by Paul Ruffle
This essay is based on the introduction to molecular clouds in chapter 8 of my PhD thesis.
Fig. 1: The Eagle Nebula (M16), comprising cold gas and dust, illuminated by ultraviolet light from a nearby cluster of massive, hot, young, luminous stars. Image taken in November 2004 with the Advanced Camera for Surveys aboard NASA's Hubble Space Telescope (HST). Credits: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
The space between the stars, or interstellar medium (ISM) is far from empty, and although the average density is only about one particle per cubic centimetre (compared to n ∼ 1019 cm-3 for gas at room temperature and pressure), in the Galactic mid-plane material other than known stars accounts for ∼60 per cent of the mass. Molecular clouds are cool dense regions of the ISM composed of molecular hydrogen (∼90%), helium (∼10%), and small amounts of other molecules, with temperatures of 10 to 20 K and densities of the order of n(H2) = 104 cm-3 (compared to a typical laboratory vacuum of n ≈ 107 cm-3). As discussed in more detail in Our Dusty Universe, dust particles of carbon or silicate material comprise about 1 per cent of a cloud's mass, and are the cause of extinction within the cloud.
Molecular clouds are typically 100-106 Msun, and around 100 pc in size. The free-fall time for such clouds is ∼106 yr. The majority of clouds in the Milky Way lie in a broad Molecular Ring encircling the Galactic centre with an inner radius of about 3 kpc and an ill-defined outer radius extending to beyond 20 kpc. They also account for roughly one-half of the total gas mass within the Sun's orbit. They are the largest gravitationally bound objects in the Galaxy, and the largest known objects in the Universe made of molecular material. Molecular clouds are the only places where star formation (and planet formation) is believed to occur. The other types of interstellar clouds, in which hydrogen is atomic, are too warm and diffuse to allow stars to form.
Molecular hydrogen is difficult to detect, so the molecule most used to trace H2 is the next most common cloud molecule, carbon monoxide. The ratio between CO luminosity and H2 mass is roughly constant, with a H2/CO ratio of around 105. The 2.6 and 1.33 mm CO lines are the prime means of investigating molecular clouds, as hydrogen does not normally emit at radio frequencies. Well over 100 other molecules have been detected by their radio emission, due to transitions between their different rotational states, driven by collisional excitation. The energy differences of these transitions are such that wavelengths fall in the submillimetre and millimetre range, i.e. the gigahertz region of the radio spectrum, where ground based observations are possible (preferably at high altitudes with low humidity), due to reasonable transparency of the Earth's atmosphere at these wavelengths.
Most of the molecules are organic, i.e. based on carbon, but inorganic species, such as silicon oxide (SiO) have also been detected. The isotopomers of many molecules have also been detected (e.g. 12CO, 13CO and C18O), enabling isotope ratios to be established in the ISM. Deuterated molecules have also been detected in molecular clouds, where chemical reactions substitute a deuterium atom for one of hydrogen. Estimates of deuterium fractionation lead to a determination of the underlying D/H ratio, which is of cosmological interest, as the amount of deuterium in the Universe has been steadily decreasing (due to generations of stars destroying it), since nucleosynthesis occurred ∼100 seconds after the Big Bang.
Molecular clouds are often clumpy, with several denser regions, n(H2) ∼ 105
cm-3, containing 100 to 1,000 solar masses. For the largest of clouds (103-106 Msun), known as Giant Molecular Clouds (GMCs), these dense cores contain infrared sources, HII regions and maser sources. The peak CO temperatures also observed, suggest that these regions are the sites of massive star formation, with maser emission often characteristic of regions surrounding protostars or prestellar cores. Magnetic fields have also been detected, where magnetic pressure appears more influential than turbulence in slowing star formation within a cloud, although gravity finally wins. There are also isolated small clouds ∼ 500 Msun of molecular hydrogen, where low mass stars form. An average spiral galaxy, like the Milky Way, is thought to contain about 1,000 to 2,000 GMCs in addition to numerous smaller clouds.
Perturbations in the ISM, such as supernova shock waves and stellar winds are thought to amplify inhomogeneities in molecular clouds, leading to the formation of these denser clumps of gas. These denser regions undergo gravitational collapse, but as the potential energy of the clump is converted into kinetic energy the temperature rises, with the increased kinetic energy opposing continued contraction. However, for further collapse to occur that will lead to star formation, this additional energy must be radiated away. Molecular hydrogen is inefficient at cooling a cloud, but other molecular species, particularly CO, radiate effectively via collisional excitations, thereby cooling the clump and allowing further contraction that ultimately leads to the formation of prestellar cores. This raises the question as to how matter in the early Universe, devoid of metals, was able to contract and cool to form the first stars.
Star forming molecular clouds are thought to be disrupted by the effects of any massive stars that form in them, and before a significant fraction of the cloud mass can become stars. Further star formation may also stop after a relatively small number of stars have been born, because the stellar nursery is blown away due to the radiation and stellar winds from the newly formed stars. The hottest (i.e. more massive) of these heat the surrounding molecular gas, break up its molecules, and drive the gas away. The previously hidden young stars become visible, and the molecular cloud and its star-forming capability cease to exist. As a result GMCs may only last for 10 to 100 million years before they dissipate.
Fig. 1 illustrates the above with an image of the Eagle Nebula (M16), which comprises cold gas and dust, illuminated by ultraviolet light from a nearby cluster of massive, hot, young, luminous stars. The column shaped cloud is silhouetted against the background glow of more distant gas. The denser clouds of hydrogen gas and dust, at the top of the image, have survived the intense ultraviolet light from the hot young stars longer than the surrounding less dense gas. The bumps and fingers of material, in the centre of the image, are examples of possible stellar birthing areas, where fledgling stars can grow as they feed off the surrounding gas cloud. The young cluster's intense starlight may be inducing star formation in some regions of the cloud. Examples can be seen in the large, glowing clumps and finger-shaped protrusions at the top of the image. The stars may be heating the gas and creating a shock front, as seen by the bright rim of material tracing the edge of the nebula at top, left. As the heated gas expands, the intense pressure compresses the gas, leading to star formation. This scenario may continue as the shock front moves through the nebula.
The effect of shock waves causing star formation is illustrated in the extreme situation of GMCs in colliding galaxies. The collision compresses the interstellar gas, and the resultant hot gas drives shock waves into the clouds triggering rapid star birth throughout the clouds. In this case the several hundred thousand stars that form from the cold molecular gas in such clouds use up most of the gas before it has time to be heated and dispersed. The result of such violent events is the nearly complete conversion of GMCs into rich star clusters, each containing up to 1 million stars. Observations by the Hubble telescope suggest that many of these newly born star clusters remain bound by their own gravity and evolve into globular clusters, like those observed in the Milky Way's halo.