How do stars die? What do they become at the end of their long and brilliant existence? The answers rely partly on computer modeling and partly on what’s seen in the sky—the usual marriage of theory and observation. The details of this topic, quite frankly, are once again tricky in that no one has witnessed a nearby star die since the invention of the telescope more than 4 centuries ago. Guided by theoretical predictions of how stars ought to behave near (and after) death, astronomers search the Universe, seeking evidence of objects resembling the predicted hulks.

The best models hold that the final stages of stellar evolution depend critically on the mass of the star. As a rule of thumb, low-mass stars die gently, whereas high-mass stars die violently. The dividing line between these two very different outcomes lies around 8 times the mass of the Sun. Since a Milky Way census shows that hardly 1% of all stars have more than this mass, our Sun and the far majority of stars are members of the low-mass category. Only rare stars much larger than our Sun are grouped in the high-mass category.

The demise of our Sun is destined to be straightforward and unspectacular. The Sun’s core will become extremely hot and compact as it heads toward its end state. A single cm³ of stellar core matter would equivalently weigh a ton on Earth. That’s 1000 kg of matter compressed into a volume the size of a pea. Yet, even at these very high densities, collisions among nuclei are insufficiently frequent and violent to raise the temperature to the extraordinarily high 600 million kelvins needed to ignite a new round of nuclear reactions, and thereby change carbon into any of the heavier elements. There’s simply not enough matter in the overlying layers of the smaller stars to bear down any harder. The density reaches maximum compression, the temperature stops rising, and oxygen, iron, gold, uranium, and many other elements cannot be created in low-mass stars.

Planetary Nebulae Small stars like our Sun manage to work themselves into quite a predicament in their old age. Their carbon core is, for all intents and purposes, dead. Helium just outside the region of carbon ash continues to transform into more carbon, while hydrogen in the intermediate layers above converts into more helium. This onslaught of heating slowly pushes away the outermost layers to even greater distances. The expected result is an object of distinctly odd posture having two separate parts—both of them comprising stage 12 of Table 3-2. Called a planetary nebula, it’s predicted to have a halo of warm, rarefied matter veiling a hot, dense core. Figures 3.29 and 3.30 show two examples.

FIGURE 3.29 — This planetary nebula, called the Ring Nebula, resides ~5000 light-years away in the constellation Lyra. Its apparent size equals about one-thousandth that of the Moon, though it's actually much larger than our Sun or Solar System, in fact spanning ~1.5 light-years. The nebula is shown here in true color, but is too dim to see with the naked eye. The arrow points to its central, white-dwarf remnant. (STScI)

FIGURE 3.30 — Called the Spirograph, this planetary nebula has expanded beyond its red-giant phase only during the past few thousand years, revealing a bright core, or white-dwarf remnant, at its center. This so-called planetary nebula, now >10 times the size of our Solar System, was once a middle-aged object like our Sun. It’s a good example of the likely fate of Ole Sol ~5 billion years from now. (STScI)

Nebula is Latin for “mist” or “cloud” of great extension and extreme tenuity, but planetary nebulae shouldn’t be confused with the even larger galactic nebulae noted earlier in this STELLAR EPOCH. Galactic nebulae are signposts of recent stellar birth; planetary nebulae are indicators of impending stellar death. The adjective “planetary” is also misleading, for these celestial objects aren’t related to planets in any way. Their designation dates back to the 18^th century, when astronomers could barely distinguish among the myriad faint, fuzzy patches of light in the nighttime sky, and some observers mistook them for planets. Later studies clearly demonstrated that the nebula’s fuzziness results from shells and rings of warm gas surrounding a small luminous object. Modern telescopes well resolve planetary nebulae, enabling us to recognize their true nature.

Odd or not, nearly 1000 examples of planetary nebulae have been discovered in our Galaxy alone. Though they sometimes appear to have a ring surrounding a bright core, their halo-shaped appearance is often an illusion owing to their emitting gas accumulated along our line of sight (Figure 3.31). Direct observations confirm the theoretical predictions that their expelled matter consists of a mostly spherical envelope in the act of gently escaping from the core of an aged red-giant star. Exceptions abound and weird patterns are common, as the star’s rotation and the irregular environment into which they expand often distort the receding gaseous shell, making some planetary nebular shapes, well . . . nebulous. A few planetaries even have illuminated jets, streams, and spirals of gas emanating from near their aged cores, sporting peculiar geometries thus far unexplained (Figure 3.32).

FIGURE 3.31 — A planetary nebula, with a spherical outer shell or 3-dimensional envelope sketched at right, appears to the eye as a small star with a halo around it. Very little of the ring emits along the central line of sight over path A, whereas more gas emits from the edges along paths B and C, causing such objects to often appear ring-like. The real object at left is called Abell 39 and is ~6500 light-years away and ~5 light-years across. (AURA)

FIGURE 3.32 – The Cat’s Eye planetary nebula, ~3300 light-years away and ~0.3 light-years across, is an example of a much more complex planetary nebula, possibly produced by a pair of binary stars (unresolved at center) that have both shed envelopes. (STScI)

One final note on planetary nebulae: The recessional motions of a red-giant star's outermost layers are initially caused mainly by nuclear burning in the middle layers between the stellar core and its periphery. Later, the steady outward movement of the envelope results from the process of electrons recombining with newly formed atoms, thereby emitting photons. Repeating the process over and over, these photons then ionize new atoms farther out, which eventually recombine to emit more photons, which in turn ionize new matter, and so on. This runaway process of atom ionization, electron recombination, and photon emission serves to steadily push portions of the gaseous envelope to greater and greater distances from the core.

Red Giants --> White Dwarfs The evolution of a nebula’s expanding envelope isn’t very interesting thereafter. It simply continues spreading out as time passes, becoming evermore diffuse and cool, and gradually merging imperceptibly with the interstellar medium. Its most important role is to enrich space with additional helium atoms and possibly some carbon atoms as well.

Continued evolution of the core remnant at the center of a planetary nebula is also rather boring. Formerly concealed by the atmospheres of red-giant stars, such cores appear once their flimsy envelopes have receded and thinned. These cores are relatively small, glowing objects, highly abundant with very hot carbon, but not nuclear burning. They shine only by their stored energy, though their small size and intense heat guarantee a white-hot appearance. Not much bigger than planet Earth, these shrunken carbon cores—balls of nuclear wastes, really—are called white-dwarf stars. They, too, are seen throughout our Galaxy.

Analysis of radiation emitted by white-dwarf stars shows their properties to agree well with the computer models of elderly, low-mass stars. Scores of dwarfs are found at the very centers of planetary nebulae, but only one per nebula. Table 3-2 lists their properties at stage 13, and part of the dashed line in Figure 3.33 depicts the evolution from red giant to white dwarf. The long trek across the HR diagram results from the steady transformation of a large, cool (red-giant) star into a small, hot (white-dwarf) star. This large evolutionary change between stages 9 and 13 is caused by the expansion and dispersal of a red giant's outermost layers, the result being that the only thing remaining is its former core, namely the white-dwarf star.

FIGURE 3.33 — The change of a red-giant star (stage 9) to a white-dwarf star (stage 13) creates an evolutionary path clear across the HR diagram. (Lola Chaisson)

FIGURE 3.34 — The Sirius B white-dwarf star (speck of light at right) is seen here as a companion to the much larger and brighter Sirius A star. The hexagonal shape of the image of Sirius A is not real; the "spikes" are artifacts caused by the support struts of the telescope. (AURA)

Not all white-dwarf stars are found as cores of planetary nebulae. Several hundred additional ones have been discovered “naked” in our Galaxy, their envelopes apparently expelled to invisibility long ago. Figure 3.34 shows the most famous of all white dwarfs, Sirius B, the dim binary companion to the much brighter Sirius A (yet about which, as noted in the previous section, nagging problems remain). Detailed observations show this white-dwarf star to have the following properties, compared to the Sun:

• mass = 1.1 solar mass
• radius = 0.007 solar radius
• luminosity = 0.003 solar luminosity
• surface temperature = 4.5 solar surface temperature
• average density = 30,000 solar core density

Note that Earth's size equals 0.009 solar radius, making our planet larger than the star Sirius B! Thus this white dwarf has more than the mass of our Sun packed into an object smaller than our planet; no wonder its density is nearly a million times anything familiar to us on Earth.

Dark Clinkers Again So astronomers readily identify red-giant stars, planetary nebulae, and white-dwarf stars in the nearby cosmos. At different stages in their old age, each of these objects seems to match the overall disposition predicted by the theoretical calculations for ancient low-mass stars. Once again, though, we shouldn’t expect to witness the act of envelope expulsion during the course of a single human lifetime. Several tens of thousands of years are typical for a red giant’s atmosphere to recede sufficiently for a white dwarf to become visible.

Nothing exciting befalls dwarf stars thereafter. For all practical purposes, these “stars” are dead. They continue to cool, becoming dimmer with time while slowly transforming from white dwarfs to yellow dwarfs and then red dwarfs. Their temperatures and luminosities change according to the dashed line near the bottom of the HR diagram of Figure 3.33. Their final state is that of a black dwarf—a cold, dense, burned-out ember in space. Such stellar corpses have reached stage 14 of Table 3-2, the graveyard of stars.

No one knows how many black dwarfs populate our Galaxy—which isn’t surprising since they’re unlit. They’re also in that hard-to-probe size range between normal bright stars, on the one hand, and atoms and molecules, on the other. Even if these dark clinkers could somehow be detected, we would probably find few of them. The total duration of a low-mass star is very long, typically comparable to or longer than the age of the Galaxy. So don’t expect them to solve the dark-matter problem. Our Milky Way hasn’t likely yet endured enough for many low-mass stars to have completed the whole stellar-evolutionary trek from birth to death. Perhaps none has.

High-mass Death Different fates await stars having >8 times the mass of our Sun. By and large, these bigger stars evolve much like their low-mass counterparts up through the red-giant stage, with only one difference. All the evolutionary paces occur more quickly for the high-mass stars because their greater mass enables them to generate more heat. As before, it’s gravity that creates that heat and the subsequent energy flows speed all evolutionary events. That’s why the biggest stars, consuming fuel at prodigious rates, endure for shorter periods, in fact well less than a billion years. Some of those containing tens of solar masses last for as little as 10 million years, or 1000 times less than the Sun—a mere cosmological wink of an eye, yet still >100,000 human lifetimes.

At the red-giant stage, the core of a high-mass star is able to attain the 6x10⁸ K to begin fusing carbon into even heavier elements. Again, mass is the key. Truly massive stars generate stronger gravitational forces than solar-type stars, and the added gravity can crush matter in the core to a high enough density to ensure frequent and violent collisions among the gas particles.

Theoretical models indicate that highly evolved stars of large mass have several internal layers where various nuclei burn simultaneously. The insides of such stars even more so resemble an onion, as illustrated by Figure 3.35. At the relatively cool periphery just below the surface, hydrogen fuses into helium. In the middle layers, helium and carbon fuse into heavier nuclei. Just above the core, magnesium, silicon, sulfur, and many other heavy nuclei are present, and some of these in turn fuse into even heavier nuclei. As the temperature dramatically rises with depth, the ash of each burning stage becomes the fuel for the next stage. The core itself is full of iron nuclei, rather complex pieces of matter each containing 26 protons and 30 neutrons, midway between the lightest and heaviest of all known nuclei.

FIGURE 3.35 — Cutaway diagram of the interior of a highly evolved star having a mass greater than 8 solar masses. Though this is a simplified sketch, in many ways the interior resembles the layers of an onion. (Prentice Hall)

Each of these fusion cycles, during which nuclei for new elements are created at various depths in a star’s interior, is induced by periods of stellar instability. The core cools somewhat, contracts a little, heats some more, fuses heavy nuclei, and depletes its fuel, after which the cycle starts over by contracting again, heating again, fusing again, and so on. At each stellar-burning stage, energy is released as a by-product of the fusion process, effectively supporting the star (at least for a while) against gravity. And at each stage, as the star’s interior evolves, its fusion rate accelerates. For example, for a star ~20 times more massive than the Sun, hydrogen burns for ~10 million years, helium for ~0.5 million years, carbon for ~1000 years, oxygen for ~1 year, and silicon for about a week.

With iron accumulating in the core, complications quickly develop for this sick and dying star in less than a day. Nuclear physicists say that iron has the “highest nuclear binding energy,” meaning that iron is the most stable of all the elements. Consequently, nuclear events involving iron don’t produce energy; iron nuclei are so compact that they can only consume energy. That’s because further contraction actually breaks down the iron back into helium, absorbing energy instead of emitting it. In lay terms, iron nuclei play the role of fire extinguisher, suddenly damping the stellar inferno, at least at the core. With the buildup of iron, the central fires quench and the nuclear events cease for the last time.

Potential for disaster now clearly exists. No longer is this very massive star upheld by nuclear fusion at its core. The star’s foundation is gone, its structural stability destroyed. Although the temperature in the iron core has by this point reached several billion kelvins, the strong and suddenly unopposed gravitational pull of the great mass of overlying matter ensures disaster in the very near future. Unless nuclear events continue unabated, trouble is a certainty for any such defunct star.

Once gravity overwhelms the pressure of the hot gas, the star implodes, falling in on itself. The implosion doesn’t take long, perhaps only minutes after cessation of core kindling; this isn’t a gentle contraction as much as a catastrophic collapse. Internal temperatures and densities then rise phenomenally, causing (in part) the star to rebound instantaneously like a coiled spring, detonating parts of the core while jettisoning all the surrounding layers. The details of how such a massive star physically rebounds like this aren’t well known, but the outcome surely is: Much of its mass—including a variety of heavy elements cooked within—expels into neighboring regions of space at speeds initially reaching tens of thousands of km/s (or nearly 100 million mph). The expulsion is much, much more violent than that for a planetary nebula; this is a titanic event, since much of an entire star has literally exploded. All stars much larger than our Sun are slated to perish in this way. Such a spectacular death rattle is known as a supernova.

Supernovae Nova is Latin for "new," though celestial novae aren’t really new stars at all. Their sudden brightening only made them appear so to observers centuries ago. Figure 3.36 is a photograph of a nova—a star that quickly brightens while ejecting a small fraction of its matter. The origins of such stellar expulsions aren’t entirely understood, although they’re probably caused by intense gravitational tides exerted on some stars within multiple-star systems. The result is a temporary instability causing a violent eruption of matter from the star's surface. Observed to brighten by ~10,000 times the luminosity of our Sun, novae eventually dim back to normal after many months to a year. Some such stars—“recurrent novae—have been observed to repeatedly brighten several times over the course of several decades.

FIGURE 3.36 — Small amounts of hot matter thrown from an old star can brighten the star. Novae, such as this one called Nova Persei, experience small-scale expulsions of their surface gases. This photo was taken about 50 years after its initial outburst, showing that the star survived the blast. (Palomar Observatory)

Supernovae are much more violent than novae. They’re likely the most tumultuous events in any galaxy, indeed among the most energetic in all the Universe. The exploded stellar debris is intensely hot and altogether can radiate a flash equal to >10⁹ times the brightness of our Sun. This amounts to a single star suddenly rivaling the brightness of nearly our entire Milky Way Galaxy within a few hours after its outburst. Eventually, as in any explosion, the surge subsides and the debris cools, but not before the galactic neighborhood has been irradiated with plenty of potent energy and heavy elements.

Astrophysicists are unsure of precisely when and how supernovae explode because a nearby star hasn’t erupted in this way since early in the 17^th century. The last one to do so in our Milky Way occurred in 1604, shortly before the invention of the telescope. Nor are the theoretical models entirely clear on the intricate details of the explosion. Glibly stated: How does most of an entire star not just implode catastrophically, but completely reverse that sudden implosion by exploding dramatically?

Supernova models imply that, while heavy elements such as carbon, nitrogen, oxygen, sodium, magnesium, silicon, and much of everything up to iron are produced in stellar interiors, the explosion itself is responsible for elements heavier than iron. At the moment of detonation and for ~15 minutes thereafter, intermediate-weight nuclei are fiercely jammed together, thus creating some of the heaviest of all nuclei; neutron capture creates the rest. Many of the rare elements are synthesized at this time, including silver, gold, uranium, and plutonium. Matter most valued by society on Earth, including all the precious metals, therefore originated in the very last gasps of shattered stars once big and bright, though now dead and disintegrated. Ironically, the heaviest elements, including all the radioactive types, are made only after their parent stars have perished. But because the time available for making those heavies is so brief, elements heavier than iron are billions of times less abundant than most light nuclei—which is precisely what makes many of them so valuable.

The sprinkled debris of erstwhile stars then mingles with fresh interstellar hydrogen and helium made during the earliest PARTICLE EPOCH of the Universe. This messy mixture of all the elements can then undergo contraction, heating, and fusion yet again, thus fabricating 2^nd-, 3^rd-, and N^th-generation stars in a seemingly endless cycle of birth, death, and rebirth. Our own Sun is at least a 2^nd-generation star, for it already contains heavy elements, including lots of iron. Since these heavies couldn’t have been made in a low-mass, relatively cool star like the Sun, they must be the products of formerly massive stars that exploded long ago.

How do we know that stars really do create heavy elements in this way? Can we be sure that the theory of stellar nucleosynthesis is correct? One piece of circumstantial evidence is in hand and one item of direct support is telling, in addition to the obvious wreck of the exploded debris itself as seen at numerous places on the sky. First, the rate at which various nuclei are captured and the rate at which they decay are known from laboratory experiments of the past few decades; some of this work was done to support America’s nuclear weapons program. When all these rates are incorporated into sophisticated computer models, which also take account of the temperatures, densities, and compositions at many layers within a massive star, the relative amounts of each type of synthesized nucleus match remarkably well the known abundances of intermediate-weight elements up to and including iron. Thus, despite the fact that no one has ever directly observed atomic nuclei in the act of production—other than the trace debris collected after nuclear bomb tests on Earth—the agreement between theory and observation is striking. We can thus be reasonably sure that Nature’s way of making elements is well understood, given our knowledge of nuclear physics and stellar evolution.

Second, close study of one type of nucleus—a rare and unstable one named technetium—provides direct evidence that heavy-element formation really does occur in massive stars. This nucleus, well heavier than iron, is known from laboratory measurements to have a radioactive half-life of ~200,000 years. This is a very short time astronomically speaking, hence the reason why no one has ever found even traces of naturally occurring technetium on Earth; all of it decayed long ago. (It can, however, be studied as a newly created by-product of nuclear reactions in laboratory experiments.) By contrast, the identification of technetium in the spectrum of many red-giant stars implies that it must have been created within the past million years or so. Elemental production is indeed underway in stars today.

FIGURE 3.37 — The "light curve" of typical supernovae. The maximum brightness or intensity can sometimes reach that of a billion suns.

And finally, the blast signatures of a detonated supernova and that of a nuclear bomb are virtually identical. The flash of a supernova (called its “light curve”—see Figure 3.37) displays a rapid rise in luminosity near the moment of explosion, followed by a steady decrease in brightness over the next few weeks and then a notably slower dimming for years thereafter. The peculiar reduction of the light owes mostly to the radioactive decay of unstable nuclei produced in the fireball itself. Studies of the flash and decline of light in the aftermath of thermonuclear weapons tests on Earth imply that the process is one and the same—though, thankfully, the man-made version of much less magnitude. As destructive as bombs are, they help us understand Nature. Destructive as supernovae are, they help enrich Nature.

Observational Evidence Supernovae are not just idle predictions of theoreticians. Plenty of evidence has been amassed that cosmic explosions have occurred throughout the ages. One of the most heavily studied supernova remnants is the Crab Nebula, so aptly named largely because its appearance resembles that type of marine animal (Figure 3.38). About 6000 light-years from Earth and in the constellation Taurus, its glowing debris is strewn over ~6-light-year extent—the result of nearly having blown itself to smithereens. Now greatly dimmed, the Crab, as it’s known for short, can be seen only through a telescope. But the measured motions of its expelled matter—still now racing outward at ~1000 km/s (or ~2 million mph)—imply a brilliant explosion whose light must have been easily seen with the naked eye nearly 1000 years ago.

Although nova indeed means “new,” modern astronomers realize that the sudden brightening of supernovae, briefly changing from near invisibility to great prominence, only made them seem so. The original explosion of the precursor to the Crab Nebula was so spectacular that old Asian and Arab manuscripts claim its brightness surpassed that of Venus and even rivaled that of the Moon in the year A.D. 1054. Native Americans also saw it, having left engravings of the event in the rocks of what is now the mid-western United States.

FIGURE 3.38 — This remnant of an ancient supernova is the Crab Nebula (catalogued as M1), for its glowing debris today resembles that type of marine species. The nebula now covers an area only one-fifth the diameter of the full Moon, but at its distance of ~6000 light-years the debris is actually scattered over several light-years and is rich in heavy elements. (ESO)

The image of the Crab Nebula certainly does give the impression of an exploded debris field. What's more, in recent decades astronomers have proven just that and also measured the ejected matter. Figure 3.39 is a superposition of a positive image of the Crab Nebula taken in 1960 and a negative image taken in 1974. If the filamentary structure of the gas were not in motion, the positive and negative images would overlap perfectly. But they don’t. Clearly, some gas has moved outward in the intervening 14 years. By knowing the total distance traveled by the gas in this interval of time, a velocity of several thousand km/s has been derived for the expelled debris. These outward motions confirm that an explosion must have occurred just about a millennia ago.

FIGURE 3.39 — Positive and negative photographs of the Crab Nebula taken 14 years apart do not superpose with precision, proving that the former star's debris is still moving away from the site of the explosion. (Harvard College Observatory)

Numerous other massive stars must have self-destructed in earlier times. Written documents refer to relics of at least a dozen supernovae in our Galaxy within the past several thousand years. The nighttime sky harbors additional evidence for many wispy remnants of former stars that must have blown up well before the advent of recorded history. The closest of these was probably the Veil supernova, whose remains lay a mere 1500 light-years from Earth, as shown in Figure 3.40. Astronomers can still see that remnant well enough to measure its expansion velocity and thus to infer that its progenitor star must have detonated around 18,000 B.C. Based on the amount of matter strewn throughout its debris field, this supernova likely shone brighter than the full Moon.

Figure 3.41 is another supernova remnant in our Milky Way. This one, called the Gum Nebula, has expansion velocities implying that a star blew up around 9000 B.C. Given the close proximity of the exploded star (only ~1000 light-years), we can only speculate what impact such a suddenly luminous orb might have had on the myths, religions, and cultures of Stone Age humans.

FIGURE 3.40 — This supernova remnant, called the Veil Nebula, is much older than the Crab Nebula. Its expelled gases extend across >150 light-years. Note the complex gas motions, as the ejected matter pushes out into the surrounding interstellar space. (AURA)

FIGURE 3.41 — The glowing gases of the Gum Nebula supernova remnant are spread across anamazingly large 60 arc degrees. The closest edge of the expanding shell is only 300 light-years away. (AURA)

The most recent supernovae observed in our Galaxy caused a sensation during the Renaissance and helped overthrow the leading philosophy of the time. Oddly, the earlier Crab Nebula had apparently gone unnoticed, or at least unrecorded, by Europeans several centuries before. Perhaps the influence of the Church was so strong and its dogma of immutability so rigid that faithful (or fearful) citizens simply put it out of their minds. But the sudden appearance and subsequent fading of very bright stellar objects in the years A.D. 1572 and 1604 were unavoidably noticed—the latter visible during the day for weeks—and together they shattered the Aristotelian idea of an unchanging Universe beyond the Earth. Little did anyone then realize that these brilliant flashes in the heavens—now known as Tycho’s and Kepler’s stars, respectively—provided the mental seedlings for the eventual emergence of the scenario of cosmic evolution, wherein the concept of change is central and ubiquitous in the Universe.

Nowadays, hundreds of supernovae have been sighted in other galaxies—many of them momentarily as bright as their parent galaxies. Astronomers patrolling the skies on any given night often notice a sudden brightening of a portion of some faraway galaxy, enabling them not only to verify that high-mass stars are common to all galaxies, but also to refine the predictions of the stellar-evolutionary models. Figure 3.42 is an example of photographs taken a few months apart and clearly showing a sudden brightening large enough to rival the normal luminosity of an entire spiral arm of this distant galaxy. If a month-long videotape could be made of a distant galaxy cluster, we would see supernovae popping off like minute and silent flashbulbs in a darkened stadium. Disconcertingly, however, humankind has been unable to inspect a supernova closely since a massive star in our own Milky Way hasn’t viewably detonated in this way during all of the previous four centuries when telescopes have been in use.

FIGURE 3.42 — A supernova can be seen exploding in this far-away galaxy at the moment the photograph on the right was taken. The photograph on the left is the normal appearance of the galaxy. (AURA)

SN1987A Much more recently, astronomers were treated to a spectacular supernova in the Large Magellanic Cloud, that small galaxy ~170,000 light-years away yet orbiting our own Milky Way. Amateur astronomers in Chile first spotted in it the winter of 1987 and within a few hours most of the world’s telescopes, on the ground and in orbit, had focused on the suspect object. SN1987A, as it’s called, was one of the most dramatic cosmic changes observed in the past 400 years. Apparently, a 15-solar-mass, B-type supergiant star with the peculiar catalog name of SK-69 202 had detonated, thereby momentarily outshining all other stars in that dwarf galaxy, as shown in Figure 3.43 before and after the explosion. By and large, the observed properties of this celestial explosion agree well with our computer models of stellar evolution, including the detection of a short burst of neutrinos that reached Earth nearly coincident with the initial flash of light. As noted below, these neutrinos—the same kind of poorly understood particles implicated in the solar mystery discussed earlier—might well have triggered the rebound of the infalling star, causing the stupendous explosion whose remnants astronomers are still studying today.

FIGURE 3.43 — The supernova SN1987A (arrow) had exploded near this nebula (called 30 Doradus, a star-forming region in the Large Magellanic Cloud) only minutes before the photograph on the right was taken. The photograph on the left is the normal appearance of the star field prior to the explosion. (AURA)

Among the important information provided by SN1987A are these:

• Light radiation that beamed forth displayed a rather baffling pattern. After fading gently, the supernova brightened rapidly; within hours of its initial detection, it had become ~100 times brighter than its progenitor, SK-69 202. It then continued to increase, though more slowly, for the next few months, before fading ~4 months after its initial outburst. This erratic behavior might be due to the decay of radioactive material supplied to the supernova's expelled cloud, or it might have been caused by opacity differences in the surrounding material. The debris is now being tracked by the Hubble Space Telescope at ~6-months intervals, showing real changes in the remnant as much of the former star races outward from the explosion at a fast clip of ~3000 km/s (or ~6 million mph). Within a few months of the explosion, its debris had reached Solar System dimensions, and within a decade it was spread across nearly a light-year.
  
• Radio waves were picked up by Australian radio telescopes during the first few days after the explosion, but few have been detected since. This long-wavelength radiation was non-thermal, resulting from fast-moving electrons spiraling in magnetic fields shortly after the outburst; eventually the particles slowed and the field became dilute as the remnant spread out. After initially decreasing, the intensity of ultraviolet radiation steadily rose for a few months and was monitored by the International Ultraviolet Explorer satellite in Earth orbit. This short-wavelength radiation was probably blocked by dust and used to heat the debris during the first few days, after which it shone through. Neither x rays nor gamma rays were detected by an array of U.S., Soviet, and European spacecraft, perhaps again due to obscuration by dust and debris of the remant. X rays and gamma rays will likely arrive at Earth at a later date, as the dissipating shell of brilliantly glowing debris slams into the surrounding interstellar medium, thus causing additional fireworks. Indeed, in 1999, the fastest-moving ejecta began doing so.
  
• A brief (~13-second) burst of neutrinos was recorded nearly simultaneously by underground detectors in Japan and the U.S. The neutrinos are predicted to arise as the electrons and protons in the star's collapsing core merge to form neutrons and neutrinos, as discussed more in the next section of the STELLAR EPOCH. Despite some unresolved details in SN1987A's behavior, detection of its neutrino pulse is considered a brilliant confirmation of theory; in fact, this singular event might well herald a new age of astronomy, since this is the first time that astronomers have received information from beyond the Solar System by any means other than electromagnetic radiation.

Nearby Supernovae Not everything about supernovae is well understood. Certainly their infrequence in our part of the Milky Way is a bit disturbing. Knowing the rate at which stellar evolution is thought to occur and estimating the number of massive stars in the Galaxy, we expect a supernova to occur in an observable location (away from the dusty parts of the Milky Way’s plane) every century or so. Yet, only 6 such galactic supernovae have been recorded in the past 1000 years, and none at all in the past 400 years. Since it’s hardly likely that any such garish explosions could have been missed since the last one several centuries ago, the Milky Way seems long overdue for a blast from the past. Unless massive stars explode much less frequently than predicted by theory, we should be treated to one of Nature’s most spectacular exhibitions any day now. Let’s just hope it’s not too spectacular—or too close!

Supernovae may be more than splendid light shows. Should a massive star detonate in the galactic suburbs where our Sun resides, it could well inundate Earth for months with radiation and matter harmful to life. An initial pulse of high-energy x and gamma rays, followed quickly by neutrinos as well, would impact our planet suddenly, certainly destroying (or badly damaging Earth’s ozone layer), probably making our atmosphere radioactive, and possibly allowing sunlight to fatally fry most life on the surface. The topmost layers of massive stars are also physically ripped off and sent flying into space as extremely fast-moving elementary particles, known as cosmic rays, which would arrive somewhat later and for an extended period (perhaps decades) of bombardment. Such violent events might have triggered episodes of mass extinction as revealed by Earth’s fossil record over the past few hundred million years—yet another fertile area of research in which astronomy and biology meet in the interdisicipline of astrobiology. Knowledge of the nearby stars—especially their masses—is thus of more than just passing interest. An ability to predict the manner in which nearby stars die is downright critical. Of particular concern is the possibility that one of our stellar neighbors might explode as a supernova, although we probably couldn’t do much about it even if one did.

Statistics of the stars in our galactic neighborhood imply that one supernova can be expected within ~300 light-years of the Sun once every 0.5-million years, or within ~30 light-years once every 0.5-billion years. Too close for comfort? Fortunately, none of Earth’s currently nearby stars is massive enough to die by self-detonation. (Betelgeuse, the big red star in the Orion constellation, is probably the best nearby candidate to go supernova, and it's ~420 light-years away.) Luckily for us, they all seem destined to perish, as will our Sun, via the more placid red giant --> white dwarf route.

Fascinatingly, a viewable massive star has almost surely already exploded, but the light from this stupendous event has yet to reach us. Owing to the finite speed of light, some of the brightest stars now seen above could have actually blown up centuries ago and we wouldn’t yet know it. The 10-solar-mass star Rigel, for instance and for all we know, looking proud and mighty ~800 light-years away, could have detonated during Earth’s Middle Ages and that “message” would still be winging its way toward us. Its “companion” in the Orion constellation, the 15-solar-mass red-giant Betelgeuse, only ~420 light-years distant, might have already done so more recently. These are certainly candidates to explode someday, as are the north star Polaris, the red supergiant Antares, and mighty Deneb, the brightest star in the constellation Cygnus.

Should such a supernova abruptly appear in the sky, we can be certain every major piece of astronomical instrumentation will immediately focus in the direction of this, the grandest of fireworks. Some major observatories, such as the Harvard-Smithsonian Center for Astrophysics, have established “supernova alert teams,” where several astronomers stand ready to commandeer, within an hour’s notice, all ground-based telescopes and orbiting spacecraft operated by that institution. A memorable false alarm on a Labor Day weekend thirty years ago even helped to smooth out some communications hazards, should a supernova inconsiderately time its earthly arrival on a human holiday! The team’s prime objective is the study of the early phases of a supernova outburst, especially the various types of emitted radiation stretching from relatively harmless radio waves to potentially lethal gamma rays.

Evolution Writ Large Supernovae are extraordinarily energetic, easily and efficiently piling up nearby matter into dense clumps, much as a plow effortlessly pushes around snow. A single supernova can sweep up far more mass than it formerly contained as a star. For example, the explosion of a 10-solar-mass star would launch a shock wave extending out to ~60 light-years and gathering up about 8000 solar masses of interstellar matter.

Shock waves can act as a strong evolutionary driver. Whether the result of expanding nebular gas produced by stellar birth or that of more violently expelled gas produced by stellar death, shock waves can create “second-generation” stars, some of which form new nebulae, others in turn explode, in either case giving rise to still more shock waves, and so on. Like a chain reaction, old stars trigger the origin of new ones ever deeper into an interstellar cloud—a sequential wave of star formation, aided and abetted especially by the most massive stars living fast and dangerously. Observations of young stars in the vicinity of supernova remnants do imply that the gentle births of new stars is often initiated by the explosive deaths of others.

Wherever O- and B-type stars exist, we can suppose that less massive stars are still in the act of formation. It takes longer for the less massive stars to form, thus we shouldn’t expect to find many A-, F-, G-, K-, or M-type stars, provided that the star formation mechanism really was triggered less than a million years ago. The whole Canis Major region (Figure 3.20), and many others like it, are probably vast breeding grounds—sites of invisible interstellar cloud fragments and protostars, as well as young, visible, massive stars—triggered by supernovae. Alignments of stars are also observed near the outside rims of several molecular clouds; groups of stars nearest such clouds appear to be the youngest, while those farther away appear to be older, much as expected.

Figure 3.44 schematically sketches what might be happening—an ongoing process that is likely even more complicated than described earlier in this STELLAR EPOCH. Each O- or B-type star forms quickly, lives briefly, and dies explosively. Each generates a shock wave of gas that pushes outward into interstellar space, forming more stars that then explode anew as part of a continuous, ongoing process of star formation probably spanning many generations of stars. This really does resemble evolution writ large.

FIGURE 3.44 — The process of star birth (a) and shock waves (b), followed by more star birth and additional shock waves (c), is likely to produce a continuous cycle of star formation in many areas of our Galaxy. Like a chain reaction, old stars trigger the formation of new stars ever deeper into an interstellar cloud. (Prentice Hall)

If we think broadly enough and over long enough timescales, stars begin to seem as replicative as bugs in a Petri dish. Perhaps a bit of a stretch, stars nonetheless provide good examples of physical evolution. As stars naturally change over time, their interiors develop steeper gradients in temperature and elemental composition; their cores heat up and their heavy elements form. Stellar size, color, brightness, and makeup all change, while progressing from protostars at “birth,” to mature stars at middle “life,” and on to red giants, white dwarfs, and pre-supernovae near “death.” At least as regards energy flow, matter circulation, internal gradients, and non-equilibrium thermodynamics while undergoing change, stars have much in common with life.

None of this is to say that stars are alive, a common misinterpretation of such an eclectic stance. Nor do stars evolve in the strict and limited biological sense—a subject best examined more closely later, in the sixth, BIOLOGICAL EPOCH. Yet close parallels are apparent, including selection, adaptation, and perhaps even generational offspring among the stars. It’s all reminiscent of a Malthusian-inspired scenario, hereby liberally stated:

Galactic clouds spawn clusters of stars, only a few of which (the more massive ones unlike the Sun) cause (via supernovae) other, subsequent populations of stars to emerge in turn, with each generation’s offspring showing slight variations, especially among the heavy elements contained within. Waves of star formation propagate through many such clouds like slow-motion chain reactions over eons of time—shocks from the death of old stars triggering the birth of new ones—neither any one kind of star displaying a dramatic increase in number nor the process of regeneration ever being perfect. Those massive stars selected by Nature to endure the fires needed to produce heavy elements are in fact the very same stars that often foster new populations of stars, thereby both gradually and episodically enriching interstellar space with greater elemental complexity on timescales measured in millions of millennia. As always, the necessary though perhaps not sufficient conditions for the growth of complexity depend on the environmental circumstances and on the availability of energy flows in such galactic domains.

On and on, the cycle churns. Build up, break down, change—a kind of stellar “reproduction” minus any genes, inheritance, or overt function, for these are the value-added qualities of biological evolution that admittedly go well beyond stellar evolution.