[6.0] Fossil Stars (2): Neutron Stars & Black Holes

v3.3.0 / chapter 6 of 10 / 01 jan 18 / greg goebel

* Although most astronomers were not keen on the ideas of neutron stars and black holes, by the late 1960s the existence of neutron stars had been confirmed. That strongly suggested that black holes existed as well, and from that time evidence has accumulated that they do. The study of such superdense objects is now an important and accepted part of astronomy.

Crab Nebula



* In 1967, a graduate student at the University of Cambridge in the UK named Jocelyn Bell (born 1943) discovered an interesting radio source in the constellation Vulpecula that emitted a sharp, intense pulse of radio energy on a period of every 1.33731109 seconds. The period was extremely precise, with a variation of no more than 1 part in 10 million.

The initial reaction in the astronomy community was one of surprise, since no such regular sources had ever been discovered before. Some speculated that the sources might actually be beacons set up by a distant civilizations, and so the sources were initially known in some circles as "LGMs", for "Little Green Men". However, the radio bursts were over a broad range of frequencies, which would have made the emitter an inefficient artificial beacon, and before long three more such sources were found in widely separated regions of the sky. The sources clearly seemed to be of natural origin, and were named "pulsars".

Bell's academic adviser, Anthony Hewish (born 1924), wrote a careful analysis of pulsars. The sharpness of the radio pulses emitted by the pulsars suggested very small objects, maybe about 15 kilometers in diameter. If the object were larger, radio waves emitted from more distant regions of the object would arrive after 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 implied an object of stellar mass, as anything lighter would simply tear itself apart. The only thing that could meet such constraints was a neutron star: Fritz Zwicky had been completely vindicated.

Hewish won the Nobel prize in 1974 for this discovery. Fred Hoyle criticized the award since he felt it failed to recognize Jocelyn Bell's contribution, and Hoyle's defenders believe that was why he was not included in the 1983 Nobel prize award to Chandrasekhar and Hoyle's colleague Fowler -- though as discussed later, there may well have been other reasons. In any case, once astronomers knew what to look for, they studied the Crab Nebula again and found a pulsar at its center, emitting bursts of radio energy at a rate of about 30 times per second. Precise measurements of the rotation rate of this pulsar showed that it was slowing down ever so slightly, 36.5 billionths of a second per day, as it radiated away energy.

Another pulsar was found in a much closer supernova remnant in the constellation Vela. The Vela supernova occurred a few thousand years ago and this pulsar has lost much of its energy and slowed down. It is now emitting radio bursts about 12 times a second. Over a thousand pulsars have been identified, with periods ranging from ten seconds to a peculiar family of pulsars with periods on the order of a millisecond. Interestingly, the Crab pulsar is unusually bright, and surprisingly far fewer pulsars were found associated with supernova remnants than expected.



* As had been suggested decades before and was clearly indicated by the Crab Nebula and Vela pulsars, a neutron star is the remnant of a Type II supernova explosion, specifically from the collapse of a star with a mass of from 8 to 15 Suns. The neutron star that results from such a catastrophe has a number of interesting characteristics:

The intense magnetic field and the flow of charged particles account for the radio pulses as well. The particles trapped in the magnetic field are focused by poorly-understood processes into focused floods of radiation from these two "hot spots". The magnetic poles are not necessarily aligned with the spin axis of the neutron star, and since the star is spinning rapidly the hot spots sweep around like the searchlight beam on a lighthouse.

The beams of young neutron stars, like the Crab Nebula or Vela pulsars, radiate energy all along the electromagnetic spectrum, from radio waves to gamma rays. As the pulsar ages and loses energy, however, the neutron star cools off and only radio emission occurs.


The rate at which the spin of a pulsar slows down indicates its rate of energy emission, and even though the beams are intense they only account for a small fraction of the energy emission of the pulsar. Most of the rest of the energy emission is likely in the form of the pulsar wind and other unseen radiation. After about ten million years, the pulsar slows down and no longer has enough energy to emit pulses. The pulsar now becomes invisible to Earth observation, unless it is part of a star system and can be detected by gravitational effects. Since the lifetime of a pulsar is relatively short in cosmic terms, there are likely hundreds of millions of radio-quiet neutron stars in our Galaxy.

* Since the conditions in a neutron star are very difficult to duplicate on Earth, nobody is exactly sure just how big a neutron star can be. One indirect argument is based on the fact that as a neutron star becomes more massive, it must become stiffer to maintain itself, and the speed of sound through the star increases accordingly. Above six solar masses, the speed of sound exceeds that of light, which is ruled out by Einstein's theory of relativity.

Six solar masses is only an upper bound on the size of a neutron star. More practical calculations estimate the upper limit as three solar masses. No objects confirmed as neutron stars are known that are larger than two solar masses; the mass is typically about 1.35 solar masses.

Neutron stars also appear to be associated with some GRBs. As mentioned earlier, there are two classes of GRBs -- those with burst durations of over 2 seconds, and those with durations of fractions of a second, usually about 0.3 seconds. The short duration GRBs don't appear to leave afterglows. The consensus on them is that they are binary systems of neutron stars that finally collide into each other, producing a spectacular explosion.



* Neutron stars are often found in close binary systems with large normal companions. That might seem implausible, since neutron stars are born in supernova explosions that would seem likely to blast away a companion star. In fact, such companions often survive the explosion. The companion star swells as it ages, and for a close binary system the companion may eventually start losing mass to the neutron star. The mass spirals down to the neutron star in an orbiting accretion disk of hot plasma that can in some circumstances radiate brightly in the X-ray spectrum, creating what is known as an "X-ray star" or "X-ray binary".

The accretion of mass from the companion onto the neutron star may also occasionally lead to X-ray "flares", when the mass builds up to a critical level and initiates fusion. Usually the mass is hydrogen or helium, the helium accumulating more as the mass of the companion is stripped away to deeper levels.

However, if the neutron star is accreting helium, then each burst of helium fusion leaves a residue of carbon behind. Ultimately, the carbon will build up to a level where it undergoes fusion itself, creating a "carbon flare" a thousand times more powerful than a helium flare that could possibly disrupt the accretion disk around the neutron star. Such a carbon flare was tentatively observed for the first time by the NASA Rossi X-ray Timing Explorer satellite on 9 September 1999, and astronomers are very interested in obtaining data from a second, since observing the reformation of an accretion disk after the outburst would give detailed insights into the physics of such explosive binary systems.

* The mass spiraling into the surface of the neutron star can also "spin it up", increasing the star's rotation rate and restarting pulsar action. This mechanism is the origin of the otherwise baffling "millisecond pulsars". The first was discovered in 1982, spinning away at 642 times a second, much to the shock of astronomers. Well over a hundred have been found since then. The fastest known spins at 716 times per second, not that much faster than the first one -- this appears to be close to a "hard limit", possibly because at any greater rotation rate, a pulsar will emit increasing amounts of "gravitational radiation". Such "gravity waves", predicted by General Relativity, would rob the pulsar of rotational energy and slow it down.

In a few rare cases the companion star itself evolves into a neutron star, and the result is a binary neutron star system. The first such system was identified by Russell Hulse (born 1950) in 1974, then a grad student at the University of Massachusetts at Amherst working at the Arecibo radio telescope in Puerto Rico, which is a huge dish dug into a shallow between a group of hills. Hulse had developed a computer algorithm to help sort out pulsar signals from radio telescope data that allowed the identification of pulsars ten times fainter than those discovered to that time,

Hulse found a pulsar designated PSR B1913+16 for its sky coordinates and found it was pulsing 17 times a second. However, the pulsar timing varied in a way that drove Hulse to distraction, until he realized it had an eight-hour orbit around an invisible second body. He called his adviser at Amherst, astronomer Joseph H. Taylor (born 1941), and Taylor came down to Arecibo to see what was going on. Analysis of the orbit showed that the second body was a nonpulsing neutron star. The object became known as the "Hulse-Taylor binary". Taylor worked with a number of colleagues to show that the two neutron stars were slowly spiraling towards each other at a rate defined by Einstein's theory of general relativity. It was the first experimental evidence for gravity waves, and in fact much of the interest in binary neutron stars lies in their usefulness as "laboratories" to test general relativity. Hulse and Taylor shared the Nobel prize for physics in 1993 for their discovery.



* The millisecond pulsars are not the only strange types of neutron stars. In the spring of 1979, a number of spacecraft fitted with gamma-ray detectors picked up a very powerful GRB, so powerful that it pegged the detectors. That was unusual, but all it really suggested was that the GRB was nearby, at least in cosmic terms. What was really unusual was that there was a second GRB from the same direction of the sky a little over 14 hours later, followed by 15 more faint bursts followed. Several more recurring burst sources were detected, with these objects placed in a new class -- outside of the classes of long and short GRBs -- of "soft gamma-ray repeaters (SGRs)". The first was designated "SGR 0526-66".

Astronomers had managed to box in the location of SGR 0526-66 to a supernova remnant in the Large Magellanic Cloud, 170,000 light-years away. Although the bursts might have occurred in a closer object that just happened to be in the line of sight to the LMC, the presence of the supernova remnant and the lack of anything else of significance in the coordinate box made it very likely the source was actually in the LMC. The only problem was that the LMC was about a thousand times farther away than what astronomers had originally estimated for the distance of the burst, and so the initial "superburst" was far more powerful than previously believed.

There was a variation in the bursts over a period of 8 seconds, which suggested a neutron star spinning once every 8 seconds. The problem was that neutron stars usually become radio pulsars, and the spin rate was extremely low for a neutron star of its age -- the age determined by backtracking the rate of expansion of supernova remnant back to its origin. The object was also continuously emitting a strong flow of X rays.

Current thinking about SGRs suggests they are powered by ultrastrong magnetic fields. Radio pulsars emit radio waves because they have strong magnetic fields, between 10^12 and 10^13 gauss; in comparison, a household magnet generates a magnetic field of about 100 gauss. The strong magnetic field is partly inherited from the original star, concentrated by its collapse into a neutron star, with charged particles circulating inside the neutron star also contributing to the magnetic field. The rotating magnetic field of the spinning neutron star radiates away electromagnetic energy as radio waves, with this loss of energy gradually slowing down the rotation of the pulsar. Eventually it loses enough energy to stop producing radio waves.

If a neutron star is born with a fast spin rate, less than about 10 milliseconds, computer simulations show that it produces a very strong dynamo effect, producing an intense magnetic field up to a limit of about 10^17 gauss. Such an object is referred to as a "magnetar". The intense magnetic field of the magnetar tends to operate as a relatively abrupt brake on its rotation, slowing it down to a long period in a few thousand years. That explained the unusually low rotation rate of the SGR 0526-66 source.

Another clue to the nature of the SGRs was that the burst pattern seemed to statistically resemble the patterns of earthquakes. This fit with the magnetar model as well. The intense magnetic field could occasionally distort a patch of the crust to the extent that it fractured, releasing an intense burst of magnetic energy that led to gamma-ray emission.

The initial superburst of SGR 0526-66 was generated by a different mechanism. On normal stars like the Sun, the magnetic field gets tangled up over time and then goes through a rapid period of rearrangement that causes solar flares. A similar process occurs in a magnetar, but it is far more intense and involves some very exotic physics, creating a fireball that is trapped in the magnetic field until it hell literally breaks loose, resulting in the huge burst of gamma rays. It may be the case that short events believed to be associated with GRBs, thought by some to be colliding neutron stars, are actually very distant SGRs producing a superburst, with the repetitive bursts lost in the background.

The magnetar theory was not taken very seriously until the late 1990s. Observations of SGRs in 1998 and 2004 provided much more data. They were extremely powerful, overloading the detector systems of science spacecraft and even disturbing the Earth's upper atmosphere, causing communications interference. They were not inherently more powerful than the 1979 SGR, they were just closer, located in the Milky Way. More have been observed since then, helping confirm the nature of SGRs as magnetars -- and also affirming that at least some of the short GRBs are actually distant SGR superbursts, and not colliding neutron stars.

Astronomers have also observed a handful of "anomalous X-ray pulsars (AXPs)" whose X-ray production seems to be due to strong magnetic fields. Astronomers believe they are magnetars as well. It is unclear if SGRs and AXPs are simply different phases in the life of a magnetar, or if they represent slightly different evolutionary paths. What is much more certain is that they are transient phenomena. Magnetars are short-lived by cosmic standards. Within about 10,000 years of their creation, their magnetic field fades to normal levels and they cease to emit bursts.



* Once neutron stars were discovered, astronomers began to wonder if singularities could exist as well. Theoretical physicists were already reasonably sure that they did. Interest in the concept had revived in the early 1960s and the concept had been fleshed out in much more detail, using computer power not available to Oppenheimer and Snyder a quarter century earlier. One theoretician, John Archibald Wheeler (1911:2008) of Princeton, had given them the name of "black holes".

That was partly as a slightly rude joke, Wheeler having a quiet but quirky sense of humor, and French physicists refused to use the phrase for a long time since the translation "trou noir" is not just slightly rude. Wheeler would not stop there either, going on to create the slogan "a black hole has no hair", meaning that once a black hole was created, it was impossible to determine what kind of matter it was created from -- lead or rocks or used cars or chocolate bars, it would precisely identical in any case. There was also some resistance to this phrase for a time, though now it is used with little self-consciousness.

Ironically, in 1958 Wheeler had squared off against Oppenheimer in a public debate on the existence of singularities, with Wheeler attacking the notion and Oppenheimer defending it. In a few years Wheeler came around and enthusiastically conceded to Oppenheimer at a 1963 conference, only to find that the worn-out Oppenheimer paid no attention. Wheeler was deeply disappointed, but over the next decade he had the satisfaction of watching astrophysicists, many of them his own students, nail down the theoretical foundations of black holes.

While they did prove that black holes had no hair, they also discovered that could have some variation in properties. Initially, theoretical models of black holes stipulated that they have no spin and no charge. Better models showed that if the body that formed the hole had been rotating, its angular momentum would be conserved through its collapse into a black hole. Not only would the hole become an ellipsoid instead of a simple sphere, but it would "drag" space around it in its vicinity, forcing rotation on any object falling towards it.

The new models also showed that if a black hole was formed out of charged matter it would retain the electric charge and produce an electric field. In fact, the electrical behavior of a black hole proved very interesting, since analysis showed that the hole could be modeled as though it was covered with a conductive "membrane" with a specific conductivity that supported electrical currents. Of course, anybody who tried to touch that "membrane" would simply disappear inside the hole and never be seen again, assuming that the tidal forces near the hole didn't simply tear him apart first.

a hole in space

* There was still the problem of actually finding a black hole, which was tricky on the face of it because they didn't emit any light. Astronomers have discovered phenomena in the cosmos that suggest that black holes do in fact exist. One is the existence of violent events associated with binary star systems and galactic cores. Such events require huge amounts of energy, and one of the most efficient ways to generate this energy is through matter falling into a black hole.

Another is the existence of binary star systems where a bright star is losing mass to a hidden companion, with the lost mass generating intense energy into the X-ray wavelengths. Analyses of some of these X-ray binary systems show that the hidden companion has a mass and size that could only be accounted for by a black hole.

One of the first bright X-ray sources in the sky, known as "Cygnus X-1" and discovered in the early 1970s, appears to be a blue supergiant star losing mass to a hidden companion of about ten solar masses. This hidden companion is strongly believed to be a black hole due to its large mass and small size. Similar binary star systems have been observed since that time.

Interestingly, in 2000, a collaboration of astronomers reported detecting solitary black holes. The collaboration was searching for massive objects in our own Galaxy using sky surveys to detect gravitational lensing events. Two events were recorded that pointed to objects a few thousand light-years away and about six times more massive than the Sun.

Observations of the cores of galaxies often show that there are objects hidden there with masses of thousands of millions of Suns but only the size of a planetary system. The Hubble Space Telescope has detected at least twenty in nearby galaxies, while radio telescope arrays have pinned down dense objects in about 100 nearby galaxies. The NASA Chandra orbiting X-ray observatory has also detected distant X-ray sources in the distant Universe that appear to be supermassive black holes.

However, all that is known in these scenarios is that there is a dense body involved whose specific characteristics are unknown, except for bounds on size and mass. These bounds can suggest the presence of a black hole, but the physics of black holes lie on the limits of physical theory, and though theoretical calculations can be surprisingly accurate, they have also in many cases proved dead wrong: while the size and mass limits might imply a black hole in theory, nature might have other ideas.

A number of alternate theories have been proposed to explain such objects; one of the more interesting one points to the slight energy inherent in the vacuum of space itself and postulates that the accumulation of that energy in the collapse of a mass will build up a "degeneracy pressure" that halts the collapse, resulting in a "black star" instead of a "black hole". So far, the astronomical community prefers black holes over the alternate models.

Even if black holes exist, neutron stars are also clearly involved in energetic events in some binary systems, and telling the difference between a binary system interacting with a neutron star and one interacting with a black hole is difficult.

* Black holes are in principle extremely efficient at converting infalling mass into energy. As objects are drawn toward the boundary of no escape, or "event horizon", they are accelerated to near the speed of light, and acquire tremendous kinetic energy, much of which is released when the flow becomes turbulent, possibly by simple friction of collisions but more likely by magnetic interactions between the charged plasmas in the flows.

The amount of energy conversion increases if the black hole is spinning, and can reach a theoretical maximum of 42%, far more than can be obtained by hydrogen fusion. The turbulent plasma falling into a black hole generates high-energy radiation in the form of X-rays. X-ray binaries such as Cygnus X-1 demonstrate intense emission consistent with such processes.

The distribution of the radiation emitted by X-ray binaries is in the form of a continuous black body spectrum. The black-body spectrum of an X-ray binary reveals a source temperature of about 10^7 K, which corresponds to the temperatures expected for matter falling into a black hole. The amount of energy released corresponds to the absorption of 10^-9 to 10^-8 solar mass per year, which matches the rate at which mass is being lost by the visible star.

That's 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 accelerating infalling matter to up to half the speed of light at impact. Conversion efficiencies are about 10% of the infalling mass, which is similar to that expected for a typical black hole. In some cases, the hidden companion is clearly a neutron star. This is the case for pulsars, since they generate pulses from their hot spots. Since a black hole has no surface, it cannot have a fixed hot spot. However, the lack of pulse activity does not necessarily prove the hidden object is a black hole.

* The most significant hint that a hidden companion is a black hole is its mass. There is no known limit on the mass of a black hole. This is not the case for white dwarfs, which have a limit of 1.4 solar masses, and neutron stars, which have a limit of about 3 solar masses. This implies that any hidden companion in a binary system that is larger than 3 solar masses is a black hole. A number of X-ray binaries have been found where the mass of the hidden companion is larger than three solar masses, with the measured mass of the hidden companions actually ranging from 4 to 12 solar masses.

Still, as mentioned, theory may be wrong, and we need to know more. The absolutely distinguishing feature of a black hole is its lack of a solid surface. All it has is an event horizon into which matter falls, never to be seen again. One of the interesting implications of the lack of a solid surface is to consider what happens if hot plasma falls through a black hole's event horizon before the plasma can radiate away its energy. In this case, the energy simply vanishes, being manifested only as an increase in mass of the black hole. This process, known as "advection", can limit the energy conversion efficiency of a black hole.

In contrast, if hot plasma falls onto a neutron star, all its energy has to be radiated away, either from the plasma or from the surface of the neutron star. This means that if energy appears to be disappearing into a hidden companion, that companion is likely to be a black hole. Astronomers have been hunting for X-ray binaries with just such a characteristic.

Observations of some binary systems and galactic cores have strongly hinted that energy is disappearing without a trace in this way. Much work remains to be done, and though uncertainty remains, it is yet another piece of evidence that encourages astrophysicists to believe they are in fact on the right track.



* As an interesting footnote to the story of black holes, the well-known British physicist Stephen Hawking (born 1942) has suggested that in the creation of the Universe there could have been regions where pressures and densities were so high that very small "primordial" black holes, even with masses of far less than a kilogram, could have been created.

Hawking also suggested that such "miniholes" could actually "evaporate". Modern field theory proposes that the entire fabric of the Universe is filled with "virtual particle pairs", consisting of an antiparticle and a particle, that are spontaneously being created and then recombining so fast that they cannot be directly detected. Hawking proposed that if such a virtual pair, such as a positron and an electron, were created near the event horizon of a black hole, one of the particles might disappear into the black hole and be lost forever, and the other 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 analysis showed that such a process would rob the black hole of energy to create the emitted particle. For a large-scale black hole derived from stellar collapse, this process would have a negligible effect, and such black holes will vastly outlive the active life of the Universe: a black hole with a mass of two Suns will take 1.2E67 years to evaporate, and the interval gets longer for bigger holes at a steep rate.

However, by the same coin the rate of evaporation increases steeply as the hole grows smaller, and at the end the hole will evaporate abruptly in an energetic burst of gamma rays. All primordial black holes that had masses less than that of a mountain or mid-sized asteroid have evaporated by now, while those with masses a few times bigger should be "timing out" in our current era, generating increasing amounts of energy as they shrink down to their energetic finale. Such miniholes will have diameters about the size of an atomic nucleus.

Hawking's miniholes remain a interesting speculation. Detecting large scale black holes is hard enough at present. Tracking down miniholes and the gamma ray bursts they emit when they evaporate is not practical for now, though some particle physicists believe that they may be able to generate very tiny "quantum" black holes, much smaller than a nucleus. There have been concerns that such quantum black holes might start to grow and become a threat to the planet, but according to Hawking's theory, these holes will evaporate in an instant.

There is of course the worry that theory is dead wrong at times, and in fact when a physics lab proposed to create miniholes, a protest was made in a court of law to block the effort. The case was thrown out: if it is indeed possible to create quantum black holes in a particle accelerator, then they are also being created on a continuous basis by the impact of high energy "cosmic rays" in the upper atmosphere. Energetic cosmic sources, well more powerful than any human-made particle accelerator, are also commonplace in the Universe, and stable miniholes were possible, such sources would be creating them in large quantities, to spew them out in the Universe. We're still here.