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.
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.