The Milky Way and Beyond (27 page)

The chemistry and physical conditions of the interior of a molecular cloud are quite different from those of the surrounding low-density interstellar medium. In the outer parts of the dark cloud, hydrogen is neutral. Deeper within it, as dust blocks out an increasing amount of stellar ultraviolet radiation, the cloud becomes darker and colder. Approaching the centre, the predominant form of gaseous carbon changes successively from C
⁺
on the outside to neutral C (C
⁰
) and finally to the molecule carbon monoxide (CO), which is so stable that it remains the major form of carbon in the gas phase in the darkest regions. At great depths within the cloud, other molecules can be seen from their microwave transitions, and more than 150 chemical species have been identified within the constituent gas.

Because of the comparatively low densities and temperatures, the chemistry is very exotic, as judged by terrestrial experiments; some rather unstable species can exist in space because there is not enough energy to convert them to more-stable forms. An example is the near equality of the abundances of the interstellar molecule HNC (hydroisocyanic acid) and its isomer HCN (hydrocyanic acid); in ordinary terrestrial conditions there is plenty of energy to allow the nitrogen and carbon atoms in HNC to exchange positions and produce HCN, by far the preferred species for equilibrium chemistry. In the cold clouds,
however, not enough energy exists for the exchange to occur. There is less than one-thousandth as much starlight within a cloud as in the interstellar space outside the cloud, and the heating of the material in the cloud is provided primarily by cosmic rays. Cooling within the cloud occurs chiefly by transitions between low-lying levels of the carbon monoxide molecule.

The emission lines from C
⁺
, C
⁰
, and CO show that the edges of the molecular clouds are very convoluted spatially, with stellar ultraviolet radiation able to penetrate surprisingly far throughout the cloud despite the absorption of dust. Stellar radiation can apparently enter the cloud through channels where the dust (and gas) density is lower than average. The clumpiness of the interstellar material has profound effects on its properties.

In the inner regions of molecular clouds an important event takes place: the formation of stars from the gravitational collapse of dense clumps within the nebula. Initially the cloud consists of a chaotic jumble of smaller clouds, each of which is destined to be an individual stellar system. Each system has a rotary motion arising from the original motions of the material that is falling into it. Because of this spin, the collapsing cloud flattens as it shrinks. Eventually most of its mass is in a rotating condensation near its centre, a “protostar” destined to become one or more closely spaced stars. Surrounding the protostar is a rotating disk larger than the solar system that collapses into “protoplanets” and comets.

These ideas are given encouraging confirmation by observations of molecular clouds in very long wavelength infrared radiation. Some of the brightest infrared sources are associated with such dark dust clouds; a good example is the class of T Tauri variables, named for their prototype star in the constellation Taurus. The T Tauri stars are known for a variety of reasons to be extremely young. The variables are always found in or near molecular clouds; they often are also powerful sources of infrared radiation, corresponding to warm clouds of dust heated by the T Tauri star to a few hundred kelvins. There are some strong infrared sources (especially in the constellation of Orion) that have no visible stars with them; these are presumably “cocoon stars” completely hidden by their veils of dust.

One of the remarkable features of molecular clouds is their concentration in the spiral arms in the plane of the Milky Way Galaxy. While there is no definite boundary to the arms, which have irregularities and bifurcations, the nebulae in other spiral galaxies are strung out along these narrow lanes and form a beautifully symmetric system when viewed from another galaxy. The nebulae are remarkably close to the galactic plane; most are within 300 light-years, only 1 percent of the Sun's distance from the centre. The details of the explanation of why the gas is largely confined to the
spiral arms is beyond the scope of this book. Briefly, the higher density of the stars in the arms produces sufficient gravity to hold the gas to them.

Why doesn't the gas simply condense into stars and disappear? The present rate of star formation is about one solar mass per year in the entire Galaxy, which contains something like 2 × 10
⁹
solar masses of gas. Clearly, if the gas received no return of material from stars, it would be depleted in roughly 2 × 10
⁹
years, about one-sixth the present age of the Galaxy. There are several processes by which gas is returned to the interstellar medium. Possibly the most important is the ejection of planetary nebula shells; other processes are ejection of material from massive O- and B-type normal stars or from cool M giants and supergiants. The rate of gas ejection is roughly equal to the rate of star formation, so that the mass of free gas is declining very slowly. (Some gas is also falling into the Galaxy that has never been associated with any galaxy.)

This cycling of gas through stars has had one major effect: the chemical composition of the gas has been changed by the nuclear reactions inside the stars. There is excellent evidence that the Galaxy originally consisted of 77 percent hydrogen by mass and that almost all of the rest of the constituent matter was helium. All heavy elements have been produced inside stars by being subjected to the exceedingly high temperatures and densities in the central regions. Thus, most of the atoms and molecules on Earth, as well as in human bodies, owe their very existence to processes that occur within stars.

A different type of nebula is the hydrogen cloud, or the H I region; this region is interstellar matter in which hydrogen is mostly neutral, rather than ionized or molecular. Most of the matter between the stars in the Milky Way Galaxy, as well as in other spiral galaxies, occurs in the form of relatively cold neutral hydrogen gas. Neutral hydrogen clouds are easily detectable at radio wavelengths because they emit a characteristic energy at a wavelength of 21 cm (8 in).

Neutral hydrogen is dominant in clouds that have enough starlight to dissociate molecular hydrogen into atoms but lack hydrogen-ionizing photons from hot stars. These clouds can be seen as separate structures within the lower-density interstellar medium or else on the outer edges of the molecular clouds. Because a neutral cloud moves through space as a single entity, it often can be distinguished by the absorption line that its atoms or ions produce at their common radial velocity in the spectrum of a background star.

If neutral clouds at a typical pressure were left alone until they could reach an equilibrium state, they could exist at either of two temperatures: “cold” (about 80 K) or “warm” (about 8,000 K), both determined by the balance of heating and cooling rates. There should be little material in between. Observations show that
these cold and warm clouds do exist, but roughly half the material is in clouds at intermediate temperatures, which implies that turbulence and collisions between clouds can prevent the equilibrium states from being reached. Cold H I regions are heated by electrons ejected from the dust grains by interstellar ultraviolet radiation incident upon such a cloud from outside. Cooling is mainly by C
⁺
because passing electrons or hydrogen atoms can excite it from its normal energy state, the lowest, to one slightly higher, which is then followed by emission of radiation at 158 micrometres. This line is observed to be very strong in the spectrum of the Milky Way Galaxy as a whole, which indicates that a great deal of energy is removed from interstellar gas by this process. Cold H I regions have densities of 10 to 100 hydrogen atoms per cubic cm. Warm H I regions are cooled by excitation of the
n
= 2 level of hydrogen, which is at a much higher energy than the lowest level of C
⁺
and therefore requires a higher temperature for its excitation. The density of 0.5 atom per cubic cm (1 cubic cm = .06 in) is much lower than in the colder regions. At any particular density there is far more neutral hydrogen available for cooling than C
⁺
.

The reflection nebulae are interstellar clouds that would normally be dark nebulae but whose dust reflects the light from a nearby bright star that is not hot enough to ionize the cloud's hydrogen. The famous nebulosity in the Pleiades star cluster is of this type. It was discovered in 1912 that the spectrum of this nebula mimics the absorption lines of the nearby stars, whereas bright nebulae that emit their own light show their own characteristic emission lines. The brightest reflection nebulae are illuminated by B-type stars that are very luminous but have temperatures lower than about 25,000 K, cooler than the O-type stars that would ionize the hydrogen in the gas and produce an H II region.

The extent and brightness of reflection nebulae show conclusively that dust grains are excellent reflectors in the broad range of wavelengths extending from the ultraviolet (as determined from observations from space) through the visible. Optical observations suggest that about 60–70 percent of the light is reflected rather than absorbed, while the corresponding fraction for Earth is only 35 percent and for the Moon a mere 5 percent. Grains reflect light almost as well as fresh snow, more because of their favourable size (which promotes scattering rather than absorption) than their chemical composition. Calculations show that even graphite, which is black in bulk, reflects visible light well when dispersed into small particles.

Nebulae that are full of ionized hydrogen atoms are H II regions. (These regions are also called diffuse nebulae or emission nebulae.) The energy that is responsible for ionizing and heating the hydrogen in
an emission nebula comes from a central star that has a surface temperature in excess of 20,000 K. The density of these clouds normally ranges from 10 to 100,000 particles per cubic cm (1 cubic cm = .06 in); their temperature is about 8,000 K.

Like molecular clouds, H II regions typically have little regular structure or sharp boundaries. Their sizes and masses vary widely. There is even a faint region of ionized gas around the Sun and other comparatively cool stars, but it cannot be observed from nearby stars with existing instruments.

The largest H II regions (none of which occur in the Milky Way Galaxy) are 500 light-years across and contain at least 100,000 solar masses of ionized gas. These enormous H II regions are powered by clusters of massive hot stars rather than by any single stellar body. A typical H II region within the Galaxy measures about 30 light-years in diameter and has an average density of about 10 atoms per cubic cm. The mass of such a cloud amounts to several hundred solar masses. The only H II region visible to the naked eye is the beautiful Orion Nebula. It is located in the constellation named for the Greek mythological hunter and is seen as the central “star” in Orion's sword. The entire constellation is enveloped in faint emission nebulosity, powered by several stars in Orion's belt rather than by the star exciting the much smaller Orion Nebula. The largest H II region in terms of angular size is the Gum Nebula, discovered by Australian astronomer Colin S. Gum. It measures 40° in angular diameter and is mainly ionized by two very hot stars (Zeta Puppis and Gamma Velorum).

High-resolution studies of H II regions reveal one of the surprises that make the study of astrophysics delightful. Instead of the smooth structure that might be expected of a gas, a delicate tracery of luminous filaments can be detected down to the smallest scale that can be resolved. In the Orion Nebula this is about 6 billion km (4 billion miles), or about the radius of the orbit of Pluto around the Sun. Even finer details almost surely exist, and there is evidence from spectra that much of the matter may be gathered into dense condensations, or knots, the rest of the space being comparatively empty. Unrestrained gas would
fill a vacuum between the visible filaments in about 200 years, an astronomical instant. The nebular gas must be restrained from expansion by the pressure of million-degree tenuous material between the filaments. Its pressure, however, is comparable to that in the visible “warm” (8,000 K) gas of the H II region. Hence, the density of the hot material is several hundred times lower, which effectively prevents it from being observable except in X-rays. The space throughout the plane of the Milky Way Galaxy is largely filled with this hot component, which is mainly produced and heated by supernovae.