Wednesday, May 2, 2012
How to make a black hole
Black holes are, well, odd. They are the place where common sense goes to die. Physics takes such a beating when it comes to black holes that when it comes to the central point of a black hole, its singularity, physicists are often left smiling and nodding as their equations are chewed up and turned into useless infinities.
But don't let this make you think that we know nothing about these cosmic anomalies. We have gone from the days of black holes being nothing more than mathematical oddities to one of the most intriguing areas of studies in modern cosmology.
The idea of an object so dense as to have a mass greater than the escape velocity of light was first proposed in 1783 by John Michell, but it wasn't until 1916 that mathematical evidence for such an object was proposed. Using Einstein's Theory of General Relativity (which was published a year earlier), Karl Schwarzchild was able to show that black holes were, at the very least, mathematically consistent. But for the longest time, the process that led to stellar mass black holes was a mystery. It was believed to involve dying stars, but little else was known.
With an increasing array of tools that are constantly becoming more sensitive and precise, physicists have been able to assemble a basic recipe for a stellar mass black hole. Before I go on, I should clarify what I mean by a stellar mass black hole. There are multiple kinds of black holes, each sharing the same physics but with a difference of scale. From the supermassive black holes lurking in the cores if nearly, if not all major galaxies to the hypothetical primordial black holes left over from the big bang, there is more variety than the uninitiated would expect. The best studied form is the stellar black hole. These are all made through a similar process in the core of dying stars.
But not any star can form a black hole. In fact, the theoretical lower limit of a star large enough to produce one of these stellar black holes is around 20 times the mass of our own sun. Anything smaller just doesn't have the mass required to form a black hole. They will either cool down into a white dwarf like our sun or form another bizarre stellar remnant such as a neutron star. However, there are a lot of stars in the universe. In our galaxy, the Milky Way, there is predicted to be around 100 million stellar black holes.
Now that we have a star of the right size, how does it become a black hole? The answer lies in the core of a star. Stars work by fusing matter together using their enormous gravity. Normally, the nuclei of atoms are kept apart thanks to the electromagnetic force. But with enough energy, the electromagnetic force can be overcome allowing things such as the strong nuclear force to take over. When this happens, the atomic nuclei are forced together into a new, more massive nuclei. Thanks to a little equation known as E=MC2, some of that mass is converted to energy. This left over energy is what powers a star.
Stars primarily are composed of hydrogen and, as such, fuse more hydrogen into helium than anything else. Our sun alone fuses 700 million tons of hydrogen into 695 million tons of helium every second. The remaining 5 million tons is the left over energy that powers our sun and helps to keep us from, you know, dying. But not all stars fuse at the same rate, it all depends on mass. A star twice as massive as our sun will fuse hydrogen at ten times the rate of our sun, while a star twenty times will fuse 36,000 times as quickly. The larger the star, the shorter the life time.
But a star never uses up all its hydrogen. It is only in its core that the environment is right for nuclear fusion. As the star's life continues, eventually the available hydrogen in its core begins to dwindle. Soon helium has to be fused into carbon, carbon into neon, and so on. The denser material settling at the core with the easy to fuse hydrogen being pushed further out. Each step takes less time then the last with every element forming a ring around its denser counterpart. Eventually, if a star is massive enough, it will fuse atomic nuclei all the way up to iron, but this is as far as it can go.
Unfortunately for such massive stars, iron resists fusion and outside extreme conditions such as supernovae and hypernovae, iron nuclei cannot be fused into heavier elements. For such massive stars, this is the beginning of the end. For lighter stars that cannot fuse up to iron, the process is quite similar, the only difference is that the masses required to fuse up to iron are lacking and a lighter element will become the dense core that cools or collapses into one of a variety of stellar remnants.
The iron core, as I mentioned, cannot be fused inside a star. Stars are kept from collapsing by the energy released through the fusion of its mass. Since the iron cannot fuse, the star begins to lose its internal support, placing even greater force on the core. This force, coupled with the cores already massive size, begins to cause the electrons that have been stripped from their atoms due to the extreme temperature and forces to be crammed together when the iron nuclei. Thanks to the Pauli exclusion principle, these particles cannot inhabit the same locations in time-space and begin to form a counter pressure known as degeneracy pressure. This pressure helps to keep the star together, but only for a time.
Once this degenerate matter core reaches a mass of at least 1.4 solar masses, it can no longer hold back the massive pressures and the pull of gravity. The core then collapses in an extraordinary way. In a thousandth of a second this core collapses at speeds of around 45,000 miles a second, shrinking a core thousands of miles across to just a few miles across. The resulting vacuum causes the remaining shell of the star to collapse, adding further pressure, before rebounding. The star would continue to collapse, rebound and shrink again if not for the affects of a ghost like particle known as the neutrino.
Neutrinos are very weakly interacting particles. About 65 billion of these particles are passing through every square centimeter of the Earth every second and nearly all of these particles pass through it as if there was nothing there. But in the conditions found inside a dying star, the 10 to the 58th power neutrinos released in a ten second burst from the core are enough to shred the outer star leading to a supernova. It is only here, in the final moments of a dying star, that the core of a massive enough star collapses into a black hole.
First the degenerate core will see such extreme pressure and gravity to force the electrons and protons together into neutrons. This new material, neutronium, is the component of neutron stars. If there is enough mass, the core will continue to collapse all the way into a black hole. The resulting black hole will then begin to feed on the remnants of its parent star with an accretion disk forming around it. Some of this material may find a stable orbit around the black hole while the rest is heated up through friction to the point of emitting light across the electromagnetic spectrum.
With thanks to Phil Plait, Ph.D. and his book Death From The Skies.