EXOTIC "STARS"

What remains in the aftermath of a supernova explosion? Is the entire star just blown to bits and ejected into the surrounding interstellar medium? Astronomers aren’t quite sure, though most computer models predict that some portion (perhaps 20%) of the star survives. Like planetary nebulae that expel matter less violently and bequeath a white-dwarf core remnant, most supernovae are also expected to leave behind tiny, severely compressed cores. The matter within this centralized cinder is unlike anything found on Earth, indeed it comprises one of the strangest states of matter in all the Universe.

Neutron Stars During the moment of implosion of a massive star, just prior to its explosion, all the electrons in the core violently collide with protons. Free, unbound electrons were there all along given the plasma state extant throughout the star's interior, but the protons are newly freed when some of the heavy nuclei disintegrate under the phenomenal onslaught of the whole star collapsing. The result is an elementary-particle reaction that races through the core of the massive star, converting within seconds all the electrons and protons into neutrons and neutrinos:

electron + proton --> neutron + neutrino.

Though the neutrinos rapidly leave the scene at nearly the speed of light, they are suspected by many theorists to play a major role in triggering supernovae. The reason is that neutrinos transport much of the energy of the collapsed core to the overlying layers of the star, deposit it there, and cause the rest of the star to detonate like a colossal bomb. In contrast to our Sun where the neutrinos escape in seconds without hesitating, the collapsed cores of more massive stars are so much denser that the neutrinos are actually stopped in their tracks before escaping the star—but only momentarily, as their accompanying energy is sufficient to blow the rest of the star to high heaven.

Made of nuclei much heavier than the neutrinos, the shattered debris from everything above the star’s core departs at fast speeds, yet much less than the velocity of light. These are the wisps and filaments seen today still racing outward from old supernova remnants. Only the core remains intact as an ultradense ball of pure neutrons hardly more than a few kilometers across—about the size of an asteroid. Astronomers breezily call this core remnant a neutron star, but it’s not really a “star” in the true sense of the word since all its nuclear reactions have ceased forever.

The theory of stellar evolution predicts that neutron stars are very small, though very massive. Composed simply and solely of neutron particles crammed into a tight-knit sphere ~10 km across, a neutron star is not much larger than a typical city (Figure 3.50). Despite its diminutive size, each unexploded core remnant usually contains a few times the mass of our Sun, making neutron stars extraordinarily compact. Their average density is estimated to reach at least a quadrillion times that of Earth’s rocks. Not merely a huge density, this is an incredible density, nearly a billion times denser than the already super-compact, white-dwarf stars; a single thimbleful of neutron-star stuff would weigh a hundred million tons. In fact, the density of a normal atomic nucleus (~1014 g/cm3) is not much greater; neutron stars are just about as compressed as the matter within the nuclei of normal atoms (which is why some theorists regard neutrons stars as ginormous nuclei). Such an extraordinary density was already encountered in the story of cosmic evolution during the first, PARTICLE EPOCH of the Universe (consult Table 1-1). The weird neutron stars might therefore act as “primeval laboratories,” enabling scientists to study the physical conditions prevalent just after the start of the Universe.

FIGURE 3.50 FIGURE 3.50— Neutron stars are not much larger than many of Earth's major cities. Here, in this fanciful comparison, a typical neutron star is juxtaposed against Manhattan Island.

Once stars explode as supernovae, all nuclear events end. Calculations predict that the remnant neutron stars are probably solid objects, more like planets than stars. We might imagine standing on one, provided it has cooled enough, though it wouldn’t be easy; a neutron star’s gravity is unbelievably powerful. A person weighing an Earthly 70 kg (~150 pounds) would weigh the equivalent of ~109 kg (or a million tons) while on a neutron star. Actually, standing on one wouldn’t even be possible, for the severe pull of gravity would instantly squash a person to the thickness of a postage stamp. Gravity is so strong on a neutron star that the entire population of the world, if shipped there, would be crushed to a volume the size of a pea!

What’s more, newly formed neutron stars must rotate extremely rapidly, with periods averaging fractions of seconds; that’s because objects that shrink down almost inevitably spin up. And finally, any magnetic field embedded in the original stellar core is amplified during the compression of the progenitor star, reaching field strengths on the order of trillions of times those of Earth's magnetic field (and even millions of times those in solar flares). Odd objects to say the least, neutron stars represent states of matter unimaginably different from what we’re used to.

Evidence of Neutron Stars Can we be sure that objects as strange as ultradense neutron stars actually exist? The answer is again yes. In the past few decades, radio and x-ray astronomers have made some remarkable discoveries, proving that neutron stars are for real. These observers regularly monitor rapidly blinking stars, or pulsars for short, that emit short, repetitive bursts of radiation lasting ~0.01 second apiece. Each pulse contains a packet of energy, after which there is nothing; then another pulse arrives, again and again ad infinitum. We humans are insensitive to such rapid flashes (even when they occur optically), making it impossible to observe a pulsar’s flickering by eye, but suitably designed instruments can record the quick machine-gun-like signals. The time intervals between pulses are so astonishingly uniform—good to one part in 105 (thus the equivalent of losing ~1 second in 10 years)—that the repeated emissions can be used as a highly accurate clock. Figure 3.51 is a typical recording of the periodic radio radiation from such a peculiar object. Yet even these cosmic timepieces are not perfect, for they too change. Over billions of years, pulsars gradually slow down and die.

FIGURE 3.51 FIGURE 3.51 — Pulsars emit periodic, very accurately timed, bursts of radiation. This recording, taken from a video screen, shows the regular change in the intensity of the radio radiation emitted by one such object—the core remnant at the center of the Crab Nebula. (NRAO)

More than 1000 pulsars have been charted among the cindral remains of supernova remnants, though not all such debris fields have pulsars buried within them. Perhaps the latter are examples of those stars that did blow themselves totally to smithereens. The most prominently studied pulsar resides close to the center of the Crab Nebula, the site of a tumultuous explosion observed nearly 1000 years ago and ~6000 light-years away—therefore whose explosion actually took place ~7000 years ago. Figure 3.52 compares two optical photographs of the pulsar at the core of the Crab Nebula. In one frame, the pulsar is on; in the other, it's off. By determining the speed and direction of travel of the ejected matter observed for that supernova remnant, researchers have been able to trace the debris backward, pinpointing the location in space at which the progenitor star exploded. There the supernova core remnant is expected to be located. And that’s exactly the region in the nebula from which the pulsing signals arise. Apparently, pulsars are indeed the dregs of once-massive stars—in the case of the Crab, one that now launches a signal ~30 times each second.

FIGURE 3.52 — The pulsar at the core of the Crab Nebula blinks on and off with a period of ~0.03 second, or ~30 times/second. At the left, it's off, while at the right, it's on. (AURA)

Astrophysicists reason that the only physical process consistent with such precisely timed signals is a small, spinning source of radiation. Only rapid rotation—not pulsation—can cause the high degree of regularity of the observed pulses. Typically once around on its axis in about a second makes for a pretty amazing cosmic object, even more so for the Crab at ~30 times faster. And only an object <10 km in diameter can account for the sharp, crisp quality of each pulse; radiation emitted by a larger object would arrive at Earth at slightly different times, blurring the pulse to a droning hum. Normal stars, even white dwarfs, would be completely torn apart by such rapid rotation; they’re too big in size and too weak in gravity to hold themselves together. It’s hardly surprising then that the best theoretical model of a pulsar envisions a small, compact, spinning neutron star that periodically flashes radiation toward Earth. The experimentalist’s “pulsar” and the theoretician’s “neutron star” are synonymous.

Figure 3.53 is a stunning image of another supernova debris field, this one called Cassiopeia A, the site of a stellar explosion whose radiation first reached Earth about 300 years ago. Roughly 10,000 light-years from Earth, the whole region of still glowing gases shown here measures some 10 light-years across. The small turquoise dot at the center might be a neutron star created in the blast, the sole survivor of the explosion.

According to leading theoretical ideas, a “hot spot” on or near the surface of a neutron star, or perhaps in the atmosphere above it, continuously emits radiation in a sort of narrow “searchlight” beam. Figure 3.54 is an artist’s conception of this model. The spot could be akin to a violent surface quake or atmospheric storm much like flares on the Sun or volcanoes on Earth. Most likely, it’s a localized region near the neutron star’s poles, where charged particles, possibly ripped from a companion star and accelerated by intense magnetism to extremely high energies, emit radiation along the neutron star’s axis—not entirely unlike the active galaxies whose accretion disk around a central black hole spews forth fast, magnetically guided electrons into huge lobes along polar axes orthogonal to the disk. Although the energized spot sprays radiation steadily into space, the star’s spin rate of typically once per second guarantees that the emitted radiation in any direction behaves like a hail of discrete bullets. The radiation sweeps through space like a revolving lighthouse beacon or a spinning lawn sprinkler. Arriving at Earth, perhaps thousands of years later, the rapid pulses are captured by telescopes—provided that the star is oriented so that its beam sweeps past our planet. The details of the theoretical model are sketchy and controversial, for researchers have little hard information about the behavior of matter with densities great enough to border on the surreal.

FIGURE 3.53 FIGURE 3.53 — This image is actually a composite of three images taken by telescopes in orbit: optical light (shown in yellow) was acquired by the Hubble Space Telescope, x-ray radiation (blue and green) with the Chandra Observatory, and infrared radiation (red) with the Spitzer Space Telescope. The turquoise dot a center is probably a pulsar. (NASA)

FIGURE 3.54 FIGURE 3.54 — This artist’s conception of a popular theory of neutron-star emission can account for many of the observed properties of pulsars. As depicted, charged particles, accelerated by the magnetism of the neutron star, flow along the magnetic field lines, producing radio radiation that beams outward. (Prentice Hall)

Ultracompression Neutron stars are indeed outlandish objects. Even so, modeling stipulates them to be more or less stable, much like most other stars. In this case, however, neutron stars are not balanced by the inward pull of gravity battling the outward pressure of hot gas. As best we can tell, neutron stars have no hot gas. Instead, the outward force derives from the crystalline nature of the tightly packed neutrons themselves; quantum theory (the "Pauli principle") restricts the number of neutrons occupying any one space and that’s sufficient to buoy the star against its own gravity. Side by side and virtually touching one another, the neutrons form a solidified ball of matter resembling, but much denser than, the compacted electrons in a white-dwarf star. Compositional details differ with some models implying a crusty surface of hard iron ash, others claiming whole new superconducting materials, and still others proposing a core of unspecified “strange matter” that levitates the iron crust above it, but 20th-century science could not reach a consensus on these true oddballs. Any object having strong magnetism and rapid rotation will have an unsure status since modern physics is still unable to solve this "magnetohydrodynamic" problem, even with high-speed computers and assumptions of spherical symmetry. The essential feature of neutron stars is that their hyper-compressed neutrons generate enormous pressure that not even gravity can counter—with one notable exception.

Hypotheses have been advanced that galaxies ought to naturally house stellar core remnants with masses so large that the inward pull of gravity can in fact crush even a seemingly incomprehensible sphere of pure neutrons. According to some theories, should enough matter (>3 solar masses) be packed into a small volume (<1 kilometer), the collective efforts of gravity can overwhelm any countervailing force. In this case, gravity becomes powerful enough to compress an entire star into an object the size of a planet, a city, a pinhead, or even smaller!

As the core of a formerly massive star shrinks, the gravitational pull in its vicinity eventually becomes so great that light itself is unable to escape beyond its event horizon; the light would return much as baseballs fall back to Earth when thrown into the air. Such freakish objects are expected to emit no light, no radiation, no information whatsoever—providing grist for both natural scientists’ wildest models and science-fiction writers’ boldest dreams. Incommunicado, such a massive star effectively collapses onto itself and utterly vanishes—into a “hole” typically less than a kilometer across, but a hole nonetheless into which all nearby matter falls, trapped by its own gravity perhaps forever. These are the most bizarre end points of stellar evolution, encountered earlier on much larger scales in the GALACTIC EPOCH as candidates for the vibrant cores of active galaxies. These are the celebrated black holes.

Stellar Black Holes Here again is the sequence of events expected if these late stages of stellar evolution are valid: A star having >8 times the Sun’s mass ends its burning cycle by exploding as a supernova. Much of the star’s original content is ejected as fast-moving debris. Provided ~3 solar masses of matter remain behind in the core, the unexploded remnant collapses catastrophically, the whole core diving below the event horizon in less than a second. The core simply winks out—not merely becoming invisible but literally disappearing—leaving a small dark region from which no radiation or matter, indeed nothing at all, can escape. This is the way black holes are born as blackened domains in space. These bizarre end states of stellar evolution are not really objects as much as holes—black holes in the fabric of spacetime.

So what’s the story? Do black holes really exist, or are they nothing more than figments of theorists’ fertile imaginations? Maybe all massive stars are blown to bits when they explode as supernovae, never leaving much of a remnant core at all. Or perhaps another, as yet undiscovered force is capable of competing with gravity despite these extreme conditions of ultra-condensed matter. Each of these possibilities would preclude the existence of black holes—though what remained in their place would still be awfully bizarre. Just how much observational evidence is there for black holes?

In the previous GALACTIC EPOCH, black holes were invoked as the most likely answer to the vast energy pouring forth from the centers of galaxies and quasars. Despite the term black holes, the environments around such regions are expected to be intense sources of emission, the result of matter falling into the clutches of the holes themselves, heating and radiating all the while. In earlier times, when the Universe was more violent and not much matter had yet collected into organized structures, the conditions were ripening for the formation of supermassive black holes at the hub of the emerging galaxies. Astronomers now reason that what we perceive today at most galaxy centers—though little of it’s seen directly—are the galactic remnants of erstwhile holes, millions to billions of times more massive than for mere stellar remnants, some apparently still gulping matter. So it seems that the two kinds of black holes—the smaller stellar variety and the much larger galactic variety—form in different ways, though our understanding of the latter is especially shaky. In truth, despite a growing consensus in the astronomical community that black holes are probably real, we are unsure of how good the theoretical models are.

Black holes may be invisible, but that doesn’t mean they lack gravity; they still house mass and thus still exert gravity. Accordingly, astronomers can test for holes’ existence by probing the gravitational force in and around the space near them. For example, the motion of a spacecraft or of a nearby celestial body could conceivably be used to study the nature of a black hole. Any object outside an event horizon should behave just as though a massive, visible object resided at the site of the hole. In other words, all conventional (electromagnetic) means of assessing a black hole are moot, yet its gravity persists. If astronomers could find some small, neighboring black holes, perhaps we could examine, if not them directly, then at least their environments. The closer they are to us, preferably in our own Milky Way, the better we should be able to resolve their external features, infer their internal mass, and test our ideas about them.

Surely, our civilization doesn’t have the capability to maneuver spacecraft into the neighborhood of suspected black holes, even if we knew their exact locations. However, the Galaxy is populated with many binary-star systems whose two members orbit about one another. Yet for many of these only one star is visible, the other betrayed only by indirect means such as periodic spectral shifts or starlight dimming. Of course, each unseen companion could be just a dwarf star hidden in the glare of a bright stellar partner. Or the unseen object could be shrouded by interstellar dust, making it invisible to equipment on Earth, but not necessarily indicative of a black hole. And these alternatives are probably true for the majority of binaries having an unrevealed member, in which case the invisible candidates are not black holes.

A few of these atypical binary systems, however, display peculiar properties strongly suggestive of black holes. Pioneering observations, made by Earth-orbiting satellites as long ago as the 1970s, revealed binaries that emit copious amounts of x-ray radiation. This high-frequency emission cannot easily penetrate dust, making it unlikely that galactic debris has camouflaged one of the systems’ partners. More recently, advanced satellites equipped to pinpoint x rays from suspected targets have been monitoring just this type of radiation from several binary systems that show only one star visually. For each, the x-ray radiation arises from 106 K gas flowing from a large visible star toward a small, invisible companion. Furthermore, each invisible member harbors several solar masses and spans <100 km. That’s several times the mass of our Sun all compacted within the size of a large city. These properties have all the earmarks of black holes.

Figure 3.55 shows an area of the sky where astronomers have reasonably good evidence for the nearest and best-case “classical” black hole of stellar (i.e., not galactic) dimensions. This part of the sky was known to the ancients as the Cygnus region, a name still used today. The rectangle outlines the celestial system of interest, ~7000 light-years from Earth. The main features of many observations of this system are as follows:

  • Spectroscopy of the visible radiation implies the bright object—a blue B-type star with the catalog name of HDE226868—to be only one member of a binary-star system whose orbital period is nearly a week and size is ~0.2 astronomical unit
  • Other spectral-line observations suggest that hot gas is flowing from the bright star toward an unseen companion, called Cygnus X-1
  • This invisible object has a mass of ~10 solar masses, as derived from knowledge of the binary system's orbital size and period
  • X-ray radiation, emitted from the immediate neighborhood of Cygnus X-1, suggests the presence of scalding gas, perhaps as hot as 107 K (see Figure 3.56)
  • Rapid time variations of this x-ray radiation imply a size for Cygnus X-1 itself of <100 km across.
FIGURE 3.55 FIGURE 3.55 — The brightest star in this photograph is a member of a binary system whose unseen companion, called Cygnus X-1, is thought to be a good candidate for a black hole. (The rectangle outlines the field of view illustrated in the next figure.) (Harvard Observatory)

FIGURE 3.56 FIGURE 3.56 — Invisible x rays emitted near the Cygnus X-1 source can be analyzed by changing the detected x rays into electronic signals, which can then be viewed on a video screen from which this picture was taken. (The field of view here is outlined by the rectangle in the previous figure.) (Smithsonian)

A tentative model for Cygnus X-1 stipulates that much of the gas drawn from the visible star flows onto a nearby, tire-shaped oval of matter. Although invisible black holes can neither be actually seen nor hardly imagined, Figure 3.57 is a conception by an artist who has worked closely with leading researchers. It shows an accretion disk where the superheating occurs and the x-ray radiation is launched, often variably thus implying that the unseen companion is very compact—presumably a neutron star or black hole. If the latter, some of this gas inevitably streams toward the black hole, sucked ever deeper into its whirlpool and eventually trapped forever by it.

FIGURE 3.57 FIGURE 3.57 — Artist's conception of a binary system containing a large, bright, visible star, and an invisible, x-ray emitting black hole. This particular painting depicts the Cygnus X-1 region discussed in the text. (Lola Chaisson)

A handful of other, stellar black-hole candidates are frequently monitored in or near our Galaxy, each of them displaying observational traits similar to those of Cygnus X-1. And far beyond, extremely intense gamma-ray bursts have been detected nearly daily by civilian and military satellites during the past few decades, possibly the result of two neutron stars first colliding and then triggering a supernova (or “hypernova”) that instantaneously releases monstrous flashes of narrowly collimated energy just before the whole mess core-collapses into a black hole. But nagging problems plague most of these interpretations as black holes, not the least being that all the suspected holes in binary systems have masses close to the neutron star-black hole dividing line of a few solar masses. When the effects of rotation and magnetism are fully incorporated into the physics of dead stars—tasks untenable at present—there’s a chance that the dark objects in question may turn out to be more mundane neutronlike stars and not foreboding black holes at all. Or perhaps something else entirely.

Alternatives “Quark stars” come to mind as cutting-edge alternatives to black holes—and that may be where they will stay as none has yet been unambiguously found anywhere in Nature. Such strange objects, purportedly at least 1000 times denser than neutron stars, or ~1015 (or a quadrillion) times more compact than in the core of our Sun, might nonetheless be a substitute for black holes. A quark star would form when neutronic matter in a neutron star cracked under unbearable strain, splitting many of the neutrons back down into their constituent quarks while compressing the star to the absolute limits of known physics, yet without, however, plunging it indefinitely into a black hole that seems beyond physics. Strange quarks, strange stars; this matter, if real, has reverted to the form in which it was born less than a microsecond after the big bang.

That said, the payoff would be great if astronomers can demonstrably prove the existence of a nearby black hole. It would become a virtual laboratory in space for the study of bizarre states of matter ruled by quantum gravity. Such peculiar phenomena so foreign to our everyday senses are thought to mimic, more than any other astronomical entity, the decidedly unearthly conditions prevalent near the very start of the Universe. Ultimately and ironically, these burned-out corpses of massive stars may someday help us comprehend one of the foremost quandaries in all of science—the emergence of that singular, superhot, superdense state from which the Universe itself originated long ago.


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