How Was Cygnus X-1, the First Black Hole, Discovered?
Cygnus X-1, companion object to the blue super giant star HDE 226868, is located in the constellation Cygnus at 19 hours 58 minutes 21.9 seconds Right Ascension and 35 degrees 12’ 9” Declination. Not only is it a black hole, but the first one to be discovered. What exactly is this object, how was it discovered, and how do we know it is a black hole?
Black holes were first mentioned in 1783 when John Michell, in a letter to the Royal Society, talked about a star whose gravity was so great that light did not escape its surface. In 1796 Laplace mentioned them in one of his books, with calculations as to dimensions and properties. Throughout the intervening years they were called frozen stars, dark stars, collapsed stars but the term black hole was not used until 1967 by John Wheeler from Columbia University in New York City (Finkel 100).
Discovery of Cygnus X-1
Astronomers at the U.S. Naval Research Lab discovered Cygnus X-1 in 1964. It was further researched in the 1970’s when the Uhuru X-Ray satellite was launched and examined over 200 X-Ray sources with over half of those in our own Milky Way. It spotted several different objects including gas clouds, white dwarfs, and binary systems, Both noted that X-1 object emitted X-Rays, but when people went to observe it they discovered that it was not visible on any plane of the EM spectrum save for X-Rays. On top of that, the X-Rays flickered in intensity every millisecond. They looked towards the closest object, HDE 226868, and noted that it had an orbit which would indicate that it was a part of a binary system. However, no companion star was located within proximity. In order for HDE to remain in its orbit, its companion star needed a mass larger than a white dwarf or a neutron star. And that flickering could only arise from a small object that could undergo such rapid changes. Perplexed, scientists looked towards their previous observations and theories to try to determine what this object was. They were shocked when they found their solution in a theory that many regarded as a mere mathematical fancy (Shipman 97-8).
Einstein and Schwarzchild
The first mention of a black-hole-like object was in the late 1700s when John Mchill and Pierre-Simon Laplace (independent of each other) talk about dark stars, whose gravity would be so large as to prevent any light from leaving their surfaces. In 1916 Einstein published his General Theory of Relativity, and physics was never the same. It described the universe as a space-time continuum and that gravity causes bends in it. The same year the theory was published, Karl Schwarzschild put Einstein’s theory to the test. He attempted to find the gravitational effects on stars. More specifically, he tested the curvature of space-time inside a star. This became known as a singularity, or an area of infinite density and gravitational pull. Einstein himself felt that this was merely a mathematical possibility, but nothing more. It took more than 50 years until it was regarded not as science fiction but as science fact.
Components of a Black Hole
Black holes consist of many parts. For one, you must imagine space as a fabric, with the black hole resting on top of it. This causes space-time to dip, or bend, into itself. This dip is similar to a funnel in a vortex. The point in this bend where nothing, not even light, can escape it is called the event horizon. The object causing this, the black hole, is known as the singularity. The matter surrounding the black hole forms an accretion disk. The black hole itself is spinning rather rapidly, which causes the material around it to achieve high velocities. When matter reaches these velocities, they can become X-rays, thus explaining how the X-rays come from an object which takes all and gives nothing.
Now, the gravity of a black hole does cause matter to fall into it but black holes don't suck, contrary to popular belief. But that gravity does stretch space-time. In fact, the closer you get to the black hole the slower time goes by. Therefore, if one could maneuver the environment around a black hole, it could be a type of time machine. Also, the gravity of a black hole does not change how things orbit around it. If the sun were condensed into a black hole (which it can't, but go along with it for the sake of argument) our orbit would not change at all. The gravity isn't the big deal with black holes, its the event horizon that ends up being the difference maker (Finkel 102).
Interestingly, black holes do radiate something called Hawking radiation. Virtual particles form in pairs near the event horizon and if one of them gets sucked in then the companion leaves. Through conservation of energy, this radiation will eventually cause the black hole to evaporate, but a possibility of a firewall could cause complications that scientists are still exploring (Ibid).
Birth of a Black Hole
How could such a fantastic object form? The only means that can cause this come from a supernova, or a highly massive explosion as a result of star death. The supernova itself has many possible origins. One such possibility is from a super giant star exploding. This explosion is a result of hydrostatic equilibrium, where the pressure of the star and the force of gravity pushing down on the star cancel each other out, is off-balanced. In this case, pressure cannot compete with the gravity of the massive object, and all that matter is condensed to a point of degeneracy, where no more compression can occur, thus causing a supernova.
Another possibility is when two neutron stars collide with each other. These stars, which as their name implies are made of neutrons, are super dense; 1 spoonful of neutron star material weighs 1000 tons! When two neutron stars orbit each other, they can fall into a tighter and tighter orbit until they collide at high speeds.
Ways to Detect Black Holes
Now, the careful observer will note that if nothing can escape a black hole’s gravitational pull, then how can we actually prove their existence becomes difficult. X-rays, as previously mentioned, is one mode of detection, but others exist. Observing a star's motion, such as HDE 226868, can shed clues to an invisible gravity object. In addition, when black holes suck up matter, the magnetic fields can cause matter to jet out at the speed of light, similar to a pulsar. However, unlike pulsars, these jets are very quick and sporadic, not periodic.
Now that the nature of the black hole is understood, Cygnus X-1 will be easier to comprehend. It and its companion orbit each other every 5.6 days. Cygnus is 6,070 light years away from us according to a trig measurement by the Very Long Baseline Array team led by Mark Reid. It is also about 14.8 solar masses according to a study by Jerome A. Orosz (from the San Diego State University) after examining over 20 years of x-ray and visible light. Finally, it also has a diameter about 20-40 miles and spins at a rate of 800 hz as reported by Lyun Gou (from Harvard) after taking the previous measurements of the object and working the mathematics in the physics. All of these facts are in accordance for what a black hole would be if located within proximity of HDE 226868. Based on the speed X-1 moves through space, it was not generated by a supernova for otherwise it would travel at a faster velocity. Cygnus siphons material from its companion, forcing it into an egg shape with one end tailing into the black hole. Material has been seen entering Cygnus but eventually it red shifts significantly then vanishes into the singularity.
Black holes continue to mystify scientists. What is exactly going on at the point of the singularity? Do black holes have an end to them, and if so does the matter it intakes exit there (this is called a white hole), or is there actually no end to a black hole? What will be their role in an accelerating expanding universe? As physics tackles these mysteries, it is likely that black holes will become even more mysterious as we investigate them further.
“Black Holes and Quasars.” Curious About Astronomy? May 10, 2008. Web.
“Cygnus X-1 Fact Sheet.” Black Hole Encyclopedia. May 10, 2008. Web.
Finkel, Michael. "Star-Eater." National Geographic Mar. 2014: 100, 102. Print.
Kruesi, Liz. "How We Know Black Holes Exist." Astronomy Apr. 2012: 24, 26. Print.
---. "Researchers Learn Details of Cygnus X-1's Black Hole." Astronomy Apr. 2012: 17. Print.
Shipman, Harry L. Black Holes, Quasars, and the Universe. Boston: Houghton Mifflin, 1980. Print. 97-8.
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© 2011 Leonard Kelley