A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H<sub>2</sub>), and the formation of H II regions. This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H<sub>2</sub> is carbon monoxide (CO). The ratio between CO luminosity and H<sub>2</sub> mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies.
Within molecular clouds are regions with higher density, where much dust and many gas cores reside, called clumps. These clumps are the beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse.
Research and discovery
thumb|left|Astronomer Henk van de Hulst first theorized hydrogen could be traceable in interstellar space using radio signals.
The history pertaining to the discovery of molecular clouds is closely related to the development of radio astronomy and astrochemistry. During World War II, at a small gathering of scientists, Henk van de Hulst first reported he had calculated the neutral hydrogen atom should transmit a detectable radio signal. This discovery was an important step towards the research that would eventually lead to the detection of molecular clouds.
thumb|right|Jansky and his rotating directional radio antenna (early 1930s), the world's first radio telescope
Once the war ended, and aware of the pioneering radio astronomical observations performed by Jansky and Reber in the US, the Dutch astronomers repurposed the dish-shaped antennas running along the Dutch coastline that were once used by the Germans as a warning radar system and modified into radio telescopes, initiating the search for the hydrogen signature in the depths of space.
The neutral hydrogen atom consists of a proton with an electron in its orbit. Both the proton and the electron have a spin property. When the spin state flips from a parallel condition to antiparallel, which contains less energy, the atom gets rid of the excess energy by radiating a spectral line at a frequency of 1420.405 MHz.
Occurrence
thumb|left|Molecular cloud [[Barnard 68, about 500 ly distant and 0.5 ly in diameter]]
Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of it. The bulk of the molecular gas is contained in a ring between from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on a timescale shorter than 10 million years—the time it takes for material to pass through the arm region.
thumb|Circinus molecular cloud has a mass around 250,000 times that of the Sun.
Perpendicularly to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height, Z, of approximately 50 to 75 parsecs, much thinner than the warm atomic (Z from 130 to 400 parsecs) and warm ionized (Z around 1000 parsecs) gaseous components of the ISM. The exceptions to the ionized-gas distribution are H II regions, which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars; and as such they have approximately the same vertical distribution as the molecular gas.
This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of the gas is highly irregular, with most of it concentrated in discrete clouds and cloud complexes. Most of the gas is found in a molecular state. The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in a fast transition, forming "envelopes" of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary. Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance.
thumb|left|The star T Tauri with NGC 1555 cloud nearby
Most of the gas constituting a molecular cloud is molecular hydrogen, with carbon monoxide being the second most common compound.
thumb|left|The Elephant's Trunk Nebula is an elongated dark globule. The globule is a condensation of dense gas that is barely surviving the strong ionizing radiation from a nearby massive star.Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the collision theory have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures. This process begins when approximately 2% of the mass of the cloud has been converted into stars. Stellar winds are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability. The burning of hydrogen then generates enough heat to push against gravity, creating hydrostatic equilibrium. At this stage, a protostar is formed and it will continue to aggregate gas and dust from the cloud around it.
One of the most studied star formation regions is the Taurus molecular cloud due to its close proximity to earth (140 pc or 430 ly away), making it an excellent object to collect data about the relationship between molecular clouds and star formation. Embedded in the Taurus molecular cloud there are T Tauri stars. These are a class of variable stars in an early stage of stellar development and still gathering gas and dust from the cloud around them. Observation of star forming regions have helped astronomers develop theories about stellar evolution. Many O and B type stars have been observed in or very near molecular clouds. Since these star types belong to population I (some are less than 1 million years old), they cannot have moved far from their birth place. Many of these young stars are found embedded in cloud clusters, suggesting stars are formed inside it.]]
A vast assemblage of molecular gas that has more than 10 thousand times the mass of the Sun is called a giant molecular cloud (GMC). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser than a GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps. A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation. Recent studies have suggested that filamentary structures in molecular clouds play a crucial role in the initial conditions of star formation and the origin of the stellar IMF.
The densest parts of the filaments and clumps are called molecular cores, while the densest molecular cores are called dense molecular cores and have densities in excess of 10<sup>4</sup> to 10<sup>6</sup> particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with ammonia. The concentration of dust within molecular cores is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae.
GMCs are so large that local ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion molecular cloud (OMC) or the Taurus molecular cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of the Sun coinciding with the Gould Belt. The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space.thumb|Distribution of molecular gas in 30 merging galaxies
Small molecular clouds
Isolated gravitationally-bound small molecular clouds with masses less than a few hundred times that of the Sun are called Bok globules. The densest parts of small molecular clouds are equivalent to the molecular cores found in GMCs and are often included in the same studies.
High-latitude diffuse molecular clouds
In 1984 IRAS identified a new type of diffuse molecular cloud. These were diffuse filamentary clouds that are visible at high galactic latitudes. These clouds have a typical density of 30 particles per cubic centimetre.thumb|The [[Serpens South star cluster is embedded in a filamentary molecular cloud, seen as a dark ribbon passing vertically through the cluster. This cloud has served as a testbed for studies of molecular cloud stability.]]
List of molecular cloud complexes
- Sagittarius B2
- Serpens-Aquila Rift
- Rho Ophiuchi cloud complex
- Corona Australis molecular cloud
- Musca–Chamaeleonis molecular cloud
- Vela Molecular Ridge
- Radcliffe wave
- Orion molecular cloud complex
- Taurus molecular cloud
- Perseus molecular cloud