Molecular clouds

Definition

Molecular clouds are dense regions of gas and dust in space, primarily composed of hydrogen molecules. They serve as the primary sites for star formation, providing the necessary conditions for gravity to collapse and form stars. These clouds are typically cold, with temperatures around 10-20 K, and their high density allows them to shield molecules from dissociation by ultraviolet radiation, making them crucial for the formation of new stars and planetary systems.

5 Must Know Facts For Your Next Test

1. Molecular clouds are often called stellar nurseries because they are where new stars are born from collapsing gas and dust.
2. These clouds can have masses ranging from a few hundred to several million solar masses and can span many light-years in size.
3. Molecular clouds are typically identified using radio telescopes that detect emissions from carbon monoxide (CO), a molecule prevalent in these regions.
4. The density of molecular clouds can vary significantly, with some regions being extremely dense (up to 100,000 molecules per cubic centimeter) while others are more diffuse.
5. The gravitational collapse within molecular clouds is triggered by various factors, including shock waves from nearby supernovae or collisions with other clouds.

Review Questions

• How do molecular clouds contribute to the process of star formation?
  • Molecular clouds are essential for star formation because they provide the necessary conditions for gravity to act on dense regions of gas and dust. When parts of a molecular cloud become dense enough, they start to collapse under their own gravity. This collapse leads to the formation of protostars, which will eventually evolve into main-sequence stars. The cold temperatures and high densities within these clouds help shield molecules from ultraviolet radiation, making it possible for star formation to occur.
• Discuss the relationship between molecular clouds and HII regions in terms of star formation.
  • Molecular clouds serve as the initial site of star formation, while HII regions represent the later stages where newly formed hot stars emit intense ultraviolet radiation that ionizes surrounding hydrogen gas. Once a star forms in a molecular cloud and reaches a certain mass, it will produce radiation that creates an HII region around it. This process highlights how molecular clouds transition into HII regions as stars evolve and their impacts on their environments become pronounced.
• Evaluate the significance of molecular clouds in determining the star formation rate in galaxies.
  • Molecular clouds play a pivotal role in influencing the star formation rate within galaxies because they represent the primary reservoirs of material from which new stars form. The efficiency with which these clouds convert gas into stars directly impacts the overall star formation rate of a galaxy. Factors such as cloud density, temperature, and external influences like shock waves from supernovae can enhance or inhibit this process. Therefore, understanding molecular clouds helps astronomers assess how actively galaxies are forming new stars.

Related terms

HII regions: HII regions are areas of ionized hydrogen surrounding newly formed stars, where intense ultraviolet radiation from hot stars ionizes the surrounding gas.

Star formation rate: The star formation rate is a measure of the amount of mass converted into stars within a specific volume of space over a given time period, often expressed in solar masses per year.

Protostar: A protostar is an early stage in the formation of a star, occurring when a molecular cloud collapses under its own gravity, leading to the accumulation of material at its center.