Jeans Mass

Definition

The Jeans mass is a critical mass threshold that determines whether a cloud of gas and dust will collapse under its own gravitational attraction to form stars. If a molecular cloud's mass exceeds the Jeans mass, it is likely to undergo gravitational instability, leading to star formation, while a mass below this threshold will disperse and not form stars.

5 Must Know Facts For Your Next Test

1. The Jeans mass can be calculated using the formula: $$ M_J = \frac{(5k_B T)^{3/2}}{(G \mu m_H)^{3/2} \rho^{1/2}} $$, where \( k_B \) is Boltzmann's constant, \( T \) is temperature, \( G \) is the gravitational constant, \( \mu \) is the mean molecular weight, and \( m_H \) is the mass of hydrogen.
2. In typical molecular clouds, the Jeans mass is often several hundred solar masses, meaning clouds of that size or larger are more likely to collapse to form stars.
3. The Jeans mass depends on temperature; as temperature increases, the Jeans mass also increases because higher temperatures increase thermal pressure and counteract gravitational forces.
4. Different regions within a molecular cloud can have different Jeans masses due to variations in temperature and density, leading to localized areas where star formation is more likely.
5. Understanding the Jeans mass helps astronomers predict where star formation will occur in molecular clouds and how many stars might form within a given region.

Review Questions

• How does the Jeans mass influence the processes of star formation within molecular clouds?
  • The Jeans mass acts as a critical threshold that determines if a molecular cloud will collapse under its own gravity. When the mass of the cloud exceeds this threshold, it becomes gravitationally unstable, leading to the formation of stars. If the mass is below the Jeans mass, thermal pressure dominates and prevents collapse, causing the cloud to disperse instead. Therefore, understanding the Jeans mass allows astronomers to identify potential star-forming regions.
• Discuss how variations in temperature and density within a molecular cloud affect the Jeans mass and star formation potential in different regions.
  • Variations in temperature and density directly impact the Jeans mass across different regions of a molecular cloud. Higher temperatures increase thermal pressure, raising the Jeans mass and making collapse less likely in those areas. Conversely, denser regions tend to have lower Jeans masses, enhancing their likelihood for gravitational collapse and subsequent star formation. This variability means that some parts of a molecular cloud may actively form stars while others remain stable.
• Evaluate the implications of knowing the Jeans mass for understanding galaxy evolution and star formation rates across different environments.
  • Understanding the Jeans mass provides critical insights into galaxy evolution and star formation rates by identifying regions within galaxies that are prone to forming new stars. By analyzing how environmental factors such as temperature and density impact Jeans masses across various galactic settings—like spiral arms versus elliptical galaxies—astronomers can infer star formation efficiency and history. This knowledge helps paint a clearer picture of how galaxies evolve over time based on their ability to form stars in response to their internal conditions.

Related terms

Molecular Clouds: Dense regions in space composed primarily of hydrogen molecules, where conditions are favorable for star formation.

Gravitational Instability: A condition where the inward gravitational force exceeds the outward pressure from thermal motion, leading to the collapse of a cloud.

Thermal Pressure: The pressure exerted by particles in a gas due to their thermal motion, which counteracts gravitational collapse.