Molecular cloud
Dense interstellar clouds where stars are born.
A molecular cloud is a type of interstellar cloud dense and large enough to allow for absorption nebulae, the creation of molecules (especially molecular hydrogen, or H2), and the formation of H II regions. This sets it apart from most of the interstellar medium, which is mostly made up of ionized gas. When star formation is happening inside one, it is often called a stellar nursery. In the Milky Way, these clouds make up less than one percent of the interstellar medium's volume, but they are its densest component.
Detecting molecular hydrogen directly is tricky because it barely shows up in infrared or radio observations. Instead, astronomers use carbon monoxide (CO) as a stand-in. The ratio between how bright CO is and how much H2 is present is generally considered steady, though this assumption has been questioned when looking at other galaxies. Inside a molecular cloud, there are denser pockets called clumps, which contain a lot of dust and gas cores. If gravity is strong enough, these clumps can collapse and start forming stars.
The discovery of molecular clouds is tied to the rise of radio astronomy and astrochemistry. During World War II, Henk van de Hulst predicted that neutral hydrogen atoms should give off a detectable radio signal. After the war, Dutch astronomers took old German radar dishes along the Dutch coast and turned them into radio telescopes to hunt for this signal. Neutral hydrogen has a proton and an electron, each with a spin. When the spins flip from parallel to antiparallel (a lower-energy state), the atom releases energy as a spectral line at 1420.405 MHz, known as the 21 cm line. This line is the signature of atomic hydrogen (HI) and made it detectable from Earth.
In 1951, two teams nearly simultaneously found this radio emission from interstellar neutral hydrogen. Ewen and Purcell reported it in March, and Muller and Oort followed in May using the Kootwijk Observatory. After that, radio astronomers mapped the Milky Way's neutral hydrogen. In 1958, van de Hulst, Muller, Oort, and an Australian team published the Leiden-Sydney map, the first to show the galaxy's spiral arm structure. Astronomers then turned to looking for molecules. In 1963, Alan Barrett and Sander Weinred detected OH emission in the Cassiopeia A supernova remnant, the first radio detection of an interstellar molecule. More OH detections followed, and in 1965, Harold Weaver's team found OH emissions from the Orion Nebula and Cassiopeia. In 1968, Cheung, Rank, Townes, Thornton, and Welch detected ammonia (NH₃) in space. The next year, Lewis Snyder's team found formaldehyde, and George Carruthers identified molecular hydrogen. These discoveries led to the first detection of a molecular cloud in 1970.
Hydrogen is the most common atom in these clouds, and under the right conditions it forms H₂. But H₂ is hard to see directly because its symmetrical structure gives it weak rotational and vibrational signals. The breakthrough came when Arno Penzias, Keith Jefferts, and Robert Wilson found CO in the Omega Nebula's star-forming region. CO is much easier to detect due to its asymmetry and rotational energy, and it became the main tracer of star-forming clouds. In 1970, Penzias's team quickly found CO near the galactic center, including in Sagittarius B2, a giant molecular cloud 390 light-years from the center. This was the first molecular cloud ever detected. (This team later won the Nobel Prize for discovering the Big Bang's microwave background.) Research has grown since then; a 2022 paper reported over 10,000 molecular clouds have been found since Sagittarius B2.
Most of the Milky Way's molecular gas sits in a ring between 3.5 and 7.5 kiloparsecs (11,000 to 24,000 light-years) from the galactic center. (The Sun is about 8.5 kiloparsecs out.) Large-scale CO maps of the galaxy continue to chart these structures.
- field
- Astronomy, Astrophysics
- composition
- Primarily molecular hydrogen (H2) and carbon monoxide (CO)
- location_in_milky_way
- Concentrated in a ring between 3.5 and 7.5 kiloparsecs from the galactic center, predominantly in spiral arms
Lore & Background
A molecular cloud is a type of interstellar cloud whose density and size allow it to form absorption nebulae, molecules (most commonly molecular hydrogen, H₂), and H II regions, unlike other areas of the interstellar medium that are mostly ionized gas. These clouds are sometimes called stellar nurseries when star formation is occurring. Molecular hydrogen is difficult to detect via infrared and radio observations, so carbon monoxide (CO) is the molecule most often used to trace the presence of H₂. The ratio between CO luminosity and H₂ mass is thought to be constant, though this assumption is doubted in observations of some other galaxies. Within molecular clouds are denser regions called clumps, which contain much dust and many gas cores. If gravitational forces are sufficient, these clumps begin the process of star formation. The first detection of a molecular cloud occurred in 1970, when a team led by Arno Penzias identified CO in the Omega Nebula and then quickly detected CO in other locations near the galactic center, including the giant molecular cloud Sagittarius B2. This discovery built on earlier work: the 1951 detection of the 21 cm line from neutral hydrogen by two research groups, and the subsequent mapping of neutral hydrogen in the Milky Way’s galactic disk, which revealed its spiral arm structure. Later detections of interstellar molecules such as OH, NH₃, and formaldehyde paved the way for identifying molecular clouds.
Reader's Guide
Molecular clouds are dense interstellar regions where conditions allow molecules, most commonly molecular hydrogen (H₂), to form, in contrast to the predominantly ionized gas found elsewhere in the interstellar medium. When star formation occurs within them, they are sometimes called stellar nurseries. Their density and size also permit the formation of absorption nebulae and H II regions. Although hydrogen is the most abundant species in these clouds, its symmetrical structure makes it nearly invisible to direct observation. Instead, carbon monoxide (CO) is used as the primary tracer, as its rotational energy and asymmetry make it far easier to detect. The ratio between CO luminosity and H₂ mass is generally considered constant, though this assumption has been questioned in observations of some other galaxies. Within molecular clouds are denser regions called clumps, which contain much dust and many gas cores. If gravitational forces are sufficient, these clumps collapse to initiate star formation. The discovery of molecular clouds in 1970 built on earlier detections of interstellar molecules, including OH in Cassiopeia A and NH₃ inversion line radiation. The breakthrough came when Arno Penzias, Keith Jefferts, and Robert Wilson identified CO in the Omega Nebula, quickly followed by detections near the galactic center, including the giant cloud Sagittarius B2. This team later received the Nobel Prize in Physics for their discovery of microwave emission from the Big Bang.
Did You Know?
- Molecular hydrogen is difficult to detect by infrared and radio observations, so carbon monoxide (CO) is most often used to determine the presence of H2.
- Molecular gas clouds account for less than one percent of the volume of the interstellar medium in the Milky Way, yet are its densest part.
Frequently Asked Questions
What is a molecular cloud?
A molecular cloud is a dense, cold region of interstellar gas and dust thick enough for molecules to form and remain stable. When active star birth is taking place inside one, astronomers often refer to it as a stellar nursery. It stands apart from the more common ionized gas that fills most of the interstellar medium.
What are molecular clouds primarily made of?
The vast majority of a molecular cloud's mass is molecular hydrogen (H2), with carbon monoxide (CO) serving as the most easily observed trace species. Despite occupying less than one percent of the interstellar medium's volume, these clouds represent its densest component.
Where in the Milky Way are molecular clouds found?
They are concentrated in a ring roughly 3.5 to 7.5 kiloparsecs from the galactic center, with a strong preference for the spiral arms. This placement puts them in zones where gravitational conditions favor the slow collapse of gas into new stars.
Why are molecular clouds important to astronomy and star formation?
They provide the cool, dense environment where gravity can overcome thermal pressure and pull gas together until fusion ignites in a newborn star. Without these clouds, the surrounding interstellar medium would remain too warm and diffuse to produce new stellar systems.
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