Some may spend the day today (or at least the small hours of the morning) snatching up all those items they think they need, fighting (literally) for the “best deal” while Astronomers and Physicists at NASA will spend the day reaching out to these very people and everyone else, teaching and informing them about Black Holes hence, today is Black Hole Friday!
Well, just what is a Black Hole? It isn’t a physical object, but the extreme environment created in the aftermath of the cataclysmic end to a massive star, a Type II supernova. The concept of a Black Hole, an entity whose gravity is so intense that not even light can escape its clutches was first introduced by Albert Einstein in his General Theory of Relativity. A massive star, usually in excess of 10x the mass of the sun, ends its life as a Type II supernova. The “remnant” can form a rapidly rotating neutron star, known as a pulsar or, if the progenitor star was massive enough, a Black Hole. The first modern solution to General Relativity that would characterize a black hole was found by Karl Schwarzschild in 1916. General Relativity is Einstein’s general theory of gravity just as the Universal Law of Gravitation was Newton’s. Instead of thinking of gravity as a mutually attractive force, Einstein thought of gravity as the behavior of objects moving in accordance with the curvature of space in the vicinity of a massive object or, more precisely, the curvature of space-time, since space and time are inexorably linked. The greater the mass of an object, the greater the warping of space. The effects, described by Newton’s law gravitation still hold, that with increasing mass there would be an increased force of attraction, that it would be increasingly difficult to escape the gravitational “pull” of the object, requiring more and more energy. According to Einstein’s Special Theory of Relativity, there is a universal speed limit above which nothing can travel, the speed of light. The mass associated with a Black Hole is so great that not even light can escape its gravitational clutches. Since we can’t “see” the black hole but only its effects, we name it as such.
Consider a massive star, say 10x the mass of the sun. Such a star’s lifetime is measured in tens of millions of years while the sun’s is measured in tens of billions of years. Throughout every star’s life, the core of the star, a giant nuclear fusion reactor, is producing energy by transmuting four hydrogen nuclei (protons) into a single helium nucleus with the helium “building up” in the core’s interior. When the core hydrogen has been depleted to about 12% of its original value, the balance between gravity and outward gas pressure wanes and the core begins to collapse. During the star’s productive life, the outward pressure produced by the tremendous energy released during the nuclear fusion reactions was balanced by the tremendous mass of the star sitting on top of the core. As the core collapses, it heats up to a point where the inert helium in the inner core ignites, beginning the star’s helium burning phase. The helium fusion will produce carbon and oxygen and, like the helium before, the carbon and oxygen will build up in the core’s interior. For the sun, the process ends here but for stars with masses at least 8x the mass of the sun this process will continue up through silicon, a burning cycle that will produce a nickel-iron core. Because of iron’s endothermic properties, this is the nuclear dead end for all stars, regardless of their mass. When the silicon fusion cycle ends, a process that lasts, at most, 24 hours, a list-ditch attempt by nature to use the inert nickel-iron in the core as a new source of energy has the opposite effect as before. The iron nucleus is so tightly bound that any attempt to combine an iron nucleus with a helium nucleus (as this has been what’s happening) takes energy out of the system, having a chilling effect on the core. Within seconds, the core collapses in upon itself, rebounding on the materially degenerate inner core of nickel-iron and, in the process, releasing all the pent-up gravitational potential energy in a cataclysmic shock wave that propagates outward, obliterating the star. If a remnant remains, it will become a rotating neutron star, often referred to as a Pulsar, or a black hole where the core’s collapse continues resulting in a physical conundrum, a “singularity” where infinities and zeros abound, infinities that involve density and zeros that involve radius. If a neutron star remains, then its density would be comparable to that of an atomic nucleus, if a black hole is all that remains then, theoretically, any mass confined to a zero radius would result in an infinite density, something that comes out in the mathematics but, does it exist as such in reality, we may never know.
The evidence of black holes is observed everywhere, from the 4 million-solar-mass black hole at the center of our own Milky Way galaxy to Quasars so distant that their light left them when the universe was only 800 million years old or 13 billion years ago. That we can observe these titans at this enormous distance is a testament to their power, a supermassive black hole at their center.
Quasars are a subclass of objects known as AGNs or Active Galactic Nuclei. Active galaxies derive their power from the dark heart of a black hole at their centers. Quasars are observed across the electromagnetic spectrum from radio waves to gamma rays and are so luminous, across all electromagnetic wavebands, that they can, quite literally, be seen across the universe. When first discovered in the 1960s, they were thought to be much closer and, hence, less luminous than we know them to be today; it was inconceivable when first discovered that they would be so luminous. Were they close by and not so luminous or at cosmologically significant distances and super luminous? That was the question. Yes, we could measure their brightness but without knowing their intrinsic luminosity we would not be able to determine their distance. Not until it was determined what was driving the enormous energy output would we be able to determine their intrinsic luminosity and hence, their distance. So, what is it; how does the black hole at their center cause them to be so luminous? Astronomers have observed evidence of a supermassive black hole at the centers of most galaxies; that is more or less, a given. Some black holes are active and some are dormant. When active, or feeding, they become very bright. In fact, their is evidence that the black hole at the center of our own galaxy is becoming active. Known as Sgr-A* (Sagittarius A), this object has exhibited flaring recently, a sign that it is becoming active and has drawn the attention of many of the worlds’ astronomers and astrophysicists. Through extraordinary observations by astrophysicist Andrea Ghez with the twin 10-meter telescopes of the William M. Keck observatory, orbital velocities in excess of 5,000 km/sec have been measured for the stars whose orbits carry them closest to Sgr-A*. Black holes will only become active if a hapless star, gas or any object drifts too close or crosses the “Event Horizon” or Schwarzchihld radius, the point of no return, inside of which not even light can escape and a distance dependent on the effective mass of the black hole. If that happens, the enormous tidal gradient of the black hole will shred the star, even if it doesn’t cross the event horizon. The tidal gradient, the change in gravity as a function of distance, of the black hole is so enormous that the star will become shredded or spaghettified and its gas will fall into orbit around the black hole as an accretion disk. Think of this happening on a galactic scale, at the center of any galaxy and you have a good idea of what drives a Quasar.
As the gas spirals towards the black hole, with its enormous tidal gradient, it heats up due to tidal friction, becoming an ionized plasma, a stream of super hot electrons and protons. At first, in the outer regions of the accretion disk, the plasma is not all that hot and is emitting visible or ultraviolet light but rapidly heats up as it spirals closer and closer to the event horizon. At the event horizon, at the moment it is drawn into the black hole and gone forever, its temperature has increased to where it is measured in millions of degrees and is emitting x-rays. This explains why quasars exhibit a luminous continuum across the entire electromagnetic spectrum. The definition of electric current is movement of electric charge. The rotating ionized plasma that is the accretion disk is such with its stream of electrons and protons circulating around the black hole; it has created an enormous electric current at the event horizon. This current has associated with it a powerful magnetic field where the free electrons spiral along the magnetic field lines thus created, producing synchrotron radiation.
Many science fiction franchises such as Star Trek (Star Ship Enterprise) and Star Wars (The Millennium Falcon) build much of their hypothetical technology around certain aspects of General and Special Relativity and notions of warped space and wormholes. That Black Holes are a rip in space-time conjures up the question, well if it is a rip, perhaps we can manufacture our own, controlled rip in space-time, travel through it to distant points in the universe or perhaps backwards or forward in time without ever violating any laws of physics. Such fictional adventures are fanciful, yet there is a physical basis for these ideas. That time is relative and that black holes rip space-time, it is theoretically possible to travel “through” space to points many light years distant without ever violating the universal speed limit. Think of space as a curved surface just as you would the earth; it would be much faster to travel to say China from any point on the European continent if you could travel through the earth instead of following the curved path along its surface. This idea, known as a Schwarzschild Wormhole or Einstein-Rosen bridge (named a wormhole by Theoretical Physicist John Archibald Wheeler in 1957) was first postulated by Karl Schwartzchild, a contemporary of Einstein, in 1916. It goes without saying that we could never travel through a black hole given what happens to a star that drifts too close. These kinds of technologies, if not centuries into the future, may forever remain in our hearts and imaginations, as there are tremendous socio-political challenges that would need to be overcome before we could mount such historic adventures to the stars; we would first have to learn how to get along with each other.
The future is ours to imagine and realize; it is as limitless as that ever receding event horizon. It is a fantastic time to be alive, to be witness to these discoveries and triumphs of the human mind and spirit. Let us never stop looking up, to boldly go where no one has gone before, if not in body then in imagination!
“It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we’ve ever known.” Carl Sagan, Pale Blue Dot