Birth of Stars
How and where are stars born? Observational evidence points to the interstellar gas and dust clouds along the Galaxy's spiral arms (Figure 14.1) as being the birthplaces of stars. As some stars--such as those responsible for planetary nebulae, novae, and supernovae--approach the end of their lives, they return some of their mass to the interstellar medium. New generations of stars are thus forming from the "ashes" of previous generations.
Collapse of Interstellar Clouds
Apparently the particles composing interstellar matter are not subjected to a net force by which an excess gravitational attraction from their neighbors pulls them together. If they were, within several hundred million years all the matter in the interstellar medium would collapse and fragment into stars. Thus all the interstellar matter would have been used up early in the Galaxy's history, and no more stars could form.
The very existence of interstellar matter and its organization into clouds of up to several hundred thousand solar masses argues that the gas pressure in clouds is sufficient to balance the effects of gravity. The first step in making new stars is to compress a cloud to strengthen gravity's effect so that the cloud material can contract and fragment into smaller units that eventually collapse to form stars.
A promising way of getting this process going is the traveling compression wave, or density wave, which astronomers believe is responsible for the Galaxy's spiral arm structure (Figure 20.14). As the wave moves past a cool molecular cloud, it compresses the cloud, driving the particles closer together. Their mutual gravitational attraction is now greater than the gas pressure. If the compressed cloud has no other force that can halt contraction, collapse continues until the matter heats up, raising the gas pressure sufficiently to resist further contraction.
Another possible mechanism for compressing molecular clouds is supernovae outbursts. An expanding shock front on the leading edge of the gas shell expelled by a supernova outburst impinges on nearby clouds and compresses them by factors of 10 or more, triggering gravitational collapse. The discovery that some young stellar associations are located inside the expanding shells of old supernova remnants certainly makes this a believable mechanism.
Finally, the collapse of clouds could begin if a cloud could be cooled so that the gas pressure would go down. There are several possible ways of cooling clouds, such as dust grains radiating away energy as infrared radiation.
Regardless of what starts the process, a fragmenting molecular cloud breaks into smaller units, and the fragments attract more matter and grow in mass. The rate at which stars are created from fragments of an interstellar cloud and the number of stars of different masses formed probably depend on several factors: total mass, density, temperature, magnetic fields, and the amount of internal motion stirring the material. The mechanism forming stars favors clearly small-mass stars, since we observe many more small-mass stars than large-mass ones. Finally, it appears that only a small fraction, 1 or 2 percent, of the matter in dark clouds actually forms stars.
Bok Globules
Small, dark blobs have been photographed against many of the bright star-filled regions and luminous H II nebulosities of the Milky Way. They are called Bok globules (Figures 17.8 and 17.9) in honor of astronomer Bart Bok (1906-1983). Globules are typically a few light years in size (hundreds to thousands of times the size of the Solar System), they possess densities of cold molecular hydrogen and dust of several tens of thousands of particles per cubic centimeter, and they contain several tens of solar masses of material. A number are known in which young stars of up to a few solar masses are embedded. This suggests that they are indeed part of the star formation process. The famous Horsehead Nebula shown in Figure 17.10 is probably a Bok globule in formation.
Matter in collapsing fragments converts its gravitational potential energy into thermal energy, some of which is radiated away into space as infrared radiation. At some point, however, a significant amount of energy goes into dissociating molecular hydrogen to form atomic hydrogen; later more energy is needed to ionize all chemical species. Because this energy used to dissociate and ionize is not available as thermal energy, the collapsing fragment is prevented from achieving hydrostatic equilibrium. Consequently, in a very short time (hundreds to tens of thousands of years), the fragment collapses from a small fraction of a light year in diameter (several million solar radii) to a few thousand solar radii (Figure 11.3).
During collapse of a fragment, its matter has been growing hotter and emitting more visible light and less infrared radiation. Because it is cooler, however, dust in the surrounding stellar nebula out of which the star is forming absorbs visible photons, heats up, and reemits the energy in the infrared. Thus the stellar nebula conceals the developing star until most of the surrounding gas and dust is either attracted to it or blown out of the system by it.
There are several examples in which astronomers have apparently witnessed interstellar dust rearranging itself over a period of years to reveal, if not a developing star, the place where one or more stars will be eventually.
Protostars, First Appearance on the H-R Diagram
Eventually, the central regions of a forming star become opaque and slow the outward flow of radiation. The effect of this is to stem the loss of energy so that the temperature rises and the gas pressure increases. This causes the central region to slow from a free-fall collapse to a gradual contraction as it approaches a balance between gas pressure, which is pushing outward, and weight resulting from gravity, which is pushing inward. Now the embryo star can appear on the H-R diagram for the first time; it begins its evolution on the coolest fringes of the diagram on evolutionary tracks determined from stellar models (see the right-hand side of Figure 17.11).
Once the forming star has stabilized somewhat, it is in the red-giant region of the H-R diagram, although it is not called a red giant; it is a protostar. The temperature of a protostar's surface is about 4000 K, and energy in its deep interior is transported to the surface entirely by convection, which extends from center to surface. In this slower-contraction phase, a protostar decreases its luminosity but keeps about the same surface temperature; most of the accretion of matter has stopped.
Gravitational contraction eventually raises the temperature in a protostar's core to 1 million K or so, which is hot enough to destroy by nuclear reactions such light nuclei as deuterium, lithium, beryllium, and boron (initially present in small quantities). These are the first stages of the star's thermonuclear existence, from which it derives very little energy; the next stage is to initiate hydrogen burning.
By the time a protostar's central temperature has risen to several million degrees, the p-p chain can be ignited and hydrogen burning begins to supply some luminosity, at first in small amounts. Several million years later the protostar arrives on the zero-age main sequence in the H-R diagram, where hydrogen burning supplies 100 percent of the luminosity and contraction has virtually ceased. Astronomers define the zero-age main sequence (Figure 17.11) as the line along which protostars of different masses cease to contract (and thus are stable configurations) and derive all their luminosity by burning hydrogen. The protostar is now a full-fledged star.
How long does it take to go through the protostar stage to reach hydrogen burning? For the Sun it was about 30 million years; approximate times are listed for other stellar masses in Figure 17.11. As the figure shows, stars exceeding the Sun's mass evolve quite rapidly, while for less massive stars the protostar phase is longer than that for the Sun. It is usual among astronomers to date a star's age from the zero-age main sequence onward, since the time it has spent contracting out of the interstellar medium to the main sequence is only a small fraction of its life span, typically a few tenths of a percent.
Some stellar models indicate that protostars with masses of less than about 0.1M. never become hot enough at their centers to fuse hydrogen. They pass the lower end of the main sequence and continue contracting toward extremely high densities. These very low mass "stars" apparently bypass normal stellar evolution and proceed slowly to becoming brown dwarfs. With less than a 0.01M. such protostars may become Jovian-type planets (Jupiter's mass is 0.001M.).
There is little doubt among astronomers that rotation is also a crucial factor during the collapse of interstellar clouds and the contraction of the resulting fragments through the protostar stage into main-sequence stars. Rotation probably determines whether the results will be multiple-star systems (high rotation), binaries, stars with brown dwarf or planetary companions, or just single stars (low rotation).
Stellar Nurseries
By the time several O stars arrive on the zero-age main sequence from the collapse of a giant molecular cloud, they will produce enough ultraviolet radiation to evaporate dust grains and ionize gas in their vicinity; this process forms an H II region. Figure 17.12 is an attempt to depict the formation of an O association on one end of a giant molecular cloud. Star formation advances across the cloud by the formation of new O stars. This occurs because the O stars' emission of ultraviolet radiation (which forms the H II region) in conjunction with their stellar wind creates a shock front that compresses the cloud. This in turn initiates new fragmentation and collapse, forming more stars. When the massive O stars reach the end of their lives, they also undergo a supernova outburst that adds to the compression of the cloud and furthers star formation. Finding H II regions in dark-cloud complexes definitely demonstrates that stars are forming, as the example in Figure 17.13 shows.
We do not definitely know whether all stars that will originate in a giant molecular cloud form simultaneously. The formation of individual stars may spread over a period as long as 10 million years. However, when a molecular cloud begins to fragment in selected regions into a cluster of protostars of differing masses, the evolving stars will reach the main sequence at different times according to their mass. The more massive stars begin burning hydrogen first, and in beadlike progression the others arrive along the zero-age main sequence from upper left down to the lower right in the diagram. Stars of lower mass will lie progressively farther above and to the right of the zero-age main sequence at any instant in time after contraction starts.
The open cluster NGC 2264 shows this progression quite well in the H-R diagram in Figure 17.14. The less-massive cluster stars should eventually arrive on the lower portions of the main sequence in order of their mass. Many of the cluster's stars even have gas and dust shells. The cocoons, or shells, around these stars contain large quantities of dust apparently inherited from the original giant molecular cloud. The dust absorbs visible light from the forming star, heats up to several hundred K, and reradiates the energy as infrared radiation. Thus infrared studies can reveal what goes on in "stellar nurseries."
Additional evidence that can be used to identify recently formed stars comes from the coexistence of T Tauri variable stars in open clusters and veiling clouds, out of which they seem to have originated. The T Tauri variables have characteristics that we might expect for objects going through pre-main-sequence evolution; in particular, they lie above the main sequence in the H-R diagram. Sometimes very massive O and B stars, which are definitely quite young and already on the main sequence, are intermingled with T Tauri stars. It is presumed that T Tauri stars are in the 0.2 to 3.0M. range, contracting toward the main sequence. Typical radii measured for them are about five times that of the Sun.
About 4.6 billion years ago, after being a protostar some 30 million years, the Sun settled on the main sequence for a long, uninterrupted period of hydrogen burning. This stable phase in its life should continue for another 5 billion years. A star's time on the main sequence is the longest and most quiescent phase in its life. As hydrogen burning progresses, the energy-generating core is depleting hydrogen and converting it to helium. Because gas pressure depends on density, or number of particles, converting four hydrogen nuclei to one helium nucleus must reduce the gas pressure. Hydrogen burning will therefore be accompanied by a slight contraction of the energy-generating core and a heating up of the material. Because this increases the temperature difference between the center and surface causing a greater outflow of radiation, a star brightens slightly; the outer portion of the star also expands, increasing the radius. This is part of the reason for some of the width of the main sequence evident in H-R diagrams. As noted earlier, estimates from mathematical models for the Sun suggest that it has increased its luminosity by 20 to 30 percent during its 4.6 billion year existence as a main-sequence star. After the Sun exhausts its hydrogen fuel, it must undergo some relatively rapid changes that lead eventually to its death, as we shall see in the next chapter.