Details of the evolutionary stages described in the previous two sections were derived from simulations ("numerical experiments") performed on powerful, high-speed computers. The numbers listed in Table 3-1 and the evolutionary paths described in Figures 3.6, 3.8, and 3.9 are mathematical predictions of a multifaceted problem incorporating gravity, heat, nuclear reaction rates, elemental abundances, and several other physical conditions specifying the various states of contracting interstellar clouds. Only with modern computer technology have theorists been able to construct such specific models. Yet the accuracy of these models is only partly known, for it's currently difficult to test them observationally. Furthermore, each of the stages isn’t as clear-cut as suggested by the table; many of the stages probably overlap, just as with any gradually evolving, messy system.

How can we be sure that these theoretical predictions are valid, given that no human being has ever seen an interstellar cloud or a protostar move through all its evolutionary paces? In fact, the total lifetime of our civilization is (thus far) much, much shorter than the time needed to contract a cloud and form a star. We could never observe individual objects proceed through their full panorama of star birth. We can, however, observe different objects as they appear at different stages of their evolutionary cycle. Advanced equipment now enables us to probe interstellar clouds, protostars, and very young stars approaching the stage of long-lived, main-sequence burning. By observing many objects at various, often unrelated sites in our Galaxy, astronomers have observationally verified some of the prestellar stages described in the preceding parts of this STELLAR EPOCH.

Again—to stress a key point—the method used resembles that of archaeologists and anthropologists who dig up artifacts and bones at numerous unrelated places strewn across Earth's surface. Not having had the opportunity of living at the time of our ancestors, these researchers examine the various remnants, trying to integrate them into an overall picture of human evolution. Likewise, astronomers probe many unrelated regions of our Galaxy, striving to understand how the many varied objects in each region fit into a consistent scheme of stellar evolution. It's very much like a puzzle, and the various terrestrial bones and extraterrestrial clouds are the pieces. The picture emerges only when each piece is found, identified, and oriented properly relative to all the other pieces.

Evidence of Cloud Fragmentation and Contraction Many of the best pieces of the stellar evolutionary puzzle can be found near gaseous nebulae, which are "islands" of hot, glowing matter surrounding the most luminous O- and B-type stars. Actually it's not the nebulae that interests us here, rather they are merely signposts of recent star formation that is also likely occurring in the dense, dark, and dusty regions nearby; these are cooler regions, many with temperatures below 100 K. Clearly, the gas must be downright cold if the interstellar clouds near nebulae are to be candidate regions for contraction and protostar development. Were the gas not so cold, the necessary condition for contraction—gravitational potential energy > thermal energy—wouldn’t be met.

Figure 3.10 is an optical photograph of a rather typical region of interstellar space. The horizontal spread of stars, gas, and dust denotes the galactic plane, where most young stars are found today in our Milky Way, as well as numerous gaseous nebulae and interstellar clouds. Figure 3.11 is an enlargement of the small fuzzy "island' of brightness toward the bottom of Figure 3.10—the splendid gaseous nebula labeled M20. Again, such brilliant nebular regions illuminated by youthful stars alert us to the general environment where stars are likely to be still forming. Indeed, the dark interstellar regions near nebulae provide evidence for cloud fragmentation and protostars.

FIGURE 3.10 — A wide-angle view along the plane of our Milky Way Galaxy (which extends diagonally, from upper left to bottom center). The small fuzzy region labeled M20 toward the bottom is enlarged in Figure 3.11. (Harvard College Observatory)

FIGURE 3.11 — The interstellar region labeled M20 in the previous figure shows a beautiful gaseous nebula, called the Trifid Nebula, in addition to much dark surrounding matter. The region is estimated to be ~6000 light-years away. (AURA)

Prestellar objects at stages 1 and 2 aren’t hot enough to emit much infrared radiation. And surely no optical radiation arises from such dark, cool clouds; optical astronomers could observe star-forming regions forever and see little of interest, for there is, quite frankly, literally nothing to see in a dark cloud. Consequently, the best way to study the early stages of cloud contraction and fragmentation is to use radio telescopes to detect radiation emitted or absorbed by one or more interstellar molecules. Only long-wavelength (radio and infrared) radiation can penetrate the dense, dark clouds, thus escape the clouds and eventually (in minute quantities) be captured by telescopes at Earth.

The interstellar regions surrounding the M20 nebula in Figure 3.12 provide an example of galactic matter that seems to be contracting. This figure shows a contour map of the abundance of the formaldehyde (H₂CO) molecule, the presence of which, among several other kinds of molecules such as carbon monoxide (CO), is widespread in the area, especially throughout the dusty regions outside this nebula. Analysis of the radio radiation shows that these molecules are especially abundant near a totally opaque dark region to the south of the nebula in Figure 3.12. Further analysis of the observations suggests that this region of greatest molecular abundance is contracting, fragmenting, and generally on its way toward forming a star or a cluster of stars.

FIGURE 3.12 – Contour maps of formaldehyde (H2CO in green) and carbon monoxide (CO in red) near the M20 nebula demonstrate how these molecules are especially abundant in the dark interstellar regions outside the nebula. The contours denote increasing molecular abundance from the outside to the inside, so the molecules are concentrated at bottom right of the visible nebula. (background photo AURA)

How do astronomers know that a portion of the dark cloud outside M20 is probably contracting? By studying Doppler-shifted radio radiation (which is a velocity diagnostic) arising from suspect regions, we can perceive cloud fragments well on their way toward becoming stars—or, more likely, clusters of stars. Widths of spectral lines are especially useful in revealing the motions of the regions sampled. Figure 3.13 shows a map of the spectral-line width of the formaldehyde molecule's radiation observed toward one of the darkest clouds to the south of M20. This map was made by measuring the widths of spectral lines at various places across the dark interstellar cloud. Contours were then drawn connecting places having spectral lines of equal width. Significantly, this line-width map peaks at roughly the same place as the molecular abundance map—at the totally opaque, dark region to the south of M20.

FIGURE 3.13 — A map of the abundance (W, solid contours) and spectral-line width (delta-nu, dashed) of the formaldehyde molecule's radiation toward the south of M20 reveals some evidence of a contracting interstellar cloud. The linewidth contours increase in uniform steps from the outside to the inside of "blob B," as shown by the dashed curves here. They approximately coincide with the map of total formaldehyde abundance, displayed here as solid contours. (background photo AURA)

Similarity in the spreads of molecular motion and molecular abundance suggests that a small region of the large interstellar cloud engulfing M20 might be in a state of gravitational contraction. At the periphery of the region, narrow spectral lines are observed because the motion of infalling matter is mainly perpendicular to our line of sight; this causes little Doppler broadening of the spectral-line profiles. By contrast, toward the center of the dark region, infalling gas from the back and front of the cloud would be moving mainly parallel to our line of sight; this would produce Doppler-broadened line profiles of greater width, just as observed.

Other interpretations of the data are also possible. But the contracting interpretation just described is the simplest and most straightforward explanation of all the observations. Should it be correct, the M20 region provides a good piece of observational evidence for the beginnings of the evolutionary scenario outlined in Table 3-1. Only further observations will tell for sure.

The interstellar clouds in and around M20 provide tentative evidence for three broad phases of star formation, as depicted in Figure 3.14. In the northern part of this vast region, the nebular velocity of about 18 km/s, deduced from the analysis of radio lines emitted by ions in the hot gas, meshes nicely with the velocity of the adjacent molecular cloud. This is probably the average velocity of the gigantic, though invisible, cool interstellar cloud engulfing much of the hot nebula. This inactive (or quiescent) phase is characterized by low densities and temperatures, in the range of ~100 particles/cm³ and ~20 K.

FIGURE 3.14 — The M20 region shows observational evidence of three broad phases in the birth of a star: the inactive (or parent) phase, followed by the contracting (or infalling) phase, and finally the stellar (or nebula) phase.

Higher densities and temperatures typify smaller portions of this huge interstellar cloud. The totally obscured regions at which the molecular abundances peak (Figure 3.13) represent such denser and hotter fragments. Here the total gas density is observed to be at least 1000 particles/cm³. And the temperature measures ~100 K. These particle densities don’t refer to formaldehyde molecules alone, but are total densities, mostly comprising hydrogen molecules. The formaldehyde (and especially carbon monoxide) molecules are merely used by radio astronomers as convenient tracers of regions where molecules are generally abundant. Less than a light-year across, the contracting region noted in Figure 3.13 has a total mass of >1000 solar masses. This second broad phase of our star-formation sequence represents a cloud somewhere between stages 1 and 2 of Table 3-1.

The third phase, also shown in Figure 3.14, is exhibited by the star at the center of the M20 nebula itself. The glowing, reddish region of ionized gas results directly from a massive O-type star having formed there sometime within the past million years or so. Since the star is already fully formed, this final phase corresponds to stage 7 of our earlier evolutionary scenario.

Current observations of the M20 region don’t display evidence for any of the intermediate evolutionary stages outlined earlier. Someday, when radio and infrared observations are made with better angular resolution, we might be able to study directly the details of the protostellar objects that presumably lurk in the darkest realms of these contracting fragments. In the meantime, we must examine other objects to find examples of stages 3 through 6.

Evidence of Protostars Other regions of our Milky Way do show suggestive evidence for more advanced prestellar objects. The Orion complex, shown in the optical photograph of Figure 3.15, is one such region that houses scores of young stars in addition to many suspected protostars. Lit by several O-type stars, the bright Orion Nebula itself is partly engulfed by a vast molecular cloud. This dark cloud actually extends for hundreds of light-years beyond the frame of the image and has been studied by means of the radio radiation emitted and absorbed by carbon monoxide (CO) and formaldehyde (H₂CO) molecules. These molecules are useful probes of interstellar gas having moderate densities of ~1000 particles/cm³.

Many other molecules, such as hydrogen cyanide (HCN) and carbon monosulfide (CS), usually emit radiation from regions of even greater density. As mapped in Figure 3.15, these molecules extend over only a small part of the molecular cloud, just behind the bright nebula. As might be expected, the molecules seem to delineate a fragment of the larger molecular cloud where the density is ~100,000 particles/cm³. (Again, as noted earlier, atomic hydrogen H and molecular hydrogen H₂ are by far the most abundant gases at all stages of early star formation.) The measured extent of the fragment is a little less than 1 light-year, has a temperature of nearly 200 K, and hence can be identified as an object near stage 2 of Table 3-1.

FIGURE 3.15 — The Orion Nebula, itself ~3 light-years across and ~1500 light-years distant, is the gaseous nebula (left) situated below the three star's in the "hunter's belt" of the Orion constellation. This nebular region is enlarged (right), showing how it is partly engulfed by a vast molecular cloud, various parts of which are probably fragmenting and contracting, with even smaller sites resembling protostars. The various scales to the right and bottom of the right frame depict the extent of different molecules within a dark cloud behind the bright nebula. (Background photo AURA)

The Orion molecular cloud also harbors several smaller sites of intense radiation emitted by molecules under very special conditions. Molecules such as hydroxyl (OH) and water vapor (H₂O) have been found by radio techniques to be buried deep within the core of the cloud fragment. Their extent, shown in Figure 3.15, measures about 10¹⁰ km, or ~1000 times smaller than 1 light-year. This is just about equivalent to the full diameter of our Solar System. The gas density of these smaller regions is ~10⁹ particles/cm³, and although the temperature cannot be estimated reliably, many researchers regard these regions as objects near stage 3. We cannot currently determine if these regions will eventually form stars more or less like the Sun, but it does seem certain that such intensely emitting regions are on the threshold of becoming protostars.

In a relatively recent development, strong winds have been found to be associated with potential protostars. Radio and infrared observations of H₂ and CO molecules in the same Orion cloud have revealed clouds of gas expanding outward at velocities approaching 100 km/s (or ~200,000 mph). Furthermore, high-resolution, interferometric observations have disclosed expanding knots of H₂O emission within the same star-forming region, thus linking the strong winds to the protostars themselves. The specific causes of these winds—like those of many genuine, fully formed stars—remain unknown. But astronomers will need to consider the implications for the early Solar System; after all, our Sun was surely once a protostar, too.

When hunting for and studying objects at more advanced stages of star formation, radio techniques are less useful. The bulk of the exploration shifts to the infrared part of the spectrum because stages 4, 5, and 6 are expected to display more heat than their ancestral clouds. As the Planck ("blackbody") curve of thermal emission from warm protostars and young stars shifts toward shorter wavelengths, these objects should be more easily observable in the infrared.

Sure enough, a most interesting object within the core of the Orion molecular cloud was detected by infrared astronomers during the 1970s. This compact infrared source is outlined only by the contours in Figure 3.15, for only long-wavelength radiation can penetrate the cloud cores. Most astronomers agree that this warm, dense blob is a genuine protostar, poised on the verge of stardom.

The energy sources for the infrared objects seem to be optically luminous hot stars which, however, are hidden from view by surrounding dark clouds. Apparently, some of the stars are already so hot that they emit large amounts of ultraviolet radiation, which is mostly absorbed by a "cocoon" of dust. The absorbed energy is then re-emitted by the dust as infrared radiation. Some of the ultraviolet radiation heats and ionizes the accompanying hydrogen, whose emission can also be observed in the radio domain. In particular, the intense OH and H₂O radiation (which escapes the region) enables the clouds to cool off despite the continual heating by the stars. Some of the cloud fragments are so massive that their own gravitation is trying to contract them further. As this tendency is resisted mainly by the random motions of the molecules, it’s possible that the cooling provided by the escape of the intense radio radiation plays a significant role in the contraction process. That dust cocoons are invariably found in the dense cores of molecular clouds supports the idea that the hot stars responsible for their heating only recently emerged from the surrounding cloud.

Until the Infrared Astronomy Satellite was launched in the 1980s, astronomers were only aware of giant stars forming in clouds far away. But IRAS showed that stars are forming much nearer than anyone knew, and some of these protostars have masses comparable to that of our Sun. Figure 3.16 shows a premier example of a solar-mass protostar—Barnard 5, which has an infrared heat signature just that expected of a warm blob somewhere along the Hayashi track.

FIGURE 3.16 — An infrared image of the nearby region containing the source Barnard 5 (arrow). (NASA)

In sum, some protostars do stand out as fuzzy little telltale sources of heat, often deep inside cocoons of dust that hide them from direct optical view. What’s more, they are now known to exhibit strong “winds” and “jets” of fast-moving gas expanding outward at high velocities, typically tens of km/s. Imagery such as that in Figure 3.17 often reveals bipolar jets billowing from adolescent stars and churning up the surrounding interstellar medium. The jets, which sometimes amazingly extend for a few light-years beyond a single young star, emanate perpendicular to much smaller disks where planets are probably beginning to form. Such outflows resemble the vastly larger lobes of hot plasma seen near active galaxies such as the quasars as noted in the earlier GALACTIC EPOCH, and are yet another way that young stars “manage” their energy budgets, all the while ridding themselves of excess energy (lest they blow up). Eventually, such a star breaks through its placental envelope, its winds cast away much of the disk, and henceforth its energy is emitted less in the form of twin jets and more as a normal, uniformly bright, visual star, such as our Sun today.

FIGURE 3.17 – This view (in the inset), near the edge of the Orion molecular cloud (large image), shows the outflow from a newborn star still surrounded by nebular gas. The protostar (the dim, faint speck near center of the inset) created a pair of high-speed jets, called HH1/HH2, which extend for nearly a light-year across, perpendicular to the presumed protoplanetary disk. (STScI)

Complexities of Reality The subject of star formation is much more complicated than the previous discussion suggests. Interstellar space at any one time is populated with all sorts of clouds, fragments, protostars, stars, and nebulae. As they all interact in complex, interrelated fashion, one type of matter doubtless affects the behavior of others. For example, the presence of a gaseous nebula in or near a molecular cloud probably influences the evolution of the whole interstellar region. Most of the gaseous nebulae observed so far can easily be visualized to create expanding waves of matter driven by the pressure of stellar ultraviolet radiation from the central stars of the nebula. As the waves push outward into the surrounding molecular cloud, the interstellar gas would tend to become piled up or compressed, thus increasing the density of matter. Such a shell of gas, rushing rapidly through space, is known as a shock wave. It can push ordinarily thin matter into dense sheets, just as snow is swept up by the blade of a plow.

Many astronomers regard the passage of shock waves through interstellar matter as the triggering mechanism needed to initiate star formation in our Galaxy. Calculations imply that when a shock wave encounters an interstellar cloud, it races around the thinner exterior of the cloud more rapidly than it can penetrate its thicker interior. Shock waves don’t blast a cloud from only one direction. They effectively squeeze it from many directions, as illustrated in Figure 3.18. Atomic bomb tests have experimentally demonstrated this squeezing effect: Shocks created in the blast surround buildings, causing them to be blown together (imploded) rather than apart (exploded). Similarly, shock waves can cause the initial compression of an interstellar cloud, after which natural gravitational instabilities divide it into fragments that eventually form stars. Figure 3.19 suggests how this mechanism might be at work near M20.

FIGURE 3.18 — Shock waves tend to wrap around interstellar clouds, compressing them to greater densities, and thus possibly triggering star formation. (Prentice Hall)

FIGURE 3.19 — In this artist's conception, a cloud fragment is undergoing compression on the southerly edge of M20 as shock waves from the nebula penetrate the surrounding interstellar cloud. (Prentice Hall)

Gaseous nebulae are not the sole generators of shock waves. At least two other sources are available—the spiral-arm waves that plow through the Milky Way and the remnants of exploded stars (supernovae) to be discussed later in this STELLAR EPOCH. Perhaps each of these sources among others trigger star formation, but a supernova's great violence probably make it the most efficient way to pile up matter into dense clumps.

The photograph in Figure 3.20 shows a semicircular band of glowing gas in the Canis Major region of the sky. The bright interstellar matter along the arc is almost certainly only part of a 3-dimensional (spherical) shell, but we can’t be sure from such an optical view. Radio observations of the region reveal that other parts of the expanding shell are made mostly of invisible neutral hydrogen gas. The gas comprising the shell is apparently the remnant of an ancient stellar explosion; the size and expansion velocity of the shell's gas imply that a star blew up ~0.6 million years ago. Ever since, a wave of gas has been moving away from this point, piling up matter into high-density concentrations. Although the evidence is circumstantial, the presence of numerous O- and B-type young (and thus quick-forming) stars in the vicinity of this remnant do imply that the birth of new stars is often initiated by the violent, explosive deaths of old stars. Ironic, indeed, if the demise of old stars are the trigger needed to conceive new ones, a subject to which we shall return later in this STELLAR EPOCH.

FIGURE 3.20 — This arc of glowing gas is only part of a nearly complete shell of interstellar matter, which was probably ejected by a massive star that exploded nearly 600,000 years ago. Young stars are found on the inside edge of the shell, while additional stars are probably forming on the outer edge (marked by arrows) as the shell's shock wave piles up the matter. (Harvard College Observatory)

Circumstellar Disks Notably, but only in a few of the closest stellar nurseries such as the Orion Nebula ~1500 light-years away, circumstellar disks have recently been spotted. Powerful telescopes are needed to resolve the details, yet even a naked-eye amateur can spot this fuzzy nebula at the business end of the “sword” hanging below the “hunter’s belt” of 3 blue-white stars strikingly aligned in this winter-sky constellation (see again Figure 3.15). There, relatively new stars by the dozens, including 4 notable ones called the Trapezium group and viewable with good binoculars, glow intensely, many of them as young as 100,000 years. At higher magnification—though well beyond that discernible with even the best binoculars, in fact best located with the Hubble Space Telescope in Earth orbit—thin little oblong smudges are seen on digital images all through the Orion region, each one apparently a dirty disk. As shown in Figure 3.21, their size and scale resemble those of our Solar System, much as expected for protostars and possibly planets emerging from the turbulent mishmash of interstellar gas and dust.

FIGURE 3.21 – These two infrared images of planetary-system-sized dirty disks in the Orion region show heat and light emerging from their centers. On the basis of their temperatures and luminosities, these unnamed sources appear to be low-mass protostars on the Hayashi track of the HR diagram. (STScI)

Some protostellar objects emit intense, highly focused radiation, much of it coming from small molecules containing two or more atoms linked together. The radiation is especially intriguing because of its terrific strength and localized source. The first observations, a few decades ago, of radio radiation from one such blob were so mysterious that puzzled astronomers began calling the emitting molecules “mysterium.” They were later identified properly as hydroxyl (hydrogen plus oxygen, OH) molecules, and their enormously powerful signals are now known to be enhanced or amplified by a special “maser” process a little like that found in “lasers” on Earth.

The word laser, the name of common everyday devices used in supermarket checkout counters and compact-disk players, is actually an acronym for l ight a mplification by s timulated e mission of r adiation. Lasers are artificial gadgets that emit concentrated streams of light radiation in very narrow beams. Only within the past few decades has our civilization become smart enough to build such devices, relying as they do on both advanced technology and a good understanding of atomic and molecular physics. Lasers operate by exciting atoms or molecules in a gas and then stimulating them to emit radiation spontaneously and simultaneously. The result is a tremendous burst of radiation, much more powerfully than from ordinary light bulbs.

Masers are similar to lasers, except that they produce microwave (a special type of radio) radiation rather than optical light. Physicists know how to build them in terrestrial laboratories, though masers are very delicate gadgets, requiring special conditions and much patience to operate. When working properly, these machines are the best amplifiers known, much more effective than the ordinary transistors that do yeoman service in our personal computers and household appliances.

Interestingly enough, certain regions of interstellar space are naturally suited to produce amplified microwave radiation. Protostellar blobs apparently enjoy the special conditions required, first, to excite some molecules and, second, to stimulate them to emit intensely. The blobs’ warm temperatures and moderate densities seem ideal for this unique emission mechanism. Accordingly, the strong maser radiation observed from certain molecules—not just hydroxyl, but also water vapor and a few others—can be analyzed for additional clues about protostellar regions. Such studies comprise one of the most exciting areas of contemporary astrophysics.

Interstellar Molecules The very fact that molecules populate the near void of interstellar space is remarkable. Harsh radiation and alien environments would clearly harm the molecules’ unless they are protected, which is probably why they are invariably found in and around the dark, dense, and dusty parts of space. These are the giant molecular clouds again, our Galaxy’s largest entities that overwhelm in both size and mass, indeed often fully engulf, even the biggest nebulae such as one known as Sagittarius B2, which harbors well more than a million solar masses. About a thousand such molecular clouds are currently known in the Milky Way, many of them easily a million times the mass than our Sun. Ironically, the minute dust grains within those huge clouds not only serve to shield the fragile molecules, but also likely act as catalysts to help form them. The grains provide both a place where atoms can stick and react as well as a means of dissipating any heat generated by the reaction, which might otherwise destroy the newly formed molecules. Even so, the details of this frontier subject—astrochemistry—are still subject to debate and testing. After all, textbooks entitled General Chemistry are not general at all, but are really books on terrestrial chemistry familiar to us on Earth. The truly “general” or universal chemistry texts are only now being written based on astronomers’ findings in the wider extraterrestrial domain where extraordinarily low temperatures and densities prevail quite unlike anything on Earth.

During the past few decades, well more than 100 different types of molecules have been detected in spectra, many of them in the particularly rich Sagittarius B2 area, not far from our Galaxy' center. Given that they often radiate long radio waves, which can penetrate dust, the molecules act as important tracers of a cloud’s structure and physical properties as well as its chemistry. Carbon monoxide (CO), ammonia (NH₃), and water vapor (H₂O) are especially ubiquitous. Most intriguing, a pharmaceutical array of rather complex organic (carbon-rich) molecules has also been discovered in the darkest and densest of the molecular clouds, such as formaldehyde (H₂CO, a popular cleaning fluid and preservative), formic acid (H₂CO₂, prominent in ants and other insects), ethyl alcohol (C₂H₅OH, or galactic booze), and cyanodecapentayne (HC₁₁N, a 13-atom molecule not naturally found on Earth). Their presence has fueled speculation about life having originated in interstellar space, especially since a report by radio astronomers in the mid-1990s (yet still unconfirmed) that deep space harbors glycine (NH₂CH₂COOH), which is one of the key amino-acid ingredients of protein molecules in living cells. The likelihood that such precursors of life, indeed perhaps life itself, could have formed outside Earth comprises another new and exciting interdisciplinary subject—this one, astrobiology—to which we shall return in the fifth, CHEMICAL EPOCH. Organic molecules in space are harbingers of objects of greater complexity—to be sure, much greater, living complexity—yet to come in this Web site.

The very existence of interstellar molecules has forced astronomers to rethink and to reobserve the vast, darkened realms well beyond Earth. In doing so, we have begun to realize that this active, fertile domain is far from the void suspected by theoreticians not so long ago. Regions of space recently thought to contain nothing more than galactic “garbage”—the empty-looking darkness among the nighttime stars, polluted here and there by dusty trash—are now recognized to play a critical role in our understanding of the interstellar medium in which stars, and at least the building blocks of life, are born.

Protostar --> Star We are not done with protostars, which in this recounting have not yet become real stars. Theory suggests that protostars should be a bit unstable, as their inward pull of gravity doesn’t quite balance their outward push of hot gas pressure. The temperature is still too low to establish that “gravity-in, pressure-out” equality that guarantees stellar stability—fortunately for us since, if the heated gas managed to counter gravity before reaching the point of nuclear burning, there would be no stars. The nighttime sky would be fully abundant in dim protostars, though completely lacking in actual stars. And it’s likely that neither we nor any other intelligent life forms would exist to appreciate this distinctly duller Universe.

Computer models predict that as protostars continue to follow the dictates of gravity, the gas has no choice but to contract more, alas ever so slightly. The result is renewed heating. But even after a thousand centuries of infall, and with a core temperature of several million kelvins, not enough heat has yet built up to initiate nuclear fusion. Only when the temperature deep down in the core reaches fully 10⁷ K do the nuclear reactions commence. Atomic nuclei then have enough thermal energy to slam violently into each other and to overwhelm their own mutual repulsion by means of the very same process described earlier for the transformation of hydrogen into helium during the first, PARTICLE EPOCH. The upshot is more energy released and a halt to the contraction. A genuine star has finally formed, its principal function thereafter being the consumption of hydrogen, thereby producing helium and especially energy.

So, even as the average cosmic density and temperature of the Universe continue to decline with the cosmic expansion on the largest scales, small, localized “islands of brightness” called stars arise wherein their densities and temperatures increase. Stars buck the universal trend of decreasing temperature and density; they also go against the tendency of the Universe to become more disordered, for stars are clearly sites of rising complexity and greater order, especially as their thermal and elemental gradients steepen gradually from core to surface. This increased complexity, in turn, accompanies increased energy flows, but again, only locally where stars reside. Such energy flows are likely key to the emergence of order and structure in the Universe, including on planets and in life whose complexity is even greater than for stars, as we shall see later in the Web site.

Hearts of stars, then, are sites where atomic nuclei collide viciously, interpenetrating the realm of the nuclear force, thus releasing copious amounts of energy. Contrary to popular opinion, it’s not the nuclear reactions that create the high temperatures in stellar cores. Rather, it’s the high temperatures that allow the nuclear reactions to proceed there. Only with a sufficiently high temperature—that’s the threshold of 10⁷ K just noted—can the positively charged proton nuclei of hydrogen get up sufficient speed to ram into one another fiercely enough to allow the nuclear force of attraction to overwhelm the electromagnetic force of repulsion. As with many large-scale phenomena in the cosmos, it’s gravity that basically triggers this change—a change that causes fusion to literally light up stars—for the temperature climbs only because infalling clouds manage to convert some of their gravitational potential energy into frictional heat.

Once fully formed, a star becomes a prodigious emitter of radiation. Every second, our Sun fuses ~600 million tons of hydrogen into helium, converting the equivalent of >4 tons of matter into pure energy according to that same simple yet profound equation, E = mc². In gee-whiz terms, the Sun releases, again each second, an amount of energy equivalent to the detonation of ~10¹² atomic bombs. That’s more energy than humans have generated in all of history and the Sun does it each second. In fact, that’s enough solar energy, if suitably focused, to evaporate all of Earth’s oceans in about 6 seconds, or melt our planet’s crust in a mere 3minutes. Fortunately for us, the energy from the nuclear inferno moves up through the interior of our star and is radiated, isotropically and unfocused, equally from all parts of its surface in the guise of ordinary sunlight.

All the starry points of light seen in the nighttime sky owe their existence to nuclear fires churning deep in the cores of each and every one of them. Ponder all that astronomical activity, that sheer cosmic power, while looking upward at the stars some clear, moonless evening. Even the nocturnal quiescence of the dark sky above is home to continual change, the celestial heavens pierced by myriad, brilliant signposts of stellar birth.

The remarkable change from galactic cloud to contracting fragment to protostellar blob to nascent star takes a few tens of millions of years. Obviously a long time by human standards—in fact, tens of thousands of millennia—this is still <1% of a typical star’s lifetime. The entire process amounts to a steady metamorphosis—an evolution of sorts—a gradual transformation of a cold, tenuous, flimsy pocket of gas into a hot, dense, round star. The prime instigator in all this stellar evolutionary change is, once again, gravity. And the net effect of it all is increased energy flow and rising complexity.

Bigger Stars, Smaller Stars Once heat and gravity are balanced, a star like our Sun is stable. It experiences “storms” at its surface, in the form of flares, spots, and prominences, but these are minor irritations for an object as large as the Sun (though perhaps occasionally not so minor for some nearby planets). The star’s main agenda is to produce energy steadily for ~10 billion years. A combination of theory and observation implies that the Sun has already done so for about half this duration. So “our star” can be regarded as middle-aged, a celestial body expected to steadily emit heat and light literally morning, noon, and night for about another 5 billion years. (Its total lifetime as a star, all the way through the red giant and white dwarf evolutionary stages explained below, is projected to be nearly 12 billion years.)

Stars smaller than our Sun take more time to form from interstellar matter. They also last longer, while fusing more slowly; they resemble efficient compact cars in that they carry less fuel but burn it more effectively. For example, stars having 0.1 (i.e., 10%) of the Sun’s mass require nearly a billion years for birth and endure for as long as a trillion years. Since the latter value is much longer than the current age of the Universe, all small stars that have ever formed must still be fusing hydrogen into helium, producing a constant flux of energy for the benefit of any attendant planets.

By contrast, stars larger than our Sun tend to form faster from interstellar clouds, some in as little as a million years. The more massive stars in fact seem to do everything at a quickened pace. They burn their hydrogen fuel more rapidly and they pass through all their evolutionary paces more quickly. The reason is that their greater masses gravitationally compact the big stars strongly, causing matter within them to collide more frequently and violently, which, in turn, hastens their nuclear reactions. As a result, and somewhat surprisingly despite their huge masses, the biggest stars endure for much less than the 10-billion-year lifetime of our Sun. The most massive ones, for example, are nearly 100 times the mass of the Sun, yet last for only about 10 million years. They expend their stability with great flurry, a mere wink of an eye on the normal scale of cosmic lifetimes; they resemble gas-guzzling cars that carry more fuel but burn it less efficiently. Alas, a quickened pace is not always a desirable one, for the big stars live fast and die young. Just as it’s unhealthy for humans to rush through life, the largest stars hardly seem to settle down at all. In the end, while small stars shrivel up and fade away, stars more massive than our Sun perish by catastrophically collapsing and then exploding. Apparently some clichés have universal applicability: The bigger they are, indeed the harder they fall.

Brown Dwarfs One note of merit regards “failed stars,” for some cloud fragments never do achieve legitimate stardom. The planet Jupiter is one such case, having contracted under the influence of gravity and heated up somewhat, yet not having enough mass for gravity to crush its matter to the point of nuclear ignition. With only ~0.001 the Sun’s mass, Jupiter never evolved beyond the protostellar stage. Space might well be heavily strewn with such compact, dark “clinkers” of unburned matter frozen in time.

Theory holds that some objects having at least ~12 Jupiter masses might be able to ignite a special form of hydrogen, namely the isotope deuterium having a proton and a neutron in its nucleus, but that fusion would endure for only a short period of time. Deuterium is generally present only in trace amounts in any celestial object and this minimal fusion process stops as soon as it’s depleted of that fuel. These objects, called brown dwarfs, are distinctly more massive than Jupiter, but a good deal less massive than the Sun. In fact, nearly 100 Jupiter masses (which is again ~10% of a solar mass) are needed to generate core temperatures high enough to sustain the normal fusion process of hydrogen --> helium burning that is a hallmark of a true star. Astronomers know of no brown dwarfs in or near our Solar System, nor anywhere in the extended solar neighborhood of thousands of cubic light-years, but some are now beginning to be found in the Milky Way beyond.

Even our best telescopes have difficulty spotting such brown dwarfs in deep space. They are intrinsically very faint, glowing mostly with the heat left over from their formation, and even a smattering of interstellar dust further dims our view. Recent advances in detector technology, especially in the infrared part of the spectrum where warmth can be sensed against the cold background of space, have enabled astronomers to begin to catalog what is perhaps a whole new population of these objects. It’s not inconceivable that hundreds of billions of brown dwarfs populate our Galaxy, comparable in numbers to all the genuine stars in the Milky Way. However, only a few dozen of these elusive objects have been found to date—many in binary systems whose bright stellar member often betrays the presence of a small, dark companion—yet that’s enough to prove the reality of this intermediate stage of abortive stars.

Brown dwarfs’ inherent darkness makes them potentially relevant to one of the great unsolved problems in science: Could they be part of the missing dark matter plaguing astronomy today? Until the past few years, cosmic inventories had failed to account for these small, dim objects. In principle, brown dwarfs must contribute something to the dark-matter component of our Galaxy, and some astronomers are inclined to think that they might hide much of it. However, in practice, given that they’re made of normal, baryonic matter, brown dwarfs cannot be the entire solution. Current censuses imply that their small masses likely add up to no more than a few percent of the inferred dark matter.

A whole array of smallish, compact bodies—from dwarf stars to asteroidal rocks and planet-sized objects, as well as myriad old and dead stars encountered in the next section—could all be roaming the Milky Way in prodigious numbers undetected thus far. To repeat for emphasis: Any object having a size midway between, on the one hand, stars large enough to illuminate themselves and thus become visible from afar and, on the other hand, atoms and molecules small enough to reveal themselves spectroscopically even at great distances, would be virtually undetectable by any observational means currently available to humankind. Ironically, galactic space could indeed be laden with “interstellar basketballs” yet we have no way of knowing about them.