Molecular cloud
Dense interstellar clouds where stars are born.
A molecular cloud, sometimes called a stellar nursery if star formation is occurring within, is a type of interstellar cloud whose density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, H2), and the formation of H II regions. This contrasts with other areas of the interstellar medium that contain predominantly ionized gas. Within the Milky Way, molecular gas clouds account for less than one percent of the volume of the interstellar medium, yet it is also the densest part of it.
- 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
The history pertaining to the discovery of molecular clouds is closely related to the development of radio astronomy and astrochemistry. During World War II, Henk van de Hulst calculated that the neutral hydrogen atom should transmit a detectable radio signal. After the war, Dutch astronomers repurposed dish-shaped antennas along the Dutch coastline, once used by the Germans as a warning radar system, into radio telescopes to search for the hydrogen signature.
Reader's Guide
The discovery of molecular clouds built on earlier detections of interstellar molecules. Despite hydrogen's abundance, its detection proved difficult due to its symmetrical structure. The breakthrough came when Arno Penzias, Keith Jefferts, and Robert Wilson identified CO in the Omega Nebula. Carbon monoxide became the primary tracer of star-forming clouds. 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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