Solving the Busy Stellar Mystery of Cygnus X-1

Deep in the heart of the constellation Cygnus, the Swan lies an otherwise-invisible object called Cygnus X-1. Its name comes from the fact that it was the first galactic x-ray source ever discovered. Its detection came during the Cold War between the U.S. and Soviet Union when sounding rockets began to carry x-ray-sensitive instruments above Earth's atmosphere. Not only did astronomers want to find these sources, but it was important to distinguish high-energy events in space from likely events caused by incoming missiles. So, in 1964, a series of rockets went up, and the first detection was this mysterious object in Cygnus. It was very strong in x-rays, but there was no visible-light counterpart. What could it be?

Sourcing Cygnus X-1

The discovery of Cygnus X-1 was a big step in x-ray astronomy. As better instruments were turned to look at Cygnus X-1, astronomers began to get a good feel for what it might be. It also emitted naturally-occurring radio signals, which helped astronomers figure out exactly where the source was. It appeared to be very close to a star called HDE 226868. However, that wasn't the source of the x-ray and radio emissions. It wasn't hot enough to generate such strong radiation. So, there had to be something else there. Something massive and powerful. But what?

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Further observations revealed something massive enough to be a stellar black hole orbiting in a system with a blue supergiant star. The system itself could be about five billion years old, which is about the right age for a 40-solar-mass star to live, lose a bunch of its mass, and then collapse to form a black hole. The radiation is likely coming from a pair of jets that extend out from the black hole — which would be strong enough to emit strong x-ray and radio signals.

The Peculiar Nature of Cygnus X-1

Astronomers call Cygnus X-1 a galactic x-ray source and characterize the object as a high-mass x-ray binary system. That simply means there are two objects (binary) orbiting a common center of mass. There's a great deal of material in a disk around the black hole that gets heated to extremely high temperatures, which generates the x-rays. The jets carry material away from the black hole region at a very high rate of speed.

Interestingly, astronomers also think of the Cygnus X-1 system as a microquasar. This means that it has many properties in common with quasars (short for quasi-stellar radio sources). These are compact, massive, and very bright in x-rays. Quasars are seen from across the universe and are thought to be very active galactic nuclei with supermassive black holes. A microquasar is also very compact, but much smaller, and also bright in x-rays.

How to Make a Similar Object

The creation of Cygnus X-1 happened in a grouping of stars called an OB3 association. These are fairly young but very massive stars. They live short lives and can leave behind beautiful and intriguing objects, such as supernova remnants or black holes. The star that created the black hole in the system is called a "progenitor" star and may have lost as much as three-quarters of its mass before it became a black hole. The material in the system then began to swirl around, drawn in by the gravity of the black hole. As it moves in an accretion disk, it is heated by friction and magnetic field activity. That action causes it to give off x-rays. Some material is funneled into jets that are also superheated. They give off radio emissions.

Due to actions in the cloud and jets, the signals can oscillate (pulsate) over short periods of time. These missions and pulsations are what caught the attention of astronomers. In addition, the companion star is also losing mass through its stellar wind. That material gets drawn into the accretion disk around the black hole, adding to the complex actions going on in the system.

Astronomers continue to study Cygnus X-1 to determine more about its past and future. It's a fascinating example of how stars and their evolution can create strange and wonderful new objects that give clues to their existence across the light-years of space.