Molecular cloud

Molecular clouds are massive, dense regions of interstellar gas and dust. They are often called stellar nurseries because they provide the raw materials for new stars to form. These clouds make up about 50% of the total interstellar gas in a galaxy. While they occupy less than one percent of the interstellar medium's volume, they are the densest parts of it.

Most of the gas within these structures exists in a molecular state. This state allows for complex chemistry and the eventual collapse of matter into stars.

The internal structure of a molecular cloud is irregular and filamentary. It is not a uniform blob but a collection of varying densities. Large substructures called clumps form the main body of the cloud. These clumps are typically about 1 parsec in size. Inside these clumps, there are even denser regions called cores.

Cores are about ten times smaller than clumps. When gravity becomes strong enough, these cores undergo a gravitational collapse. This collapse is the primary mechanism that begins the process of star formation.

Chemistry plays a vital role in how these clouds function. The most abundant gas is molecular hydrogen, known as H2. However, H2 is very difficult to detect directly because its symmetrical shape results in weak rotational and vibrational modes. To find these clouds, astronomers use carbon monoxide (CO) as a tracer. CO is much easier to observe because of its asymmetrical structure and rotational energy.

Cosmic dust also plays a key role by providing shielding. This dust protects the internal molecular gas from being broken apart by ultraviolet radiation. Without this shielding, ultraviolet photons would cause dissociation, turning molecular gas back into atomic gas.

The history of discovering these clouds is tied to the rise of radio astronomy. In 1951, astronomers began detecting radio emissions from neutral hydrogen. Ewen and Purcell reported the 21-cm line in March of that year. Later that May, Muller and Oort reported the same detection at the Kootwijk Observatory.

This 21-cm line occurs when a proton and electron in a hydrogen atom flip their spin states. This flip releases excess energy as a radio signal at 1420.405 MHz. This discovery allowed scientists to map the spiral arms of the Milky Way for the first time.

Following the detection of hydrogen, astronomers began searching for other interstellar molecules. In 1963, Alan Barrett and Sander Weinred found OH emissions at MIT. By 1968, researchers detected ammonia (NH3) in interstellar space. In 1969, scientists identified formaldehyde and molecular hydrogen.

The final breakthrough came in 1970. Arno Penzias, Keith Jefferts, and Robert Wilson identified CO in the Omega Nebula. They also found the giant molecular cloud Sagittarius B2, located 390 light years from the galactic center.

This was the first official detection of a molecular cloud.

Molecular clouds have a specific distribution within the Milky Way. They are mostly found in a ring between the Sun and the galactic center. The Sun is located about 8.5 kiloparsecs from the center. These clouds also align with the galaxy's spiral arms.

Because they reside in the spiral arms, they must form and dissipate quickly. They inhabit a narrow midplane of the galactic disc. This layer has a scale height of only 50 to 75 parsecs. This is much thinner than the layers of warm atomic or ionized gas.

These clouds are short-lived structures in cosmic terms. They typically exist for about 10 million years before being destroyed or changing. We know this because the young stars found inside them are usually 10 to 20 million years old.

Once stars like OB stars reach 10 million years of age, the surrounding cloud material is often dispersed. The clouds are constantly being assembled and destroyed through a continuous cycle. In the Milky Way, approximately 150 solar masses of gas are assembled into molecular clouds every year. This cycle ensures that star formation remains a constant process in our galaxy.