Cygnus X-1

Cygnus X-1 is a very bright object in space that gives off powerful X-rays. It was found in 1964 during a rocket flight and is one of the strongest X-ray sources we can detect from Earth. Scientists believe it is a black hole, a place where gravity is so strong that even light cannot escape.

This object is about 7,000 light-years away and is part of a system with a huge blue star. The star and the black hole are very close to each other, and material from the star forms a hot disk around the black hole. This disk gets so hot that it shines with X-rays.

Famous scientists, including Stephen Hawking, once debated whether Cygnus X-1 was truly a black hole. In the end, observations proved that it is, and Hawking admitted he had lost his bet. Cygnus X-1 remains one of the most studied objects in the night sky.

Discovery and observation

Observations of X-ray light help scientists study space objects with very hot gas. Since Earth’s air blocks X-rays, scientists must use special instruments high above the air to see these objects.

Cygnus X-1 was found in 1964 using X-ray tools on rockets from White Sands Missile Range in New Mexico. These tools scanned part of the sky and discovered eight new X-ray sources, including Cygnus X-1 in the Cygnus area of the sky. Later, a satellite named Uhuru, launched in 1970, found many more X-ray sources and showed that Cygnus X-1’s X-ray brightness changes quickly. This means the X-ray source is very small, about the size of the planet Jupiter.

In 1971, scientists found that Cygnus X-1 is near a big, bright star named HDE 226868. They later discovered that HDE 226868 has a very heavy, unseen companion, which they think is a black hole. Since then, Cygnus X-1 has been studied a lot, and it helps scientists understand how black holes work.

Binary system

The black hole and blue supergiant star form a binary system where they orbit each other around their center of mass every 5.599829 days. From Earth, we never see the black hole hidden behind the star; the system does not eclipse. However, we are unsure about the inclination of the orbital plane to our line of sight from Earth, with predictions ranging from 27° to 65°. A study in 2007 suggested the inclination might be around 48.0° ± 6.8°, meaning the distance between the stars is about 0.2 AU, or 20% of the distance from Earth to the Sun. The orbit is nearly circular, with an orbital eccentricity of only 0.018 ± 0.002.

This system shares movement through space with a group of massive stars called Cygnus OB3, about 7,000 light-years away. This suggests HDE 226868, Cygnus X-1, and this group may have formed together. If true, the system is about 5 ± 1.5 million years old. The distance to Cygnus X-1 is estimated to be between 6,070 ± 390 light-years.

Compact object

The mass of the object appears to be more than what a neutron star can hold. Measurements suggest it could be about 14.8 ± 1 solar masses, making it most likely a black hole—a region where gravity is so strong that even light cannot escape. Anything that passes through this boundary cannot escape.

Evidence of this boundary may have been found in 1992 using the Hubble Space Telescope. As material spirals into the black hole, its light changes in a way that matches what we expect from a black hole.

The spin of this object is still uncertain. Some data suggest it spins very fast, about 790 times per second.

Formation

The star that became this black hole may have been more massive than 40 times the Sun. It likely lost much of its mass before collapsing into a black hole, possibly without a big explosion.

Accretion disk

The black hole is surrounded by a disk of hot, glowing material called an accretion disk. This disk gets very hot due to friction and shines brightly in X-rays. These X-rays can change in patterns, helping scientists estimate the black hole's mass.

The X-rays from Cygnus X-1 can change between two main states. In one state, the X-rays have higher energy. In the other, they have lower energy and change more quickly. These changes happen because of how material close to the black hole behaves.

Jets

As material falls toward the black hole, some of its energy is released in powerful streams called jets, shooting out at nearly the speed of light. These jets can push away extra material and help balance the system. One of these jets is hitting nearby space material, creating a glowing ring that we can see.

In 2006, Cygnus X-1 was the first black hole of its kind found to give off very high-energy gamma-ray signals, linking it to bursts of X-rays.

HDE 226868

HDE 226868 is a huge, bright star with a surface temperature of about 31,000 K. It is about 20–40 times the mass of the Sun and shines very brightly. The gravity of the black hole pulls and shapes this star, making its brightness change slightly as it orbits.

The star is losing material into space, and the black hole pulls some of this material toward it. Even though the star's surface is not being pulled away, much of the material it loses ends up around the black hole.

The dust and gas between us and this star make it appear dimmer and redder than it really is. Without this interference, we might be able to see it without a telescope.

Stephen Hawking and Kip Thorne

Cygnus X-1 was the topic of a fun bet between two scientists, Stephen Hawking and Kip Thorne. Hawking didn’t think black holes were in that area of space, so he made a friendly wager. He later said this was like an “insurance policy” for all his work on black holes.

In his book A Brief History of Time, Hawking shared that if black holes didn’t exist, he’d win the bet and get four years of a funny magazine called Private Eye. But if they did exist, Kip would get a year of a magazine called Penthouse. At first, they thought there was an 80% chance Cygnus X-1 was a black hole. By 1988, Hawking felt it was almost certain—about 95% sure—but the bet hadn’t been settled yet.

Later, Hawking agreed that black holes were real after more observations. In another book, Black Holes and Time Warps, Kip Thorne said Hawking came into his office in Russia and signed the bet to show he lost. The bet was actually written on December 10, 1974, as shown by the paper with both their signatures. Kip Thorne talked about this on a TV show called Nova on PBS.

In popular culture

Cygnus X-1 has been featured in many fun and creative ways in stories and music. The Canadian rock band Rush wrote a two-part song series about it. The first part, "Book I: The Voyage", is from their 1977 album A Farewell to Kings. The second part, "Book II: Hemispheres", is on their 1978 album Hemispheres. The songs tell the story of an explorer in a spaceship called the Rocinante, who travels toward the black hole and faces big challenges.

In the 1979 Disney movie The Black Hole, there is a ship named the Cygnus that studies a black hole. The name Cygnus may be inspired by Cygnus X-1, the first-known black hole of its kind. The TV show Futurama also mentioned Cygnus X-1 in one of its episodes with a funny tagline.