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Big Bertha Thing White Dwarfs, Neutron Stars, And Black Holes
White Dwarfs, Neutron Stars, And Black Holes v1.0 01 sep 99 greg goebel (gvgoebel@yahoo.com) public domainWhile stars seem unchanging in comparison with a person's lifetime, theyare in fact evolving objects that are born, mature, age, and die.After death, they leave behind stellar "fossils" as gravestones of theirexistence. The most evident of these fossils, the small stars known aswhitedwarfs, have been known for over a century. In recent decades, however,research has shown that such fossils can be more remarkable objects, knownasneutron stars, or possibly can even be "singularities", collapsing foreverand folding space around themselves to form "black holes".This document provides a survey of white dwarfs, neutron stars, and blackholes. Contents List THE DISCOVERY OF WHITE DWARFS WHITE DWARFS AND ELECTRON DEGENERACY THE STRUCTURE AND EVOLUTION OF WHITE DWARFS WHITE DWARFS AND THE AGE OF THE GALAXY BEYOND WHITE DWARFS? NEUTRON STARS DISCOVERED CHARACTERISTICS OF NEUTRON STARS MILLISECOND PULSARS AND OTHER UNUSUAL NEUTRON STARS BLACK HOLES DISCOVERED? MINIHOLES COMMENTS, SOURCES, AND REVISION HISTORY 1. THE DISCOVERY OF WHITE DWARFS Between the years 1834 and 1844, the German astronomer Friedrich W. Besselperformed a series of careful observations of Sirius, the brightest star inour sky. Sirius, sometimes called the Dog Star, is about twice as massiveasour own Sun, 25 times brighter, and is about nine light years away in theconstellation Canis Major.Bessel's careful observations revealed a wobble in the motion of Siriusacross the sky, indicating the presence of a hidden companion. However,nobody was able to locate the hidden companion until 1862, when thetelescopemaker Alvan Graham Clark spotted it while he was testing out a new largerefracting telescope. The companion became known as Sirius B, or just thePup, while the Dog Star itself became technically known as Sirius A.Sirius B proved to be a very puzzling object.The temperature of a glowing body approximates a curve known as a "blackbodyspectrum". The peak of the black body curve gives the temperature of thebody, and the hotter the body, the higher the frequency of the peak of thecurve and the more intense the radiation. Observations of the spectrum ofSirius B indicated that it was very hot, with a temperature of about 30,000degrees Kelvin (K). This high temperature meant that Sirius B was radiatinga great deal of energy per unit of surface area.However, Sirius B was about 10,000 times fainter than Sirius A. Since it wasvery bright per unit of surface area, the Pup had to be much smaller thanSirius A, with roughly the diameter of the Earth.The faintness, of course, was one of the reasons that Sirius B took decadesto find, even though astronomers knew roughly where to look. Sirius A wassomuch brighter than Sirius B that looking for the Pup was like looking for alit match next to a searchlight beam. In fact, one of the main reasons theSirius B was discovered in 1862 was because it had moved in its mutual orbitwith Sirius A so that its angular separation as seen from Earth was overthree times greater than it had been when the search began in 1844.The mass of stars in a binary system can be easily determined. The orbitalvelocity of the visible companion can be determined by the Doppler shift ofits light, and given the orbital velocity and the period of the orbit, thesize of the orbit can be determined. A simple calculation of thegravitational forces involved gives the masses of the two stars.Analysis of the orbit of the Sirius star system showed that the mass of thePup was almost the same as that of our oun Sun. This implied that Sirius Bwas thousands of times more dense than lead. As more white dwarfs werefound, astronomers began to discover that although the Pup might be bizarre,it was hardly unique. White dwarfs are common in our Galaxy. 2. WHITE DWARFS AND ELECTRON DEGENERACY In the 1920s, led by the brilliant Sir Arthur Stanley Eddington,astronomers began to understand the processes of stellar evolution.Eddington's landmark 1926 book on the interiors of stars laid down basicconcepts that evolved into our current understanding of stars.Eddington suggested that the high densities of white dwarfs were due to thecomplete ionization of the atoms in their interiors. With all the electronsstripped from all the nuclei, the nuclei could pack much more closelytogether, resulting in the extraordinary densities observed. However, theenergetics involved in this process were puzzling and contradictory, atleastby the rules of the physics Eddington had available to him.Fortunately, another branch of physics that was evolving in parallel,quantummechanics, came to the rescue. In July 1930, the 19 year old Indianastrophysicist Subraymanyan Chandrasekhar was on a sea voyage from Madras,India, to Southampton, England, and tinkered with physics to hold offboredom. Following work done by astrophysicist Ralph J. Fowler in 1926,Chandrasekhar applied quantum mechanics to the interior of a white dwarfstarand determined how it could have such enormous densities.While a star is performing fusion reactions in its core, the outwardpressureof the thermal motion of the particles in the star keeps it from collapse.When the star is depleted of materials that can support fusion reactions, itcollapses.In the case of big stars, this collapse leads to a catastrophic supernovaexplosion, while the collapse of smaller stars is much less violent. Ineither case, the star falls in on itself until halted by some obstacle.As the exhausted star can no longer produce fusion reactions, the onlyobstacle to collapse is "quantum-mechanical electron degeneracy". This isdue to the Pauli exclusion principle, a rule of quantum mechanics thatdictates that no two electrons in the same system can have the same energylevel. As the star shrinks into itself, the electrons arrange themselves ina fully occupied range of base level energy states that can accommodate nomore electrons. This creates an "electron degeneracy pressure", completelyindependent of the electrical repulsion between electrons, that resistsfurther contraction.Once the white dwarf is stabilized by electron degeneracy into a "fullydegenerate" state, it can no longer contract. As it cannot sustain fusionreactions, the white dwarf's energy is only due to the gravitationalcollapsethat formed it. Chandrasekhar's insight into degeneracy pressure finallyexplained how white dwarfs could exist. He would later win the Nobel Prizefor his work. 3. THE STRUCTURE AND EVOLUTION OF WHITE DWARFS Chandrasekhar's theoretical studies led to a better understanding of someof the characteristics of white dwarfs.The degenerate free electrons that permeate the white dwarf make the objectan excellent thermal conductor, so the white dwarf is almost "isothermal" --that is, its temperature almost uniform throughout its entire volume. Thebare nuclei in this sea of electrons act as a close approximation of anidealgas, providing a deep reservoir of kinetic energy in their random motions.The material on the surface of the white dwarf is not degenerate. Althoughthis layer is only about 50 kilometers (30 miles) thick and only constitutesabout 0.01% of the mass of the white dwarf, it nevertheless acts as aneffective insulating layer. While the temperature at the bottom of thesurface layer is about 10 million K, it is only about 10,000 K at thesurface, with the energy flow throttled by the diffusion of radiationthroughthe surface layer, and the vertical flow of heated material by convectionthrough that layer.A white dwarf star, then, has a large supply of internal energy and aninsulating surface layer to keep the energy from radiating away rapidly.Theresult is that white dwarfs cool off very slowly through most of theirlifetime.When the 5.1 meter (200 inch) Hale telescope on Palomar Mountain,California, went into operation in 1948, astronomers were finally able toperform reasonable spectroscopic observations of white dwarfs. The resultwas another surprise: 80 percent of them showed an absorption spectrum ofpure hydrogen, while most of the rest showed an absorption spectrum of purehelium.The white dwarfs exhibiting pure hydrogen absorption spectra were designatedtype DA, while those exhibiting pure helium absorption spectra weredesignated type DB. In both cases, the surface layer was homogeneous to 1part in 100,000. A small remainder had more complicated spectra and weredesignated type DC, while a tiny handful had unclassifiable spectra.The puzzling thing was that the stars the white dwarfs were derived from hadno such purity of composition. The key to the puzzle was the intensegravityof the white dwarf, about 200,000 times that of Earth, which left lightatomson the surface while heavier atoms sank. A DA white dwarf still retainssomehydrogen, and so has a surface layer of hydrogen with a sublayer of heliumabove the degenerate core. A DB white dwarf has lost most of its hydrogenand so has a surface layer of helium.The majority of white dwarfs that have been observed are isolated (or"field") stars, rather than white dwarfs in binary systems. These isolatedwhite dwarfs can be found by searching the sky for a faint blue star with alarge rate of motion across the sky. The blue color says that the object ishot, the fast motion hints that the object is nearby, and given these twofacts the faintness suggests the object is small.White dwarfs are also found in multiple star systems. In many of thesesystems, the white dwarf is draining mass off an active stellar companion, aprocess that leads to explosive outbursts, known as "novas. The subject ofsuch "cataclysmic variables" will be discussed in detail in anotherdocument.The mechanisms of stellar evolution that lead to the creation of a whitedwarf are broadly understood. A star begins its life as a collapsing cloudof hydrogen and traces of other elements. As the mass falls in on itself,the temperature at its core rises until eventually fusion reactions start,converting hydrogen to helium and lighting up the star.In time, the star runs out of hydrogen in its core, and hydrogen fusion thenproceeds in an expanding shell around the pure helium core. The star swellsin size and cools, becoming a "red giant".With further age, fusion reactions begin in the core helium, converting itinto carbon and oxygen. For a star with a mass between 2 and 8 Suns, heliumburning goes on in a quiet fashion, with a helium burning core surrounded bythe hydrogen burning shell.When the core is then exhausted of helium, a helium burning shell thenbeginsto expand outward from the center of the star in the path of the hydrogenburning shell. The star swells even more, becoming a "red supergiant".As the star evolves into a red supergiant, its outer shell becomesincreasingly tenuous, and in fact begins to evaporate into space. The starbecomes unstable, "flashing" with rapid changes of luminosity.The result is that within a few tens of thousands of years, most of thestar's mass is lost into space, creating a spherical gaseous shell, or"planetary nebula", around the remainder of the star, now reduced to about20% of its original mass. By the way, this shell is known as a "planetarynebula" because early astronomers tended to mistake them for planets, andthese nebulas have nothing else to do with planets.The mass loss may end before all the outer hydrogen envelope is lost, andthethe ultimate result is a DA white dwarf. If all the hydrogen envelope islost, the result is a DB white dwarf.The white dwarf precursor forming at the center of the planetary nebula isknown as a "planetary nebula nucleus", or PNN. A PNN with a mass of 0.6Sunswill evolve into a white dwarf in about 10,000 years, as the planetarynebulafades into space and all fusion reactions die out.At first, the white dwarf is very hot, with a surface temperature of morethan 100,000 K, and much hotter in its interior. It is so hot that anytraceof hydrogen left in its interior is quickly fused into helium, and helium isconverted to carbon and oxygen. The interior of a typical white dwarf ismostly composed of carbon and oxygen nuclei, though white dwarfs formed bysmaller stars may be mostly helium and those formed by bigger stars may beformed of oxygen, neon, and magnesium.In the early phase of their existence, the internal processes of a whitedwarf generate large numbers of neutrinos. Neutrinos hardly interact withmatter and flood out of the interior of the white dwarf, draining it ofenergy and allowing it to cool rapidly.About 10 million years after its formation, the interior of a white dwarfcools to about 30,000 K and the star no longer radiates neutrinos. Coolingslows down dramatically. At first, most of the white dwarf's energy is lostby radiation, but as the white dwarf cools further, convection processescomeinto play, mixing the surface hydrogen layer with the lower helium layer.Eventually, helium may predominate, turning a DA white dwarf into a DB whitedwarf.A white dwarf loses most of its energy a billion years after its formation.During the long cooling period, the degenerate core continues to grow at theexpense of the outer layer. Once the energy has effectively beendissipated,the white dwarf then starts to crystallize. The bare nuclei in the core ofthe object link up into a symmetrical crystalline lattice, and thecrystallization then expands outward.The transformation from a fluid to a crystal releases energy and slows downthe cooling for a short time. Once the interior becomes heavilycrystallized, however, the white dwarf's cooling proceeds more rapidly. Thewhite dwarf becomes a dim, fading cinder, the only remnant of a oncebrilliant star. 4. WHITE DWARFS AND THE AGE OF THE GALAXY One of the reasons astronomers find white dwarf stars interesting is thatthey are a key to understanding the age of the Universe. As alreadydescribed, a white dwarf is the fossil remnant of a star that has exhaustedits nuclear fuel, lost most of its mass in a planetary nebula, and cooleddown to a dim cinder.A plot of all the observed white dwarfs by their temperatures shows that aswhite dwarfs grow cooler, their numbers increase until the temperature of3,500 K is reached. Below that temperature, there are none.The reason for this is because the Galaxy is not old enough to have allowedeven the oldest white dwarfs to cool off any more than that. This meansthatif we know how long it takes a white dwarf to cool off, we can use thatknowledge to estimate the age of our Galaxy, which in turn would be a cluetothe age of the Universe itself.The primary way of estimating the age of the Universe is throughmeasurementsof its expansion rate, derived from the redshifts of distant galaxies andvarious means of determining the distance to nearby galaxies galaxies forcalibration. Using the cooling rate of white dwarfs as a stepping stone tothe age of the Universe is an entirely independent approach, and is usefulasa reality check.Theoretical studies of the cooling processes in white dwarfs gives anestimate of an age of about 9.5 billion years for the oldest white dwarfs inour Galaxy. Factoring in the time required for galaxies to form after theBig Bang and for stars to become white dwarfs gives an age for the Universeof 11 billion years. This indicates a substantially younger Universe thangiven by estimates provided by mainstream techniques, though not by an orderof magnitude.This estimate is based entirely on theory, but better estimates based ondetailed observations are becoming available. As a white dwarf cools,vibrations can arise with a period from 100 seconds to several hours, due tospasmodic releases of energy through the dwarf's outer layer that cause theentire star to oscillate.These vibrations, which appear as small variations in brightness, give cluesto the internal processes of the dwarf, just as seismic waves give clues tothe internal structure of the Earth. Understanding the internal structureofa white dwarf means obtaining a better estimate for its cooling rate.The patterns of oscillation can be complicated, with different oscillatorymodes and frequencies overlaying each other. Fourier analysis can be usedtobreak the composite oscillation down into its spectrum, or graph ofindividual frequency components, but mapping the composite oscillation cantake a day or more.This task requires extended observations from a set of networked telescopesaround the world. The Whole Earth Telescope, as it is known, has beendeveloped through the 1990s and now can provide effectively continuousobservations for extended periods of time, with data collected at a centrallocation using electronic mail.The process of studying white dwarfs through "asteroseismology", as thestudyof stellar vibrations of such dense objects has become known, is stillevolving, but practitioners feel assured that they will be able to obtainmuch more precise data on the structure and evolution of white dwarfs. 5. BEYOND WHITE DWARFS? When Chandrasekhar published his analysis of the underlying mechanisms ofwhite dwarfs in 1930, there was an implication that disturbed many of hiscontemporaries.Chadrasekhar determined that if a white dwarf has a mass of more than 1.4Suns, electron degeneracy pressure would not be able to halt its collapse.This was not a problem in itself. What was troublesome was that once the"Chandrasekhar limit" was exceeded, "nothing" could halt stellar collapse,and the collapse would never end.Eddington found this result distasteful and strongly attackedChandrasekhar'swork. Chandrasekhar was young and impressionable and the attacks werepainful, but he was encouraged by others such as the Danish physicist NielsBohr to stand his ground. Nonetheless, nobody knew exactly what to make ofthe idea that a star could collapse forever.A hint was available, however. Einstein's theory of General Relativity,published in 1919, stated that mass distorted space and time in itsvicinity.The German astronomer Karl Schwarzchild used the equations of GeneralRelativity to perform an analysis of how a star distorts space and time inits vicinity, and while doing so had discovered something odd.Schwarzchild found that for any given mass, there was a certain radius wheretime was compressed down to zero while the spatial dimensions stretched toinfinity. This radius, now known as the "Schwarzchild radius", is verysmall, about three kilometers for a star with the mass of our Sun.Schwarzchild felt that the matter was irrelevant. Chandrasekhar's analysisof white dwarfs lay in the future, and by the physics available Schwarzchildcould see no way a star could become so compressed.Albert Einstein was more uncomfortable with the Schwarzchild radius and itsimplications, but the matter still did not seem very important. Einsteindidn't get around to dealing with it until 1939, when he published a paperinthe physics press where he attempted to prove that a mass could not becompressed to its Schwarzchild radius.In the meantime, however, other studies had been and were being performed onsuperdense objects and their properties. In 1932, physicist James Chadwickdiscovered the neutron, a nuclear particle similar to the proton but with nocharge. Physicists began to tinker with the possibilities offered by theneutron, and a few astrophysicists, particularly Fritz Zwicky of theCalifornia Institute of Technology and Soviet physicist Lev D. Landau,speculated that they could be the key to stellar fossils far more dense thanwhite dwarfs.If a collapsing star were put under extreme pressure, they suggested,electrons could be forced into protons to form neutrons, creating a denselypacked sphere a few kilometers across but with stellar mass. Zwicky andWalter Baade very astutely suggested further this pressure could be causedbysupernova explosions.Such a "neutron star" was a theoretical toy at the time, but physicists liketo toy with ideas. The prominent American physicist J. Robert Oppenheimerand several of his students, most prominently Hartland S. Snyder, wrote aseries of papers in 1938 and 1939 investigating the theoretical propertiesofneutron stars.One of the interesting questions they considered was a mass limit forneutronstars, similar to the Chandrasekhar limit for white dwarfs, above which theywould collapse. Snyder, working from suggestions by Oppenheimer, performedan analysis based on General Relativity of what would happen if the neutronstar collapsed and fell through its Schwarzchild radius.The mass would tend to collapse without limit, forming a "singularity". Ifan observer was watching a clock on the surface of the collapsing star thatemitted a pulse of light at regular intervals, the pulses would becomeredderand the pulse interval would become longer, as time would slow down in theincreasing gravity field. At the Schwarzchild radius, the pulse interval ofthe clock would become infinite, as would the wavelength of the redshiftedlight.In other terms, once the clock reached the Schwarzchild radius, light couldno longer escape from it. Oppenheimer and Snyder concluded that asingularity "tends to close itself off from any communication with a distantobserver; only its gravitational field persists."The concept of a singularity, a superdense object from which no light couldescape, was a theoretical curiosity at the time. Global war put the matteron the back burner for decades. Oppenheimer went on to the ManhattanProjectto help develop the atomic bomb. In 1947, he became director of theInstitute of Advanced Studies at Princeton University, where Albert Einsteinwas a professor. There is no record of any discussions between them of thefate of collapsing stars.In the postwar period, astronomers did search for neutron stars. A targetofparticular interest was the Crab Nebula, site of a supernova explosion thatwas observed on Earth by Chinese astronomers in 1054 AD. Although opticalastronomers found a compact object at the core of the nebula, there was noway at the time for them to determine exactly what it was. 6. NEUTRON STARS DISCOVERED In 1967, a graduate student at Cambridge University in the UK namedJocelynBell discovered an interesting radio source in the constellation Vulpeculathat emitted a sharp, intense pulse of radio energy on a period of every1.33728 seconds. The period was extremely precise, with a variation of nomore than 1 part in 10 million.The initial reaction in the astronomy community was one of surprise, sincenosuch regular sources had ever been discovered before. Some speculated thatthe sources might actually be beacons set up by a distant civilizations, andso the sources were initially known in some circles as "LGMs", for "LittleGreen Men".However, the radio bursts were over a broad range of frequencies, whichwouldhave made the emitter an inefficient artificial beacon, and before longthreemore such sources were found in widely separated regions of the sky. Thesources clearly seemed to be of natural origin, and were named "pulsars".Bell's academic advisor, Anthony Hewish, wrote a careful analysis ofpulsars.The sharpness of the radio pulses emitted by the pulsars suggested verysmallobjects, maybe about 15 kilometers in diameter. If the object were larger,radio waves emitted from more distant regions of the object would arriveafter those emitted from nearer regions, spreading out the pulse.The pulse period indicated that the object was spinning rapidly, with radio"hot spots" coming into view with each spin. The rate of rotation impliedanobject of stellar mass, as anything lighter would simply tear itself apart.The only thing that could meet such constraints was a neutron star. Hewishlater won the Nobel Prize for this discovery.Once astronomers knew what to look for, they studied the Crab Nebula againand found a pulsar at its center, emitting bursts of radio energy at a rateof about 30 times per second. Precise measurements of the rotation rate ofthis pulsar showed that it was slowing down ever so slightly as it radiatedaway energy.Another pulsar was found in a much closer supernova remnant in theconstellation Vela. The Vela supernova occurred a few thousand years agoandthis pulsar has lost much of its energy and slowed down, emitting radiobursts about 12 times a second. Over a thousand pulsars have now beenidentified, with periods ranging from ten seconds to a peculiar family ofpulsars with periods on the order of a millisecond. 7. CHARACTERISTICS OF NEUTRON STARS As had been suggested decades before and was clearly indicated by the CrabNebula and Vela pulsars, a neutron star is the remnant of a supernovaexplosion, specifically from the collapse of a star with a mass of from 8 to15 Suns.The neutron star that results from such a catastrophe has a number ofinteresting characteristics. First, conservation of angular momentum as itcollapses means that the young neutron star spins very fast, at a rate ofabout 50 times a second. (This does not come close to the period of themillisecond pulsars, but that will be discussed later.)Second, for reasons not clearly understood the supernova explosion oftendoesnot occur symmetrically, giving the neutron star a "kick" that sends it outof its birthplace at velocities of more than 1,000 kilometers a second, fastenough in some cases to eject it from the Galaxy entirely. This is why someclearly young pulsars are not directly associated with supernova remnants.Third, the young neutron star is also very hot, up to a 100,000 or a millionK. The surface area of the neutron star is very small relative to its mass,and so the energy trapped during its formation can radiate away only veryslowly.Fourth, the young neutron star has an extraordinarily intense magneticfield.Since a moving magnetic field generates an electric current through aconductor, and since a young neutron star is a big powerful magnet spinningvery fast, it sets up an immense flow of electrons, positrons, and ions overits surface and scatters them into space. This flow of charged particles isknown as the "pulsar wind", analogous to the "Solar wind" emitted by ourSun.The intense magnetic field and the flow of charged particles accounts fortheradio pulses as well. The particles trapped in the magnetic field arefocused by poorly-understood processes into focused floods of radiation fromthese two "hot spots". The magnetic poles are not necessarily aligned withthe spin axis of the neutron star, and since the star is spinning rapidlythehot spots swing past like the searchlight beam on a lighthouse.The pulse beams of young neutron stars, like the Crab Nebula or Velapulsars,radiate energy all along the electromagnetic spectrum, from radio waves togamma rays. As the pulsar ages and loses energy, however, the neutron starcools off and only radio emission occurs.The rate at which the spin of a pulsar slows down indicates its rate ofenergy emission, and even though the beams are intense they only account fora small fraction of the energy emission of the pulsar. Most of the rest ofthe energy emission is likely in the form of the pulsar wind and otherunseenradiation.After about ten million years, the pulsar slows down and no longer hasenoughenergy to emit pulses. The pulsar now becomes invisible to Earthobservation, unless it is part of a star system and can be detected by itsgravitational effects.As the conditions in a neutron star are very difficult to duplicate onEarth, nobody is exactly sure just how big a neutron star can be. Oneindirect argument is based on the fact that as a neutron star becomes moremassive, it must become stiffer to maintain itself, and the speed of soundthrough the star increases accordingly. Above six solar masses, the speedofsound exceeds that of light, which is ruled out by Einstein's theory ofrelativity.Six solar masses is only an upper bound on the size of a neutron star. Morepractical calculations estimate the upper limit as three solar masses. Noobjects confirmed as neutron stars are known that are larger than two solarmasses. 8. MILLISECOND PULSARS AND OTHER UNUSUAL NEUTRON STARS Neutron stars are often found in close binary systems with large normalstars. This might seem implausible, since neutron stars are born insupernova explosions that would tear apart a companion star. In fact, suchcompanions often can survive the explosion.The companion star swells as it ages, and for a close binary system thecompanion may eventually start losing mass to the neutron star. The massspirals down to the neutron star in an orbiting disk of hot plasma, or"accretion disk", that can in some circumstances radiate brightly in theX-ray spectrum.The mass spiraling into the surface of the neutron star can also "spin itup", increasing the star's rotation rate and restarting pulsar action. Thismechanism is the origin of the otherwise baffling millisecond pulsars, thefastest of which spins at a rate of 667 times per second.In a few rare cases the companion star itself evolves into a neutron star,and the result is a binary neutron star system. Such close neutron starbinaries are of particular interest to physicists interested in gravitywaves, as they are potentially strong sources of gravitational waveradiation.Some also speculate that the mysterious "gamma ray bursts" that are detectedon an intermittent basis may be due to the infall and collision of themembers of a binary neutron star system.The millisecond pulsars are not the only unusual types of neutron stars.There are also a handful of young pulsars that have intense magnetic fields,even by pulsar standards, and pulse mostly in the X-ray region of thespectrum. These objects are known as "anomalous X-ray pulsars", or AXPs.There are also a small number of similar pulsars that occasionally generateintense bursts of low-energy ("soft") gamma rays, and are known as "softgamma-ray repeaters", or SGRs. The gamma ray bursts appear to be caused byabrupt dislocations in the surface layers of the neutron star. Someastronomers lump AXPs and SGRs together as "magnetars", but these peculiarobjects remain poorly understood. 9. BLACK HOLES DISCOVERED? Once neutron stars were discovered, astronomers began to wonder ifsingularities could exist as well. If they existed they would be very hardto detect, since Oppenheimer and Snyder had shown that no radiation couldescape from them. Modern theorists described the result as a "black hole"inspace.Detecting such a black hole was a difficult prospect. They would benecessarily small, and could emit no detectable radiation by themselves.Theonly possible way to find one was through the observation of its effects onvisible matter in its vicinity.Astronomers have discovered phenomena in the cosmos that suggest that blackholes do in fact exist. One is the existence of violent events associatedwith binary star systems and galactic cores. Such events require hugeamounts of energy, and one of the most efficient ways to generate thisenergyis through matter falling into a black hole.Another is the existence of binary star systems where a bright star islosingmass to a hidden companion, with the lost mass generating intense energyintothe X-ray wavelengths. Analyses of some of these X-ray binary systems showthat the hidden companion has a mass and size that could only be accountedfor by a black hole.One of the first bright X-ray sources in the sky, known as Cygnus X-1 anddiscovered in the early 1970s, appears to be a blue supergiant star losingmass to a hidden companion of about ten solar masses. This hidden companionis strongly believed to be a black hole due to its large mass and smallsize.Similarly, observations of the cores of galaxies often show that there areobjects hidden there with masses of thousands of millions of Suns but onlythe size of a planetary system. The only object known in theory that couldhave such great mass and compact dimensions is a black hole.However, all that is known in these two scenarios is that there is a densebody involved whose specific characteristics are unknown, except for boundson size and mass. These bounds can suggest the presence of a black hole,butthe physics of black holes lie on the limits of physical theory, andalthoughtheoretical calculations can be surprisingly accurate, they have also inmanycases proved dead wrong. Although the size and mass limits might imply ablack hole in theory, nature might have other ideas.Even if black holes exist, neutron stars are also clearly involved inenergetic events in some binary systems, and telling the difference betweenabinary system interacting with a neutron star and one interacting with ablack hole is difficult.Black holes are in principle extremely efficient at converting infallingmass into energy. As objects are drawn toward the boundary of no escape, or"event horizon", they are accelerated to near the speed of light, andacquiretremendous kinetic energy, much of which is released in collisions.The amount of energy conversion increases if the black hole is spinning, andcan reach a theoretical maximum of 42%. The turbulent plasma falling into ablack hole generates high-energy radiation in the form of X-rays. X-raybinaries such as Cygnus X-1 demonstrate intense emission consistent withsuchprocesses.The distribution of the radiation emitted by X-ray binaries is in the formofa continuous black body spectrum. The black-body spectrum of an X-raybinaryreveals a source temperature of about 10^7 K, which corresponds to thetemperatures expected for matter falling into a black hole. The amount ofenergy released corresponds to the absorption of 10^-9 to 10^-8 solar massper year, which matches the rate at which mass is being lost by the visiblestar.That is not enough to prove that the X-ray emitting object is a black hole.A neutron star can generate a great flow of X-rays as well, by acceleratinginfalling matter to up to half the speed of light at impact. Conversionefficiencies are about 10% of the infalling mass, which is similar to thatexpected for a typical black hole.In some cases, the hidden companion is clearly a neutron star. This is thecase for pulsars, since they generate pulses from their hot spots. As ablack hole has no surface, it cannot have a fixed hot spot. However, thelack of pulse activity does not necessarily prove the hidden object is ablack hole.The most significant hint that a hidden companion is a black hole is itsmass. There is no known limit on the mass of a black hole. This is not thecase for white dwarfs, which have a limit of 1.4 solar masses, and neutronstars, which have a limit of about 3 solar masses. This implies that anyhidden companion in a binary system that is larger than 3 solar masses isa black hole.Seven X-ray binaries have been found where the mass of the hidden companionis larger than three solar masses, with the measured mass of the hiddencompanions actually ranging from 4 to 12 solar masses.Still, as mentioned, theory may be wrong, and we need to know more. Theabsolutely distinguishing feature of a black hole is its lack of a solidsurface. All it has is an event horizon into which matter falls, never tobeseen again.One of the interesting implications of the lack of a solid surface is toconsider what happens if hot plasma falls through a black hole's eventhorizon before the plasma can radiate away its energy. In this case, theenergy simply vanishes, being manifested only as an increase in mass of theblack hole. This process, known as "advection", can limit the energyconversion efficiency of a black hole.In contrast, if hot plasma falls onto a neutron star, all its energy has tobe radiated away, either from the plasma or from the surface of the neutronstar. This means that if energy appears to be disappearing into a hiddencompanion, that companion is likely to be a black hole. Astronomers havebeen hunting for X-ray binaries with just such a characteristic.Observations of some binary systems and galactic cores have strongly hintedthat energy is disappearing without a trace in this way. Much work remainsto be done, and though uncertainty remains, it is yet another piece ofevidence that encourages astrophysicists to believe they are in fact on theright track. 10. MINIHOLES As an interesting footnote to the story of black holes, the well-knownBritish physicist Stephen Hawking has suggested that in the creation of theUniverse there could have been regions where pressures and densities were sohigh that very small black holes, even with masses of far less than akilogram, could have been created.Hawking also suggested that such "miniholes" could actually "evaporate".Modern field theory proposes that the entire fabric of the Universe isfilledwith "virtual particle pairs", consisting of an antiparticle and a particle,that are spontaneously being created and then recombining so fast that theycannot be directly detected.Hawking proposed that if such a virtual pair, such as a positron and anelectron, were created near the event horizon of a black hole, one of theparticles might disappear into the black hole and be lost forever, and theother would appear to have been emitted from the black hole.Since energy conservation still remains an unviolated concept of physics,even quantum physics, the emitted particle has a certain amount of energy,and that energy can't simply appear out of nothing. Hawking's analysisshowed that such a process would rob the black hole of energy to create theemitted particle.For a large-scale black hole derived from stellar collapse, this processwould have a negligible effect. As holes grow smaller and smaller, though,their rate of "evaporation" would increase. A minihole would exist for acertain time, leaking out particles at an ever increasing rate until itevaporated in a burst of gamma rays.Hawking's miniholes remain an intriguiging speculation. Detecting largescale black holes is hard enough at present. Tracking down miniholes andthegamma ray bursts they emit when they evaporate is not practical for now. 11. COMMENTS, SOURCES, AND REVISION HISTORY This document is the first in a series on stars and stellar evolution.Eventually I hope to weld the series into a comprehensive document on thelives of stars.Sources include :- "White Dwarfs: Fossil Stars", Steven D. Kawaler, SKY & TELESCOPE, August 1987, 132:135. "Taking The Pulse Of White Dwarfs", by Nather & Winget, SKY & TELESCOPE, April 1992, 374:378. "The Reluctant Father Of Black Holes", by Jeremy Bernstein, SCIENTIFIC AMERICAN, June 1996, 80:85 "Unmasking Black Holes", by Jean-Pierre Lasota, SCIENTIFIC AMERICAN, May 1999, 40:47. "The Life Of A Neutron Star", by Joshua N. Winn, SKY & TELESCOPE, July 1999, 30:38. Greg Goebel (gvgoebel@yahoo.com)www.geocities.com/CapeCanaveral/Launchpad/6000 Return to Contents List Page geovisit();
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