Cygnus X-1: A Black Hole You Can Find!
September 2009 :
Unusual celestial objects having bizarre or extreme astrophysical properties often make highly desirable observing targets for amateur astronomers wishing to find things outside the scope of the usual, customary star party “menu”. Last February in this newsletter, I wrote a piece detailing how to locate and observe the most easily seen white dwarf star, Omicron 2b Eridani. Sure, that’s a star having light we can actually see, quite unlike a black hole, which emits no light in the visible band of the electromagnetic (EM) spectrum and is therefore invisible. So how do you find something that even the Hubble can’t image, in visible light? The answer is surprisingly simple and is my topic for this month.
The conspicuous Northern Cross asterism forming the main outline of Cygnus is the key to easily isolating a specific part of the sky from which to begin our star-hop. First, note the central star of the cross, magnitude 2.2 Gamma Cygni, also called Sadr. Second, pick out magnitude 3.0 Beta Cygni, the well-known Albireo. This stunning, color-contrasting double (wide and easy – a favorite for many) marks the head or “beak” of the swan and is at the SW end of the Northern Cross, marking the bottom or “foot” of the cross; this point is nearly on the border with the constellation Vulpecula to the south. Third, locate the brightest star roughly mid-way along the line joining Albireo and Sadr – it actually lies a bit off the middle of this imaginary line, closer to Sadr than Albireo. This is Eta Cygni, magnitude 3.9, which means you need a reasonably dark and clear sky to make it out with the naked eye. That’s the easy part of the star-hop, because Cygnus X-1 lies only about 0.5 degree to the ENE of Eta, albeit invisible. The black hole has a binary companion, though: a magnitude 9.0 blue supergiant star known as HDE 226868 located at R.A. 19h 58m 22s, Dec. +35° 12’. Isolating this star will be tricky and is difficult to describe, but I’ll do my best for you readers.
A finder chart with sufficient limiting magnitude and detail would be ideal; such a chart would be available perhaps somewhere on the web or on certain sky atlas/planetarium software that some of you might have. Volume two of Burnham’s Celestial Handbook devotes pages 793-795 to Cygnus X-1. The photograph on page 795 clearly identifies HDE 226868 with a small ring encircling the star, a great help due to the fairly close proximity of another star of similar color and brightness. I’m used to picking out this star, but patterns and shapes noted within a starfield by one observer may not at all appear the same to another. Plus, it’s impossible for me to know the orientation with respect to Eta Cygni by which your scope would show you this starfield, as opposed to whatever layout I might see. (There are a few good reasons why this potential difference might be so; I’ll forego all the details here.)
Roughly 0.75 degree to the ENE of Eta Cygni lies a 5-star asterism that resembles a long, slender parallelogram having its long sides slightly tapered, with two stars at one end and three at the opposite end. Four of these stars are of similar brightnesses and, in fact, will stand out as among the brightest stars within this specific starfield. The side of this asterism facing Eta Cygni has a separate little form rising (towards Eta) that, to me, looks like a little soup bowl seen from the side. This “bowl” has its bottom touching the side of the not-quite parallelogram and is tilted markedly with respect to the other asterism – the bowl isn’t setting evenly or upright on its base. Five or six stars mark this bowl-shaped form, but not evenly – one section is not quite “filled-in”, and your ability to distinguish one end of the top line (widest part) from the other will depend on seeing what I mean by the incompleteness of the bowl’s outline. The side that looks “fuller”, with more bright stars outlining its shape, is key. Still with me? We’re almost there.
Now try to picture the top edge of this little bowl, which is almost directly facing Eta Cygni only 0.5 degree away. (You might notice a small, square-shaped asterism mid-way between the top line of the bowl and Eta Cygni; three of its corners have stars of similar magnitudes but one – near Eta – is much brighter.) Concentrate on an imaginary line which would form the top edge of the bowl, bearing in mind that I’m describing the bowl as seen from its side. You’ll see two stars fairly close to each other near the mid-point on this line. This pair will look like a wide double star, with similar white colors but slightly uneven magnitudes. A line joining these stars will be slightly off-parallel to the line forming the bowl’s top edge, but not by much. There won’t be any other conspicuous stars along this line other than the two marking each end of the very top edge of the bowl. If you can positively identify these two stars you now only have to distinguish one to know you’re seeing HDE 226868!
Note the side of the bowl that is more completely outlined by stars than its opposite side. (This should be easily done.) Go to that side’s upper end and use the imaginary line across the top of the bowl—of the pair of stars near the middle, the first one you come to working from this starting point along the top line will be the correct star. It’s the brighter of the pair and also is the one farthest from the exact mid-point of the bowl’s top line. (Whew!)
Cygnus X-1 lies perhaps 18 to 19 million miles from the visible companion star, just 0.2 AU away. Distance to the system may be roughly 6,000 light years, meaning the magnitude 9 visible star is a real powerhouse of luminosity to be seen this easily from such a great distance. How do we know a black hole is the likely object so close to HDE 226868? The visible star itself wouldn’t generate X-rays to the high intensity detected at this precise point on the sky; a massive, compact, invisible body therefore must exist very close to the star, an object responsible for such concentrated X-ray emission. The hole is probably not “stripping” matter away from the visible star; instead, the star (which is slightly variable) is expelling its own mass gradually through its solar wind. Some of this gas would naturally be captured in an accretion disk orbiting the hole. Motion of such matter as it whirls around the event horizon at a fantastic orbital speed created tremendous friction among constituent particles in the trapped gas, raising its heat to the multiple millions of degrees by which X-rays are generated so copiously.
Cygnus X-1 is probably the very best of all known “candidates” for being an actual black hole. Readers interested in learning more about its discovery, history of observation, and astrophysical research can easily find such information among numerous astronomy sources. Best of luck to those who try my suggested star-hop. I think the effort is very worthwhile. (Don’t forget Burnham’s Celestial Handbook—the “finder photograph” can be a huge help.)
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