Cygnus X-1 — The Black Hole In The Heavenly Swan

The Northern Cross (outlined) and Vega (lower right) are visible in the western sky during December evenings. This photo was taken about 8:15 p.m. The hazy band of starlight cutting through the constellation is a portion of the Milky Way. Photo: Bob King

It’s December 1 and summer is slipping away. Not the real summer – that was gone a long time ago. I’m talking about the lingering stars of summer still pinned to the western sky during the early evening. Stars like the ones that comprise the Summer Triangle and in particular the Northern Cross and Vega. Earlier this week I noticed that the Cross stands straight up in the west now with Vega sparking lower down between bare branches. A glance to the east reveals stalwarts Orion and Gemini hauling up their caches of winter gems.

Black hole candidate Cygnus X-1 is located 1/2 degree to the upper left of Eta Cygni. Use binoculars or a small telescope and the detailed map below to get there. Maps created with Stellarium

The Northern Cross is the popular name for the constellation Cygnus the Swan. The head of the cross is actually the tail of the swan, the crossbeam its outspread wings and the bottom of the cross the bird’s head. Since the Milky Way courses the full length of the Swan, Cygnus is home to a plethora of star clusters, gaseous clouds called nebulae and double stars.

It also contains the enigmatic Cygnus X-1, one of the strongest sources of X-ray radiation emanating from one of the best black hole candidates in the known universe.

Use this chart to help you find HDE and its invisible black hole. The circle is one degree across, about the amount of sky you'd see in a low power telescope.

I wish I could say it’s visible with the naked eye. I can’t, but a pair of 50mm or larger binoculars on a steady tripod or any small telescope will easily show the star, designated HDE 226868, that feeds the black hole. First spot Eta Cygni a little more than halfway down the Cross, then use the more detailed map to star-trek your way to a pair of stars 1/2 degree (one full moon diameter) to the east-northeast. The southern and brighter member of the pair is the sacrificial lamb.

Cygnus X-1 lies is a binary system 6,100 light years from Earth composed of the blue supergiant star HDE 226868 and an invisible companion with a mass 8.7 times that of the sun. The two revolve around their common center of gravity once every 5.6 days. Astronomers compute an object’s mass in a binary star by studying how the companion’s gravity affects the other star’s orbit. They can also look at the subtle change in the color of a star’s light created by the Doppler Effect as that star approaches and recedes from Earth during its orbital ‘dance’ with a companion.

Based on those methods alone, there’s something very odd about Cygnus X-1. We see a whole lot of tugging on that supergiant but no visible star as the hand. Whatever is doing the pulling is too tiny to see yet extremely massive, a perfect job for a black hole. A black hole is so compact and its gravitational pull so intense, not even light can escape its surface. That’s why they’re black!

Material from the supergiant star HDE 226868 (left), visible in small telescopes, is pulled into a disk by the powerful gravity of a black hole 8.7 times the mass of the sun. The two objects are only about 18 million miles apart or 1/2 the distance Mercury is from the sun. Credit: ESA/Hubble

The fun doesn’t end there. The supergiant emits a powerful wind of hot plasma (electrons and protons) on a much larger scale than our own sun’s solar wind. Back in 1964, scientists launched a sounding rocket high into the atmosphere with a Geiger counter. As the rocket rotated, the counter swept across the sky and picked up a heap of X-ray radiation coming from what was soon called Cygnus X-1, the first X-ray source discovered in the Northern Cross.

Further study revealed that the wind of particles from the supergiant gets siphoned off by the black hole into a rapidly spinning disk. As the material get heated by friction on its way down the hole’s throat, it’s heated to millions of degrees and emits copious amounts of X-rays. A giant star alone isn’t capable of such an outpouring of energy – another reason astronomers attribute all this excitement to a black hole with a diameter of about 15 miles.

Take a look the next time it’s clear at the sinking Cross in the west, and know that if you had X-ray vision, one of the brightest things you’d see would be a star being devoured by a tiny black bit of potent emptiness.

5 Responses

  1. John

    Superb blog post. I teach astronomy to a YMCA summer camp, and love to show them the density of stars in Cygnus compared to other patches of the sky, in order to illustrate the Milky Way. Sadly, here in my town in light-polluted central Connecticut, it’s typically too bright to see the Milky Way itself except on the clearest, darkest nights. We always make a stop at Albireo so I can show the color contrast in the double star, and now I have something else to pull out of my back pocket to share in Cygnus.

    1. astrobob

      Hi John,
      Thank you for your kind words. I also do some astronomy teaching and plan to show the HDE star to my group at the next opportunity. Even if you can’t necessarily see something directly, when you know you’re looking right at it, imagination does the rest.

  2. John

    The kids in my groups tend to be younger, so I’ll probably just mention and point it out in the sky for the class, but after the group leaves, there’s always a few who hang around after to talk. They’re the kids we need to attract to science and astronomy. Will show them now easily, thanks to the guidance you offered in how to find it.

  3. Charalambos

    Dear Bob,
    you mentioned “Material from the supergiant star HDE 226868 (left), visible in small telescopes, is pulled into a disk ”

    I’m not sure if this information is totally true, due to the fact tha you can locate jets through dopler graphs, or watching the area in x-rays or sychrodron..

    I am kindly asking if you have any interesting site or paper to suguest, I would be satisfied.
    Best regards.

    1. astrobob

      Charalambos,
      It’s well-know from studies of this binary that material from the supergiant is being pulled into a rotating accretion disk about its (likely) black hole companion. No one has photographed the disk because the system is too far away to image an object that small. That star itself is easily visible in a small scope. Jets perpendicular to the disk blast a portion of the material away into space. Here are a couple links with more information: http://1.usa.gov/PnAn52 and http://stars.astro.illinois.edu/sow/cygx1.html

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