The Milky Way and Beyond (18 page)

As the central temperature and density continue to rise, the proton-proton and carbon cycles become active, and the development of the (now genuine) star is stabilized. The star then reaches the main sequence, where it remains for most of its active life. The time required for the contraction phase depends on the mass of the star. A star of the Sun's mass generally requires tens of millions of years to reach the main sequence, whereas one of much greater mass might take a few hundred thousand years.

By the time the star reaches the main sequence, it is still chemically homogeneous. With additional time, the hydrogen fuel in the core is converted to helium, and the temperature slowly rises. If the star is sufficiently massive to have a convective core, the matter in this region has a chance to be thoroughly mixed, but the outer region does not mix with the core. The Sun, by contrast, has no convective core, and the helium-to-hydrogen ratio is maximum at the centre and decreases outward. Throughout the life of the Sun, there has been a steady depletion of hydrogen, so that the concentration of hydrogen at the centre today is probably only about one-third of the original amount. The rest has been transformed into helium. Like the rate of formation of a star, the subsequent rate of evolution on the main sequence is proportional to the mass of the star; the greater the mass, the more rapid the evolution. Whereas the Sun is destined to endure for some 10 billion years, a star of twice the Sun's mass burns its fuel at such a rate that it lasts about 3 billion years, and a star of 10 times the Sun's mass has a lifetime measured in tens of millions of years. By contrast, stars having a fraction of the mass of the Sun seem able to endure for trillions of years, which is much greater than the current age of the universe.

The spread of luminosities and colours of stars within the main sequence can be
understood as a consequence of evolution. At the beginning of their lives as hydrogen-burning objects, stars define a nearly unique line in the Hertzsprung-Russell diagram called the zero-age main sequence. Without differences in initial chemical composition or in rotational velocity, all the stars would start exactly from this unique line. As the stars evolve, they adjust to the increase in the helium-to-hydrogen ratio in their cores and gradually move away from the zero-age main sequence. When the core fuel is exhausted, the internal structure of the star changes rapidly; it quickly leaves the main sequence and moves toward the region of giants and supergiants.

As the composition of its interior changes, the star departs the main sequence slowly at first and then more rapidly. When about 10 percent of the star's mass has been converted to helium, the structure of the star changes drastically. All of the hydrogen in the core has been burned out, and this central region is composed almost entirely of inert helium, with trace admixtures of heavier elements. The energy production now occurs in a thin shell where hydrogen is consumed and more helium added to a growing but inert core. The outer parts of the star expand outward because of the increased burning there, and as the star swells up, its luminosity gradually increases. The details of the evolutionary process depend on the metal-to-hydrogen ratio, and the course of evolution differs for stars of different population types.

The great spread in luminosities and colours of giant, supergiant, and subgiant stars is also understood to result from evolutionary events. When a star leaves the main sequence, its future evolution is precisely determined by its mass, rate of rotation (or angular momentum), and chemical composition and whether it is a member of a close binary system. Giants and supergiants of nearly the same radius and surface temperature may have evolved from main-sequence stars of different ages and masses.

Theoretical calculations suggest that, as the star evolves from the main sequence, the hydrogen-helium core gradually increases in mass but shrinks in size as more and more helium ash is fed in through the outer hydrogen-burning shell. Energy is carried outward from the shell by rapid convection currents. The temperature of the shell rises; the star becomes more luminous; and it finally approaches the top of the giant domain on the Hertzsprung-Russell diagram. By contrast, the core shrinks by gravitational contraction, becoming hotter and denser until it reaches a central temperature of about 120 million K. At that temperature the previously inert helium is consumed in the production of heavier elements.

When two helium nuclei each of mass 4 atomic units (
⁴
He) are jammed together,
it might be expected that they would form a nucleus of beryllium of mass 8 atomic units (
⁸
Be). In symbols,

⁴
He +
⁴
He →
⁸
Be.

Actually, however,
⁸
Be is unstable and breaks down into two helium nuclei. If the temperature and density are high enough, though, the short-lived beryllium nucleus can (before it decays) capture another helium nucleus in what is essentially a three-body collision to form a nucleus of carbon-12—namely,

⁸
Be +
⁴
He →
¹²
C.

This fusion of helium in the core, called the triple alpha process, can begin gradually in some stars, but in stars with masses between about half of and three times the Sun's mass, it switches on with dramatic suddenness, a process known as the helium flash. Outwardly the star shows no discernible effect, but the course of its evolution is changed with this new source of energy. Having only recently become a red giant, it now evolves somewhat down and then to the left in the Hertzsprung-Russell diagram, becoming smaller and hotter. This stage of core helium burning, however, lasts only about a hundredth of the time taken for core hydrogen burning. It continues until the core helium supply is exhausted, after which helium fusion is limited to a shell around the core, just as was the case for hydrogen in an earlier stage. This again sets the star evolving toward the red giant stage along what is called the asymptotic giant branch, located slightly above the main region of giants in the Hertzsprung-Russell diagram.

In more massive stars, this cycle of events can continue, with the stellar core reaching ever-higher temperatures and fusing increasingly heavy nuclei, until the star eventually experiences a supernova explosion. In lower-mass stars like the Sun, however, there is insufficient mass to squeeze the core to the temperatures needed for this chain of fusion processes to proceed, and eventually the outermost layers extend so far from the source of nuclear burning that they cool to a few thousand kelvins. The result is an object having two distinct parts: a well-defined core of mostly carbon ash (a white dwarf star) and a swollen spherical shell of cooler and thinner matter spread over a volume roughly the size of the solar system. Such shells of matter, called planetary nebulae, are actually observed in large numbers in the sky. Of the nearly 3,000 examples known in the Milky Way Galaxy alone, NGC 7027 is the most intensively studied.

Objects called brown dwarfs are intermediate between a planet and a star and so evolve differently from low-mass stars. Brown dwarfs usually have a mass less than 0.075 that of the Sun, or roughly 75 times that of Jupiter. (This maximum mass is a little higher for objects with fewer heavy elements than the Sun.) Many astronomers draw the line between brown dwarfs and planets at the lower fusion boundary of about 13 Jupiter masses. The difference between brown dwarfs and stars is that, unlike stars, brown dwarfs do not reach stable luminosities by thermonuclear fusion of normal hydrogen. Both stars and brown dwarfs produce energy by fusion of deuterium (a rare isotope of hydrogen) in their first few million years. The cores of stars then continue to contract and get hotter until they fuse hydrogen. However, brown dwarfs prevent further contraction because their cores are dense enough to hold themselves up with electron degeneracy pressure. (Those brown dwarfs above 60 Jupiter masses begin to fuse hydrogen, but they then stabilize, and the fusion stops.)

The brown dwarf 2MASSWJ 1207334-393254
(centre)
as seen in a photo taken by the Very Large Telescope at the European Southern Observatory, Cerro Paranal, Chile. Orbiting the brown dwarf at a distance of 69 billion km (43 billion miles) is a planet
(lower left)
that has a mass four times that of Jupiter
. ESO

Brown dwarfs are not actually brown but appear from deep red to magenta depending on their temperature. Objects below about 2,200 K, however, do actually have mineral grains in their atmospheres. The surface temperatures of brown dwarfs depend on both their mass and their age. The most massive and youngest brown dwarfs have temperatures as high as 2,800 K, which overlaps with the temperatures of very low-mass stars, or red dwarfs. (By comparison, the Sun has a surface temperature of 5,800 K.) All brown dwarfs eventually cool below the minimum main-sequence stellar temperature of about 1,800 K. The oldest and smallest can be as cool as about 500 K.

Brown dwarfs were first hypothesized in 1963 by American astronomer Shiv Kumar, who called them “black” dwarfs. American astronomer Jill Tarter proposed the name “brown dwarf” in 1975; although brown dwarfs are not brown, the name stuck because these objects were thought to have dust, and the more accurate “red dwarf” already described a different type of star. Searches for brown dwarfs in the 1980s and 1990s found several candidates; however, none was confirmed as a brown dwarf. In order to distinguish brown dwarfs from stars of the same temperature, one can search their spectra for evidence of lithium (which stars destroy when hydrogen fusion begins). Alternatively, one can look for (fainter) objects below the minimum stellar temperature. In 1995 both
methods paid off. Astronomers at the University of California, Berkeley, observed lithium in an object in the Pleiades, but this result was not immediately and widely embraced. This object, however, was later accepted as the first binary brown dwarf. Astronomers at Palomar Observatory and Johns Hopkins University found a companion to a low-mass star called Gliese 229 B. The detection of methane in its spectrum showed that it has a surface temperature less than 1,200 K. Its extremely low luminosity, coupled with the age of its stellar companion, implies that it is about 50 Jupiter masses. Hence, Gliese 229 B was the first object widely accepted as a brown dwarf. Infrared sky surveys and other techniques have now uncovered hundreds of brown dwarfs. Some of them are companions to stars; others are binary brown dwarfs; and many of them are isolated objects. They seem to form in much the same way as stars, and there may be 1–10 percent as many brown dwarfs as stars.

The relative abundances of the chemical elements provide significant clues regarding their origin. Earth's crust has been affected severely by erosion, fractionation, and other geologic events, so that its present varied composition offers few clues as to its early stages. The composition of the matter from which the solar system formed is deduced from that of stony meteorites called chondrites and from the composition of the Sun's atmosphere, supplemented by data acquired from spectral observations of hot stars and gaseous nebulae. The table lists the most abundant chemical elements; it represents an average pertaining to all cosmic objects in general.

The most obvious feature is that the light elements tend to be more abundant than the heavier ones. That is to say, when abundance is plotted against atomic mass, the resulting graph shows a decline with increasing atomic mass up to an atomic mass value of about 100. Thereafter the abundance is more nearly constant. Furthermore, the decline is not smooth. Among the lighter elements, those of even atomic number tend to be more abundant, and those with an atomic number divisible by four are especially favoured. The abundances of lithium, beryllium, and boron are rare compared with those of carbon, nitrogen, and oxygen. There is a pronounced abundance peak for iron and a relatively high peak for lead, the most stable of the heavy elements.