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Focus On Cygnus X-1

A Black Hole Lurks Here

For any telescope

Cygnus X-1 was the first and brightest source of x-rays discovered in Cygnus. X-rays are photons (particles of light) that carry more energy than the light we see. Have you ever wondered what the physical difference is between red and blue? Blue light photons carry more energy than red ones. Your eye, in addition to detecting a photon, takes a measure of the energy it carries. This is how we distinguish color. But our eyes aren't sensitive to all colors -- only a small range. We can't see x-rays, for instance.

Stars emit x-ray photons normally, but they are typically much less bright in x-rays than they are in the visible part of the spectrum. So what is the source of this bright x-ray emission, which appears to be coming from an ordinary star in Cygnus? Adding to the intrigue, the x-rays have also been found to flicker many times per second. In astronomy, the cardinal rule is that the time it takes something to change in brightness is inextricably tied to the size of the object. Only very small objects can change brightness that quickly. In fact, the object in question must be only a few hundred km across -- far smaller than any star and even much smaller than the earth.

The star we see at this location is spectroscopic X-ray binary system known as V1357 Cyg. Spectroscopic binary systems are those for which the stars lie so close together from our vantage point that we cannot separate them even with the largest telescope. Spectroscopy is the technique where we use a prism or grating to separate the light from a star into its component colors, like a rainbow. When you do this for a star you see many dark lines superimposed on the rainbow. The positions of these lines are very sensitive to how fast the gas in the star is moving toward or away from us. If there are two stars whose light is mixed together we usually see dark lines from both stars. As they orbit, one star will typically be seen moving toward us with the other moving away. All of the lines of one star will shift together in one direction, while those from the second will shift the opposite way. In time the lines merge and reverse positions. From watching these lines shift we can deduce lower limits to the masses of both stars.

So what do we see in the spectrum of V1357 Cyg? One set of lines that shift back and forth every 5 days. When we analyze the motion of this single set of lines we can determine that the object that is orbiting V1357 Cyg is very massive. Massive stars are by necessity bright. So where are the spectral lines from this other star?

All these observations together lead us to a picture of a star that is orbiting an unseen, massive, yet very tiny object. At the moment, we have only one reasonable explanation for such an object -- a black hole. A black hole is the theoretical result of the collapse of a massive star during a supernova explosion. The theory predicts an object with mass, but no size. It's as if a point in space had the mass of a star. It's this lack of size that makes a black hole so interesting. As you get closer to any object, the gravitational attraction you feel increases. With a typical star you can only get so close. But for a black hole, you could get very, very close and if you did you would experience an enormous gravitational pull. As you get closer you have to go faster and faster to get away from that pull. Eventually, you reach the point where even the speed of light isn't fast enough to get away. We call this point the event horizon, and when we speak of the size of a black hole we are actually referring to this point surrounding it where nothing, not even light, can escape.

It is normal for matter to pass from one star to another in a close binary system such as that of V1357 Cyg. If matter passes from the visible star to the unseen compact object, it will form a disk of material that will slowly spiral in.  As it spirals in it orbits faster and faster. Matter even a little bit closer to the object will move faster than that a bit farther out. The resulting friction will release light energy, and the faster they go around the more energy the light will have. This is where we think the x-rays are coming from.

The physics describing a black hole appears sound, but any time we use physics to describe something we need to test the theory carefully against reality before we can have a great deal of confidence in it. Since we can't study black holes close up, this poses a problem.

Do black holes even exist? The existence of Cygnus X-1 and other binary systems like it appear to argue that they do. One thing is clear; massive, compact objects exist. Whether or not these are black holes as we theorize them to be today may be nothing more than a matter of semantics.

So How Do I Find It?
The visible star, V1357 Cyg, is fairly easy to spot because it lies near the bright star Eta Cyg. Eta Cyg is the star in the middle of the long neck of Cygnus, the swan. Look for a pair of 9th magnitude stars about 1/2o to the east of Eta Cyg. Cygnus X-1 is the brighter of the two.


Eta Cyg is the bright star on the line half way between Sadr and Albireo. North is up and east is to the left.

HD 226868 / V1357 Cyg, the visible star
Right Ascension Declination Magnitude Con Distance From Earth
19h58m21.7s 35o12'06" 8.7-8.9 Cyg 5,600 lys

Cygnus X-1 lies about 1/2o to the east of Eta Cyg (bright star at left). This is the view in a six-inch at 50x. North is down and east is to the right.