Interstellar Medium Imagine a large plot of interstellar real estate somewhere in the Milky Way. By definition, interstellar matter is that which exists beyond each of the stars—in short, matter scattered throughout the black and vast expanses among the myriad stars in our nighttime sky. Most people think nothing exists there, for, sure enough, a clear night shows only darkness among all the minute points of glowing starlight. But the darkness of outer space only affirms the limits of our human vision.

Not much matter resides in any one interstellar region, but it most surely does exist. Interstellar matter is 10²⁴ times less dense than that in either stars or planets, in fact about a million times thinner than the best vacuum achievable on Earth by pumping the air out of a cylinder. (Such laboratory vacuums still contain ~10⁶ atoms in each cm³.) Even so, interstellar space is so huge that small amounts of matter here and there can accumulate to play a significant role. It’s not unlike the prospect of becoming a multimillionaire by collecting a mere penny from every person in North America. Even minute quantities can add up to extremely large amounts, given enough space and time. All told, roughly as much mass resides in the immense realms of interstellar space as in the stars themselves.

The interstellar medium, then, includes the mostly invisible and rarefied regions from which all stars arise at birth—in any galaxy. In our own Galaxy, it comprises a nearly 1000-light-year-thick disk extending for the full 100,000-light-year width of the visible Milky Way. We also now realize, as noted later in this STELLAR EPOCH, that interstellar space is the very same domain into which many stars explode at death. It’s one of the busiest crossroads through which matter passes anywhere in the Universe.

Interstellar matter is largely a mixture of gas and dust. Much of the gas is in the form of thinly dispersed atoms (mostly hydrogen, H and a little helium, He), though frequently clusters of atoms—namely, molecules (mostly diatomic hydrogen, H₂)—are evident. The interstellar gas density averages 1 atom/cm³, except in those places where it clumps into richer groups of atoms sometimes reaching 10³ to 10⁶ times greater density. And it is in those denser “clouds” that interesting things happen, such as star formation. In general, though, the interstellar medium is so sparsely populated that harvesting all the gas in a region the size of Earth would yield barely enough matter to make a pair of dice.

As thinly spread as is the gas, interstellar dust is even more so; only one dust particle lurks in the darkness for every trillion atoms of gas. That’s much like a single dust grain residing in a volume of interstellar space equivalent to that housed in the New Orleans Superdome. By dust, we mean solid particles made mostly of heavy elements not terribly unlike the fine chalk dust that settles on blackboard ledges, or domestic dust that lurks under beds and in closets; tiny particles in a terrestrial fog or cigarette smoke are even better examples. The dust was, and still is, probably manufactured in the cool, outer atmospheres of old stars. Still, the vastness of space grants dust a role; an imaginary cylinder 1 m² in cross section and extending from Earth to Alpha Centauri would contain >10¹⁹ dust particles.

We can also think of the dust in this way: By enlarging a solid dust particle ~10⁹ times (or by collecting a billion of them in one place), it might resemble a rocky asteroid; 10¹² times, perhaps the core of primitive Earth. Small parcels can accumulate impressively in realms as expansive as the Milky Way.

Despite their rarity, dust particles make interstellar space a relatively dirty place. If we were able to capture such a parcel of interstellar matter and compress it to the typical density on Earth, the resulting gray fog would be so thick that we wouldn’t be able to see our hands in front of us. Pound for pound, space is heavily “polluted” with dust, but that dust is normally sprinkled throughout enormous tracts of galactic territory. By comparison, Earth’s atmosphere is about a million times less dusty. So, place humanity’s pollution problems into perspective; compared to the Galaxy in general and on a fair scale, Earth is a relatively clean place.

If the gas and dust of interstellar space had remained evenly dispersed forever, neither stars nor planets, and certainly not life, would have ever formed. The sky would be absolutely dark and no one would exist to know it. Fortunately, the interstellar medium is not immutable. Like everything else, it changes its disposition.

Theory suggests that matter contained within the dark regions of space will naturally fluctuate in density and eventually fragment into clumps typically spanning tens to hundreds of light-years. Because these dark regions are just that—dark—they have always been difficult or impossible for astronomers to visually inspect. Quite frankly, there’s literally nothing to see in a dark region—which, by the way, partly explains why humankind has been, until relatively recently, virtually ignorant about star formation since the birth of astronomy thousands of years ago.

Dark and dusty regions of interstellar space are inaccessible to study by optical means; they simply emit no light. Even stars behind these regions are invisible because dust diverts their radiation from reaching Earth, hopelessly scattering it like automobile headlights in a terrestrial fog. That doesn’t mean that the murky galactic recesses are totally impenetrable, however. Marvels of modern technology, such as parabolic-dish radio telescopes and heat-seeking infrared satellites, permit invisible regions to be sampled for their long-wavelength emissions that are able to penetrate the debris of interstellar space. In the same way that soldiers use infrared sensors to locate the enemy at night, and for the same reasons that airport radars operate properly in the worst winter weather, infrared and radio astronomers can detect invisible radiation from the utter darkness of interstellar space.

Analysis of the radiation emitted by interstellar matter has now confirmed theoretical predictions that parts of the near-void among the stars of any galaxy are clumped into large gassy clouds. Their overall morphology tends to resemble the irregular, fluffy clouds of Earth’s atmosphere, but there the resemblance ends. Interstellar clouds are billions of times larger than the entire Earth. They also amass and disperse, that is, come and go, billions of times more slowly than terrestrial clouds.

Radio and infrared observations have proved that interstellar clouds are not only tenuous but cold as well, often containing no more than 100 atoms/cm³ at temperatures hovering close to absolute zero. This density, though enhanced somewhat due to a cloud’s bulk, is still extremely low, in fact still lower than that of the best vacuums attainable in physics laboratories around the world; for comparison, the normal density of air on Earth is <10¹⁸ atoms/cm³. Typical cloud temperatures, ~20 K (or –250^o C), are also extremely low, for the lowest possible temperature (at which atomic motion virtually ceases) is 0 K (or –273^o C). We can thus fairly visualize an interstellar cloud as a wispy, frosty entity, but even that is an understatement.

Gravitational Competition Now imagine a small portion of an interstellar cloud, for instance a parcel of gas and dust calved from a larger cloud and much less than a light-year across. Given the cloud’s flimsiness, such a parcel doesn’t house many atoms. Yet unless the cloud is as cold as physically possible, each atom will still have some random motion owing to its heat, however minute. And each atom will be slightly influenced by the gravitational force exerted by all the other neighboring atoms, however small the mass of each atom. If only a few atoms coalesced accidentally for a moment, as depicted in Figure 3.2, their combined gravitational pull would be insufficient to bind them permanently into a distinct clump, which would then disperse as quickly as it formed. The effect of heat, even for the frigid interstellar atoms, wins this battle with gravity.

FIGURE 3.2 — Motions of a few atoms within an interstellar cloud are influenced by gravity so slightly that their paths are hardly changed before (a), during (b), and after (c) an accidental, random encounter. (Prentice Hall)

Suppose we now widen our sights to include more than just a few atoms. Instead, consider 50, 100, or even 1000 atoms. Would a group of that many atoms exert a net gravitational force strong enough to prevent the clump from dispersing as in the previous example? Just how many atoms are needed for gravity to bind them into a tight-knit assembly?

Answers to these questions cannot be obtained from a simple study of gravity alone; nor can they be found among the solutions at the back of any science textbook. Correct solutions depend, not only on gravity, but also on several other physical factors noted earlier such as heat, rotation, magnetism, and turbulence. These additional agents tend to influence the evolution of an interstellar cloud, for, although they shouldn’t be regarded as antigravity, they do compete against gravity.

Take heat, for example. Most of the slight warmth of interstellar clouds derives from seldom, yet inevitable collisions among the atoms. More frequent collisions mean greater friction and thus more heat, just as rapidly rubbing our hands together generates more warmth than doing so sluggishly. Heat gives a cloud of gas some buoyancy that tends to offset gravity. Heat is, in fact, the main reason that the Sun doesn’t collapse; the outward pressure of its heated gas counteracts the inward pull of its gravity. The amount of heat contained within an interstellar cloud is, of course, small by solar standards—typically tens of kelvins—which is why bright stars are lit up and dark clouds are not. Consequently, thermal effects that compete strongly with gravity once stars form, don’t really play a large role until after interstellar clouds contract and become hotter.

Rotation—that is, spin—can also compete with gravity. A contracting cloud having even a small spin tends to develop a bulge around its midsection. This bulge is a sure sign that some of the matter is trying to defy gravity and thus disperse. As the cloud compresses on its way to becoming a star, its spin necessarily increases, just as a figure skater rotates faster with her arms retracted—a prescriptive principle of physics known as “conservation of angular momentum.” Any rapidly rotating object exerts an outward force; the faster the spin, the greater the force, much as anyone can feel while bearing the brunt of a circular ride at an amusement park. In the case of an interstellar gas cloud, atoms near its edge are especially vulnerable to escape if the pull of gravity is insufficient to retain them; this is sketched in Figure 3.3. Should a contracting cloud increase its spin so much that gravity can no longer bind it, then the cloud would simply disband, releasing its atoms back into interstellar space. Mud flung from a rapidly spinning bicycle wheel is a good example; outward forces dominate any surface tension tending to keep the mud on the wheel. The only way an interstellar cloud can preserve itself against the threat of dissipation via rotation is to gather more and more atoms, thereby increasing its collective strength of gravity. The result is this: Rapidly rotating interstellar clouds need more mass to guarantee continued contraction toward star-like objects than do clouds having no rotation at all.

FIGURE 3.3 — A rapidly rotating gas cloud tends to resist contraction. In this way, spin can compete with gravity. (Prentice Hall)

Magnetism can also hinder the contraction of a gas cloud. Magnetic forces permeate interstellar clouds, much as they do more strongly the Sun and Earth; in all these cases, the magnetism probably arises from the motions of charged particles. In the case of Earth's Van Allen Belts, for example, the magnetic fields exert electromagnetic control over the charged particles and ions escaping the Sun. If the magnetism is strong enough, the particles will be influenced more by the magnetic force than by the gravitational force. In the case of an interstellar cloud, the tug-of-war between gravity and magnetism often causes the cloud to slowly contract in distorted ways. Since the charged particles and the magnetic fields are coupled together (just as electricity and magnetism are a twosome), the magnetic field itself follows the contraction of a cloud, as suggested by Figure 3.4. The charged particles and ions literally pull the magnetic field toward the cloud's center, especially in the direction perpendicular to the magnetism. In this way, the strength of magnetism in a cloud can become much larger than that normally permeating general interstellar space outside gas clouds, and thus occasonally resist gravity.

FIGURE 3.4 — Magnetism can hinder the contraction of a gas cloud, especially in directions perpendicular to the magnetic field (solid lines). Frames (a), (b), and (c) are schematic diagrams of the time evolution of a slowly contracting interstellar cloud having some magnetism. (Prentice Hall)

Gas turbulence, or disordered bulk motion, is also present within each cloud, yet nearly intractable to treat mathematically. Turbulence is possibly the result of collisions among clouds over eons of time or shocks pummeling the clouds as cosmic rays from exploded stars plow into them.

Observations made during the past few decades show that most interstellar clouds are very cold, spin slowly, and are only slightly magnetized and turbulent, so individually these factors shouldn’t amount to much. But theory suggests that, taken together, even small quantities of each of these agents sometimes unite to compete effectively with gravity.

So it’s not a simple case of gravity sweeping up matter to build a star. Many additional factors serve to complicate the problem, making the star-formation process challenging to understand in detail. The upshot is that even those clouds that do manage to contract often do so in highly distorted ways, greatly altering the dynamical behavior and subsequent evolution of a typical gas cloud.

We now return to our original question: How many (hydrogen and helium) atoms need to accumulate for the collective pull of gravity to prohibit a pocket of gas, once formed, from dispersing back into the surrounding interstellar environment? The answer, even for a cool cloud having no rotation or magnetism, is a very large number. In fact, nearly a thousand billion billion billion billion billion billion (i.e., 10⁵⁷) atoms are needed for gravity to bind a gaseous condensation. There’s no doubt about the truly huge magnitude of this number. It’s much larger than the ~10²² grains of sand on all the beaches of the world, even larger than the ~10⁵¹ elementary particles comprising all the atomic nuclei throughout the entire Earth. It’s large compared to anything with which we’re familiar because there’s simply nothing on Earth comparable to a star.

This number, 10⁵⁷ atoms, just about equals the mass or our Sun—which is no coincidence. We can see this by converting 10⁵⁷ hydrogen (H) atoms to a mass value: 10⁵⁷H atoms x 2x10^-24 g/H atom = 2x10³³ g. Our Sun is an ordinary, average star (if a little on the small side), implying that many stars form from galactic fragments having approximately this number of atoms. In all, stars originate from slightly larger and smaller clumps, for the range of known stars varies from ~0.1 to ~100 times the mass of our Sun. The more massive stars probably form in interstellar regions where heat, rotation, and magnetism compete strongly with gravity, requiring the clouds' eddies to attract more than the nominal 10⁵⁷ hydrogen atoms needed for successful gravitational contraction. Stars less massive than our Sun presumably form in regions having lesser heat, rotation, and magnetism. This mass range is a rather small variation in astronomical terms, but a variation nonetheless among populations of stars.

Formation of Sun-like Objects We can best study the specific steps of star formation by considering the Hertzsprung-Russell (HR) diagram, a useful plot of absolute luminosity and surface temperature of stars, named after two leading Dutch and American astronomers of the early 20^th century. The luminosity scale in Figure 3.5 is expressed in terms of the solar luminosity (namely 4x10³³ erg/s), so that our G-type Sun is plotted at the intersection of the values, 1 solar luminosity and 6000 K.

FIGURE 3.5 — The HR diagram is a useful way to summarize the observed properties of stars. Luminosity is plotted upward, surface temperature increasing to the left. The Sun is right in the midst of the main sequence, the shaded, snaky line running through the graph. (Lola Chaisson)

Most stars plotted on such an HR diagram fall along the so-called main sequence. For roughly 90% of their lifetime, stars burn rather quiescently and hence don’t change their physical conditions very much. Data points representing such stable, genuine stars remain nearly stationary on the HR diagram.

Stars do change their properties, however, especially near the beginning and end of their existence. The HR diagram is a useful aid in describing these substantial birth-and-death changes. We shall shortly describe the final end-states of stars, but let's first examine the evolution of an interstellar cloud leading up to a star's birth.

Table 3-1 specifies several evolutionary stages experienced by an interstellar cloud prior to the formation of an ordinary star such as our Sun. Characterized by varying central temperatures, surface temperatures, central densities, and sizes of the prestellar object, these seven stages trace its progress from a quiescent cloud to a genuine star. Specific numbers given in Table 3-1 and in the present discussion are valid only for the formation of stars having approximately the mass of our Sun. (In the next section we shall relax this restriction and consider the formation of any star.)

Stage 1 is just that of any ordinary interstellar cloud. Many of these dark and dusty regions—sometimes called giant molecular clouds, which dwarf all other objects in the Galaxy—are truly vast, often spanning tens of light-years across, or about 10¹⁴ km. Temperatures are usually ~10 K both within and at the edge of such large clouds, whereas densities are often not much more than ~1000 particles/cm³. Stage-1 clouds typically contain thousands of times the mass of the Sun in the form of cool atomic and molecular gas.

If a cloud is to become the birthplace of stars, it cannot remain as a stable, homogeneous blob. Interstellar clouds must eventually break up into subcondensations, often less than a light-year across. Theory suggests that fragmentation into smaller clumps of matter occurs naturally, because gravitational instabilities at various parts of an interstellar cloud force the development of inhomogeneities in the gas. A typical cloud can break up into tens, even hundreds, of fragments, each imitating the shrinking behavior of the cloud as a whole, albeit contracting even faster than the parent cloud.

In this way, interstellar clouds are thought to produce either numerous stars, each much larger than our Sun, or whole clusters of stars, each comparable to or smaller than our Sun. Indeed, there’s no evidence for stars born in isolation, one star from one cloud. In reality, most clouds give rise to a whole family, or population, of stars, the smaller ones outnumbering the larger ones (much as at the seashore where small pebbles far outnumber larger boulders). Perhaps all stars originate as members of groups. Those now appearing alone and isolated in space, such as our Sun, probably wandered away from their litter as the star cluster dissolved.

Once a fragment assumes its own identity within an interstellar cloud, it passes through a series of inevitable stages. It first begins to contract as gravity strengthens with the ever-accumulating group of atoms. It literally shrinks under the stress of its own weight. As the protostar grows more compact, the atoms collide more frequently, in turn causing the gas fragment to warm.

Stage 2 in our evolutionary scenario represents the physical conditions of just one of the many small fragments that develop within a typical interstellar cloud. Estimated to span ~0.1 light-year across, such a fuzzy, gaseous blob is still hundreds of times the size of our Solar System. Temperatures both at the core and periphery of such a fragment have risen to ~100 K, and the central density has also increased to ~10⁶ particles/cm³. That’s still colder than our 300-K room-temperature standard on Earth, but it’s warmer than the original interstellar cloud prior to its clumping. Such a region warms because gravitational potential energy of the gas particles converts into thermal energy as the fragment contracts. This newly gained heat causes the atoms to become agitated; hydrogen atoms in particular move around with velocities of about 1 km/s (or ~2000 mph) in a 200-K gas. These faster velocities ensure that the atoms collide often and aggressively. If such fragments are to continue to contract, they must constantly radiate away some of their newly generated heat, lest the cloud become stabilized against the relentless pull of gravity. After all, stars can't form in stabilized clouds.

This description is more than a theoretical scenario. Its rough outline has now been clearly, though not visually, confirmed, using specialized equipment developed during the past quarter-century. Radio and infrared observations, in particular, have produced solid evidence that huge interstellar clouds are, in fact, fragmenting into smaller clumps of gas. Pockets of slightly hotter and denser matter within otherwise tenuous, cold, and enormous clouds are now widely observed across the Milky Way.

Fragmentation might be expected to continue indefinitely, dividing again and again and ultimately yielding ever-smaller clumps impossible to form stars. Fortunately, the process halts before it's too late. Rising gas density stops the process of fragmentation from reducing all parts of the cloud without limit, lest the cloud become homogeneous again. As individual stage-2 fragments compress their gas, they eventually become compact enough to prohibit radiation from escaping. With the cloud's natural vent partially blocked, the trapped radiation causes the temperature to rise, pressure to increase, and fragmentation to cease.

Several thousand years after it first began contracting, a typical fragment has shrunk in stage 3 to a gaseous object having a diameter roughly the size of our Solar System (still ~10,000 times our Sun's size). Its central temperature has reached several tens of thousands of kelvins, a value greater than that within the hottest steel furnaces built by our civilization on Earth. The temperature at the fragment's periphery has also increased, although not nearly as much as deep in the interior. In fact, the temperature at the surface of the fragment is likely just a little hotter than a comfortable living room, not much greater than several hundred kelvins. Because the density of matter inevitably increases at the core of the fragment faster than at its periphery, the outer surface of any contracting interstellar cloud is sure to be cooler and thinner than its interior. The central density is by now ~10¹² particles/cm³, ensuring much more violent and frequent particle collisions at the core and thus producing a large temperature difference between these two zones of the fragment.

As the cloud fragment continues to develop, computer models predict much the same story: Its size diminishes, its density grows, and its temperature rises at both core and periphery. About 100,000 years after beginning its contraction, it reaches stage 4, where its center boils at about a million kelvins. Elementary particles, now mostly electrons and protons ripped from disintegrated atoms, are really whizzing around at very high velocities—on average, ~100 km/s. Despite this veritable inferno, though, the gas is still far from the 10⁷ K needed to ignite the proton-proton nuclear reactions to fuse hydrogen into helium.

Still larger than a Sun-like star, the gaseous heap at stage 4 has a diameter equal to about Mercury's orbit. Its surface temperature has risen to several thousand kelvins. For the first time, it's beginning to resemble a star—or at least a round, glowing blob. For these reasons, astronomers call such a stage-4 fragment a protostar—an embryonic object perched at the dawn of star birth.

Note that the time needed for the appearance of a protostar is only ~100,000 years. We say "only" because that's brief by cosmic standards. However, it's long by human standards, which partly explains why no one has ever seen a protostar actually emerge from an interstellar cloud.

Once the cloud fragment reaches the protostellar stage, its surface temperature is high enough for the object's physical properties to be plotted on the HR diagram. Knowing that luminosity of any glowing object varies as the square of the object's size and as the fourth power of its surface temperature (i.e., L α R² T⁴), we can calculate the luminosity. Surprisingly, it turns out to be several thousand times the luminosity of our present Sun. As shown in Figure 3.6, the protostar is much more luminous than most other stars on the main sequence. It might seem paradoxical that a protostar can have such a large luminosity even though it hasn't yet begun its nuclear burning. The reason is that despite a surface temperature only about half that of the Sun, a protostar is usually hundreds of times larger in size, thus making its total luminosity emitted from all parts of its bloated surface very large indeed.

Figure 3.6 also depicts the approximate path followed by such an interstellar cloud fragment before becoming a protostar. Researchers often refer to this early evolutionary path as the Kelvin-Helmholtz contraction phase, so named after two European physicists who first studied the theory of contracting clouds about a century ago. Figure 3.7 is a series of artist's sketches of an interstellar gas cloud proceeding along this evolutionary path.

Protostars are still a little unstable; the outward pressure of heat doesn’t yet quite balance the inward pull of gravity. Fortunately, the average temperature is still too low to make a protostar stable. We say "fortunately" because if the heated gas were able to counteract gravity before reaching the threshold of nuclear fusion, there would be no stars. The nighttime sky would glow in dim protostars but completely lack genuine stars. And it's likely that neither we nor any other intelligent life forms would exist to know any of that.

FIGURE 3.6 — Diagram of the approximate evolutionary path followed by an interstellar cloud fragment prior to arriving, as a stage-4 protostar, at the end of the Kelvin-Helmholtz contraction phase. (Lola Chaisson)

FIGURE 3.7 — Artist's conception of an interstellar cloud changing during the early evolutionary stages outlined in Table 3-1. Shown are (a) a stage-1 interstellar cloud, (b) a stage-2 fragment, (c) a stage-3 smaller, hotter fragment, and (d) a stage-4 protostar. (Not drawn to scale.) (Prentice Hall)

Thus protostars continue to contract, although ever-slower because heat steadily increases its gravity-countering effect. By stage 5, a protostar's size has shrunk to nearly 10 times the size of our Sun. Its central temperature has reached ~5x10⁶ K, although this still isn’t enough to initiate nuclear fusion. To be sure, the core of the protostar is now mostly ionized because of violent collisions among the gas particles. But the protons still don’t have enough thermal energy (i.e., high velocity) to overwhelm their mutual electromagnetic repulsions, and thus penetrate the realm of the nuclear binding force.

The protostar's surface continues to mimic the rise of the interior temperature, but only slightly; at stage 5, as Table 3-1 notes, the surface temperature has reached ~4000 K. Despite this increase in surface heat, the protostar's luminosity doesn’t continue increasing at stage 5. Although its temperature is higher, its size is smaller; both factors affect the luminosity. In this particular case, the square of the protostar's size decreases more rapidly than the fourth power of its temperature increases. The position of the protostar on the HR diagram has therefore moved down and to the left while changing from stage 4 to stage 5.

Events in a protostar's development thereafter happen more slowly while approaching the main sequence. The initial contraction and fragmentation of an interstellar cloud occur quite rapidly, but as the protostar nears the status of a full-fledged star, its time scale for change slows. Heat is the cause of the slowdown, for even gravity must struggle to compress a hot object.

Some 10 million years after its first appearance, the protostar finally becomes a genuine star. It does this by continuing to contract a bit more under the relentless pull of gravity. At stage 6, when a 1-solar-mass object has shrunk to a size of ~2 million km (roughly the size of our present Sun), its central temperature has reached 10⁷ K. At last the heat is sufficient to ignite nuclear burning: hydrogen nuclei (protons) begin fusing into helium nuclei.

Stages 5 and 6 are plotted in Figure 3.8. Stage 6 is slightly below the main sequence mainly because the star's surface temperature at this time in its development is a little less (~5000 K) than the Sun's. Here, again, the square of the newly formed star's size is decreasing more quickly than the fourth power of its rising surface temperature. The object's changing size is the dominant influence in its evolution from stage 4 to stage 6, which accounts for the decreased luminosity. This newly formed star at stage 6 is actually a little less luminous than our Sun at present, as was our Sun at its birth some 5 billion years ago.

To distinguish this later protostellar evolutionary path where luminosity decreases from the earlier Kelvin-Helmholtz contraction phase where the luminosity increases, astronomers often call it the Hayashi track. Named for a 20^th-century Japanese researcher who made major contributions to the theory of protostars, this evolutionary path is diagrammed in Figure 3.8.

Such a stage-6 star has yet to reach the main sequence, however. Although the outward push due to heat and the inward pull due to gravity are nearly balanced, this new born star is still a little unstable. One final, relatively slow adjustment must be made before the star completely settles onto the main sequence for 10 billion years of steady nuclear burning. As usual, gravity performs the task.

FIGURE 3.8 — The changes in a protostar's observed properties are shown by the path of decreasing luminosity, from stage 4 to stage 6, often called the Hayashi track. (Lola Chaisson)

During the next 30 million years or so, the stage-6 star is squeezed just a little more. In making this slight adjustment, its central density becomes ~100 g/cm³, its central temperature increases a little to ~15 million K, and its surface temperature mimics that internal heating by reaching ~6000 K. It so happens that, in this case, the fourth power of the star's surface temperature increases more than the second power of its size decreases, that is, ΔT⁴ > ΔR². Accordingly, at stage 7, a 1-solar-mass object finally reaches the main sequence just about where our Sun now resides. Pressure and gravity are then balanced for this 1-solar-mass star. Its principal function thereafter is to consume hydrogen, thereby producing helium and releasing energy.

All the evolutionary events just described occur over the course of several tens of millions of years. Obviously a long time by human standards, this is still <1% of a 1-solar-mass star's lifetime on the main sequence. Once an object begins fusing hydrogen and establishes a gravity-in/pressure-out equilibrium, it pretty much burns steadily for a long, long time indeed.

Stars of Different Masses The numerical values and evolutionary paths noted above for the birth of a 1-solar-mass object are not valid for the formation of stars having much more or much less mass. Temperatures, densities, and sizes of other prestellar objects exhibit similar trends of change, but the actual values differ, in some cases considerably. The time scales for these changes are also quite different.

For example, cloud fragments that eventually form stars more massive than the Sun approach the main sequence along a loftier track on the HR diagram. Figure 3.9 shows how these more massive objects have both larger luminosities and higher surface temperatures at all stages. Furthermore, the pace at which these massive prestellar objects move through their evolutionary stages is a lot faster than for presolar objects like the Sun. The most massive fragments contract into stars in a mere million years.

FIGURE 3.9 — Prestellar evolutionary paths for stars either more or less massive than our Sun differ from those sketched in Figures 3.6 and 3.8. (Lola Chaisson)

Mass is the cause of all these differences. Not surprisingly, massive fragments within interstellar clouds often produce massive protostars and eventually massive stars themselves. These big fragments initially have larger sizes, more gas particles, and more frequent particle collisions than those of smaller cloud fragments. And since gravity bears down more strongly in more massive objects, they thus heat up to the required 10⁷ K more rapidly than do presolar objects.

This fast embryonic pace agrees with the paradoxical way in which the most massive main-sequence stars race through their lifetimes. Their rapid evolutionary pace eventually causes problems for the biggest stars, including all those in the O and B categories. Just as hastily prepared foundations in our world often lead to collapse, these giant stars race through all parts of their life cycle, only to die catastrophically by exploding.

The opposite case prevails for stars and prestellar objects having masses less than our Sun. Cloud fragments that evolve into low-mass stars are generally smaller and thus have particle encounters less frequently. Not only do such fragments take a long time to become protostars, but these protostars also take their time changing into genuine stars. A typical M-type star, for example, requires nearly a billion years just to form.

Dark Clinkers Some cloud fragments are too small ever to become stars. Gaseous planets, such as Jupiter, are good examples. The early blob that became Jupter did indeed contract under the influence of gravity, for the resultant heat is detectable with infrared telescopes; Jupiter still emits more radiation than it receives from the Sun. But Jupiter was unable to accumulate enough mass for gravity to crush its matter to the stage of a nuclear-fusing star (it needs about 70 times more matter). This big planet became prematurely stabilized by heat, rotation, and possibly magnetism, all opposing the pull of gravity. Thus Jupiter exists now as a chunk of gaseous matter that never evolved beyond the interstellar fragment stage. Although with a smaller size, Jupiter is literally stuck at stage 3 of our analysis.

The other Jovian Planets are similar failures. If they were still collecting gas, we might soon regard them as protostars. But virtually all the matter present during the formative stages of our Solar System is gone now, swept away by the solar wind of our Sun. Stranded in space, these giant planets will continue to cool, eventually becoming compact, dark clinkers of little significance.

Vast numbers of Jupiter-like objects may well be scattered throughout the Universe—fragments frozen in time somewhere along the Kelvin-Helmholtz contraction phase. Our technology is currently only beginning to detect them, whether they are planets associated with stars or interstellar cloud fragments unattached to any star. We can telescopically see stars, and spectroscopically infer atoms and molecules, but astronomical objects of intermediate size are very difficult to detect outside our Solar System. In fact, galaxies could be chock-full of cold, dark objects ranging anywhere from pebble-size to Jovian-size without our knowing it. Astronomers often joke about “interstellar basketballs,” but in fact objects of that size might well exist in great abundance. Conceivably, these dark clinkers might help solve the missing-mass problem plaguing our Milky Way Galaxy.