole Cygnus X-1, which is positioned with its companion star in the middle of the Milky Way.


 The  first black hole in our home galaxy


 The  first evidence of the existence of Cygnus X-1 came in 1964: Two Geiger counters on board a suborbital rocket that was launched from New Mexico registered a strong X-ray source in our Milky Way. Eight years later, the American astronomer Tom Bolton discovered that this X-ray source orbits the star HDE 226868, a so-called blue giant. Bolton concluded that the invisible object Cygnus X-1 must be a black hole. This assumption was confirmed by later observations. “Cygnus X-1 is the first black hole that was discovered in our Milky Way,” says Prof. Dr. Jörn Wilms, astrophysicist at the FAU University Observatory.


 The  actual distance of the system from Earth has so far only been roughly estimated, as has the masses of the black hole and its companion star. In a Wilms project, an international team has now collected and evaluated more precise data on Cygnus X-1. ©

 The  researchers used the Very Long Baseline Array, a cluster of ten radio telescopes distributed across the United States, to make a precise parallax measurement. “©

 The  measurement is based on the principle that you can determine the distance to an object by looking at it from two different locations,” explains Wilms. “In our case, the different observation positions result from the movement of the earth around the sun.”

An animation on the video channel of the Friedrich-Alexander-Universität Erlangen-Nürnberg shows the Cygnus X-1 system and summarizes the new findings about the black hole. Source: International Center for Radio Astronomy Research.

Further away and bigger than expected


 The  scientists observed the Cygnus system over a period of six days and recorded over 2000 readings. ©

 The  result: Cygnus X-1 is significantly further away from Earth than previously assumed – about 7200 light years instead of the previously estimated 6100 light years. In astronomical dimensions it is still very close. For example, the center of the Milky Way is 27,000 light years from Earth.

A corrected size estimate of Cygnus now follows from the researchers’ new distance measurements. “We have calculated that the black hole is more than 20 times as massive as the sun. That’s exceeds previous estimates by 50 percent, ”says Wilms.

The black hole Cygnus X-1 and its companion star - the sun is included for size comparison;  artist's impression

 The  black hole Cygnus X-1 and its companion star – the sun is included for size comparison; artist’s impression

(Bild: International Centre for Radio Astronomy Research)

This finding also sheds new light on the formation of black holes in general: Up until now, it was assumed that bright stars would lose a great deal of mass to their surroundings before the supernova explosion. “Matter is practically blown away from the surface by stellar winds. In order for a black hole to become as massive as Cygnus X-1, this loss of mass must be significantly less than we thought, ”explains the project initiator.

Rotation near the speed of light

Based on the current measurement data, the researchers assume that the black hole in the Cygnus X-1 system began its life as a star that was around 60 times the size of the sun and collapsed tens of thousands of years ago. Despite its gigantic size, it orbits its companion star in just five and a half days, with the orbit being only a fifth of the distance between the earth and the sun. Cygnus X-1 rotates incredibly fast – close to the speed of light and therefore faster than any other black hole found so far.


 The  system of star and black hole emits extremely strong X-rays during its wild dance. This occurs because the companion star loses part of its mass to the black hole and forms a disk of gas that heats up to several million degrees through friction.

The first image of the black hole in the neighboring galaxy M87, captured by the Event Horizon Telescope project.

New radio telescope should sharpen the view into space

“Black holes are still some of the best-kept secrets in the universe,” says Wilms. “With our project we have been able to reveal another part of this secret.” In the coming year, construction of the Square Kilometer Array (SKA) is to begin in Australia and South Africa, which will once again exceed the sensitivity of what is currently the largest radio telescope in the world and the universe in even greater detail should map. Astro research hopes that this will provide new impulses for the understanding of exotic and extreme cosmic objects that have so far remained hidden from us.

Originalpublikation: James C. A. Miller-Jones1, Arash Bahramian, Jerome A. Orosz, Ilya Mandel, Lijun Gou, Thomas J. Maccarone, Coenraad J. Neijssel, Xueshan Zhao, Janusz Ziółkowski, Mark J. Reid, Phil Uttley, Xueying Zheng, Do-Young Byun, Richard Dodson, Victoria Grinberg, Taehyun Jung, Jeong-Sook Kim, Benito Marcote, Sera Markoff, María J. Rioja, Anthony P. Rushton, David M. Russell, Gregory R. Sivakoff, Alexandra J. Tetarenko, Valeriu Tudose, Joern Wilms: Cygnus X-1 contains a 21–solar mass black hole—Implications for massive star winds, Science, 18 Feb 2021; DOI: 10.1126/science.abb3363


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Invisible giant space bigger expected


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