CYGNUS X-1 = HDE 226868. With a name that evokes some kind of experimental spacecraft, (located in central Cygnus, along the outstretched neck of the celestial Swan) is actually one of the brightest sources of X-rays in the sky (and the first found in Cygnus). It is associated with a magnificent, massive, blue class O (O9.7) supergiant called HDE ("Henry Draper Extension") 226868 that at ninth magnitude (8.83) is unfortunately not visible to the naked eye (but can readily be seen in a small telescope). Blue supergiants do not radiate significant X-rays unless they are in binary systems. Most do it when there are TWO massive stars (as in Eta Carinae) whose winds collide. Here, though, there is clearly but ONE of these huge stars. Yet variations in the spectrum indeed reveal a companion, an invisible one that is oddly also very massive. Welcome to the best celestial example of the stellar "black hole" and to a fine example of a "high mass X-ray binary," or "HMXB."
Cygnus X-1 is the classic black hole binary. The visually bright component, a highly luminious class O supergiant called HDE 226868, is centered in the picture up and to the left of the picture's brightest star, Eta Cygni. The black hole, in tight orbit about the supergiant, is made eveident through X-rays radiated by hot gas flowing to it from the tidally distorted much larger star. The gap in the middle of the image is the result of the absorption of distant starlight by thick clouds of interstellar dust. Were they not there, Cyg X-1 (rather HDE 226868) would be visible to the naked eye. See a full labelled image and then a broader labelled view.
While sounding mysterious, black holes are really among the simpler of astronomical objects (at least until you look at them more closely). White dwarfs are the end points of most stars: dense, dead, ancient nuclear- burning cores of stars that began life with masses from about solar to roughly 8 or 10 solar. Above that limit, the stars blow up, most producing neutron stars (from their collapsed cores) like the compact companion in X Persei (a lesser version of an HMXB). Above perhaps a few tens of solar masses, the collapsing cores go past the neutron star state and are so massive that they collapse forever, creating bodies only 10-100 kilometers across that are so dense that light cannot escape, and thus disappear from view as the fabled black holes. (The "radius"of a black hole is that of the "event horizon," at which the escape velocity roughly hits the speed of light.) Though unlit, a black hole still possesses its gravity and thus acts like any other kind of binary member, shifting its visible companion star back and forth and producing tides within it to the point where the black hole -- even though it is itself invisible -- makes itself quite known. The Cygnus X-1 system is so far away that parallax measures cannot be made. However, the system seems to be a member of the loose Cygnus OB3 association of hot stars that is centered on the open cluster NGC 6871, and whose best distance estimate is around 7000 light years. Dark interstellar dust clouds along the line of sight dim the visible star's light by 3.5 magnitudes (a factor of 25!). Were the dust not there, HDE 226868 would shine at fifth magnitude (5.34) and be readily visible in a dark sky. From the distance, an estimated temperature of 30,000 Kelvin (to allow for a large amount of ultraviolet light), and the visual brightness, we calculate a striking luminosity of 385,000 times that of that of the Sun, from which we derive a radius of 23 times solar and (from stellar structure theory) a mass 40 times that of the Sun (given the uncertainties within 10 solar masses or so). Other ultraviolet estimates could allow a luminosity as high as 450,000 Suns. Over long periods, the star is variable by a few hundredths of a magnitude. The supergiant and black hole orbit each other every 5.60 days. Estimates from the effect of the compact companion on the supergiant lead to a black-hole mass of 20 (give or take 5) times that of the Sun, which in turn leads to an orbital radius of 0.24 Astronomical Units, 52 solar radii, just double the radius of the supergiant. The optically invisible companion is far too massive to be a neutron star. All that is left is to assume a black hole. The pair is so close together that the black hole raises huge tides in the supergiant, to the point that it fills its "Roche Lobe," a teardrop-shaped surface at which the gravity between the two is balanced out. As a result, the supergiant leaks its mass through the teardrop's point toward the black hole. Flowing at a rate of about three millionths of a solar mass per year, the extruded matter falls into a disk that orbits the black hole. Heated through its compression (as well as through magnetic fields) into the millions of degrees, the disk then erratically radiates the X- rays, flipping back and forth between high and low-energy states. Even gamma rays can pour out. The matter in the disk then pours "down the drain" into the tiny black hole (only 100 km across) at its center. Some of the gas, however, is ejected in opposing jets, making the system into a "microquasar" (real "quasars" being similar but supermassive black holes that occupy the cores of distant primitive galaxies). Here is a possible scenario. The original masses within the pair, which was born a mere 3-4 million years ago, must have been in the range of 80-100 and 40-50 solar masses. The larger star lost a huge amount of matter when it evolved into a supergiant (some to the lesser companion, which helped draw them closer). When internal nuclear reactions created an iron core, it then blew up as a "hypernova" (creating a rare gamma ray burst of the kind seen coming from the distant Universe). The core was so massive that it collapsed into a black hole. The once-lesser-mass star now has the greater mass, though it too has lost (and is still losing) matter. Before long it will explode as well, perhaps creating yet another black hole (though maybe a stable neutron star). Gravitational radiation will then someday draw the two together to create a more massive black hole, with yet another gamma ray burst that announces their merger to the rest of the Universe. (Cygnus X-1 appears in Jim Kaler's The Hundred Greatest Stars. Thanks to Reginald Quinto for suggesting it.)
Written by Jim Kaler. Return to STARS.