The numerical values and the evolutionary track just described are valid only for the case of a 1—solar mass star. The temperatures, densities, and radii of prestellar objects of other masses exhibit similar trends, but the numbers and the tracks differ, in some cases quite considerably. Perhaps not surprisingly, the most massive fragments within interstellar clouds tend to produce the most massive protostars and eventually the most massive stars. Similarly, low-mass fragments give rise to low-mass stars.
Figure 19.7 compares the theoretical pre—main-sequence track taken by the Sun with the corresponding evolutionary tracks of a 0.3—solar mass star and a 3—solar mass star. All three tracks traverse the H—R diagram in the same general manner, but cloud fragments that eventually form stars more massive than the Sun approach the main sequence along a higher track on the diagram, while those destined to form less massive stars take a lower track. The time required for an interstellar cloud to become a main-sequence star also depends strongly on its mass. The most massive cloud fragments heat up to the required 10 million K and become O stars in a mere million years, roughly 1/50 the time taken by the Sun. The opposite is the case for prestellar objects having masses less than that of our Sun. A typical M-type star, for example, requires nearly a billion years to form.
Figure 19.7 Prestellar evolutionary paths for stars more massive and less massive than our Sun.
Whatever the mass, the end point of the prestellar evolutionary track is the main sequence. A star is considered to have reached the main sequence when hydrogen burning begins in its core and the star's properties settle down to stable values. The main-sequence band predicted by theory is called the zero-age main sequence (ZAMS). It agrees quite well with main sequences observed for stars in the vicinity of the Sun and those observed for stars in more distant star clusters.
If all gas clouds contained precisely the same elements in exactly the same proportions, mass would be the sole determinant of a newborn star's location on the H—R diagram, and the zero-age main sequence would be a well-defined line rather than a broad band. However, the composition of a star affects its internal structure (mainly by changing the opacity of its outer layers), and this in turn affects both its temperature and its luminosity on the main sequence. Stars with more heavy elements tend to be cooler and slightly less luminous than stars that have the same mass but contain fewer heavy elements. As a result, differences in composition between stars "blur" the zero-age main sequence into the broad band we observe.
It is important to realize that the main sequence is itself not an evolutionary track—stars do not evolve along it. Rather, it is just a "way station" on the H—R diagram where stars stop and spend most of their lives—low-mass stars at the bottom, high-mass stars at the top. Once on the main sequence, a star stays in essentially the same location in the H—R diagram during its whole time as a stage 7 object. (In other words, a star that arrives on the main sequence as, say, a G star can never "work its way up" to become a B or an O main-sequence star, or move down to become an M-type red dwarf. As we will see in Chapter 20, the next stage of stellar evolution occurs when a star leaves the main sequence. A star leaving the main sequence and entering this next stage has pretty much the same surface temperature and luminosity it had when it arrived on the main sequence millions (or billions) of years earlier.
Some cloud fragments are too small ever to become stars. The giant planet Jupiter is a good example. Jupiter contracted under the influence of gravity, and the resultant heat is still detectable, but the planet did not have enough mass for gravity to crush its matter to the point of nuclear ignition. 
(Sec. 11.3) It became stabilized by heat and rotation before the central temperature became hot enough to fuse hydrogen. Jupiter never evolved beyond the protostar stage. If Jupiter, or any of the other jovian planets, had continued to accumulate gas from the solar nebula, they might eventually have become stars (probably to the detriment of life on Earth), but virtually all the matter present during the formative stages of our solar system is gone now, swept away by the solar wind.
Low-mass interstellar gas fragments simply lack the mass needed to initiate nuclear burning. Rather than turning into stars, they will continue to cool, eventually becoming compact, dark "clinkers"—cold fragments of unburned matter—in interstellar space. On the basis of theoretical modeling, astronomers believe that the minimum mass of gas needed to generate core temperatures high enough to begin nuclear fusion is about 0.08 solar masses.
Vast numbers of Jupiter-like objects may well be scattered throughout the universe—fragments frozen in time somewhere along the Kelvin—Helmholtz contraction phase. Small, faint, and cool (and growing ever colder), they are known collectively as brown dwarfs. Our technology currently has great difficulty in detecting them, be they planets associated with stars or interstellar cloud fragments far from any star. We can telescopically detect stars and spectroscopically infer atoms and molecules, but although recent advances in observational hardware and image-processing techniques have now identified several likely brown dwarf candidates (Interlude 19-1 ), astronomical objects of intermediate size outside our solar system remain very hard to see. Interstellar space could contain many cold, dark Jupiter-sized objects without our knowing it. Conceivably, they might even account for more mass than we observe in the form of stars and interstellar gas combined.
