Jeans mass

Definition

Jeans mass is the critical mass that a cloud of gas and dust must have in order to collapse under its own gravity to form stars. It relates directly to the balance between gravitational forces and thermal pressure within a molecular cloud, dictating whether regions of the cloud will contract and potentially lead to star formation. The Jeans mass is influenced by various factors, such as temperature, density, and the composition of the cloud, making it a key concept in understanding how stars are born from these massive structures.

5 Must Know Facts For Your Next Test

1. The Jeans mass formula incorporates temperature and density, where a lower temperature and higher density lead to a smaller Jeans mass, making collapse more likely.
2. If a region within a molecular cloud has a mass greater than the Jeans mass, it will likely collapse and potentially form one or more stars.
3. The Jeans mass can be calculated using the equation $$M_J = \frac{(5kT)^{3/2}}{(G\mu m_H)^{3/2}\rho^{1/2}}$$, where k is the Boltzmann constant, T is temperature, G is the gravitational constant, \mu is the mean molecular weight, m_H is the mass of hydrogen, and \rho is the density.
4. Different types of molecular clouds have different Jeans masses depending on their conditions; for instance, dense star-forming regions have higher Jeans masses than diffuse clouds.
5. When the Jeans mass threshold is exceeded in a region of a molecular cloud, it can trigger local gravitational instabilities leading to fragmentation and star formation.

Review Questions

• How does the concept of Jeans mass help us understand why certain regions within molecular clouds can lead to star formation?
  • The concept of Jeans mass explains that only regions within a molecular cloud with sufficient mass can overcome thermal pressure and collapse under gravity. If a region's mass exceeds the Jeans mass, it indicates that gravitational forces are strong enough to cause contraction despite any opposing thermal motion. This means that areas with sufficient density and low temperature are prime candidates for star formation as they can effectively transition from being stable clouds to forming new stars.
• Discuss how variations in temperature and density within a molecular cloud affect its Jeans mass and the likelihood of star formation.
  • Variations in temperature and density significantly impact the Jeans mass of a molecular cloud. A higher temperature increases thermal pressure, resulting in a larger Jeans mass which means that more mass is needed for collapse. Conversely, increased density lowers the Jeans mass, making it easier for gravitational forces to dominate over thermal pressure. This interplay creates different environments within the cloud that either promote or inhibit star formation based on local conditions.
• Evaluate how understanding Jeans mass contributes to our knowledge of star formation rates in different types of molecular clouds across the universe.
  • Understanding Jeans mass enhances our knowledge of star formation rates by allowing us to assess which types of molecular clouds are likely to form stars based on their physical conditions. By analyzing variations in temperature, density, and composition across diverse environments—from dense star-forming regions to more diffuse clouds—we can predict where star formation is actively occurring or may occur in the future. This evaluation aids in mapping out galactic evolution and identifying potential sites for new stellar populations.

Related terms

Molecular cloud: A large region of space filled with gas and dust that can potentially form stars, often cooler and denser than surrounding areas.

Thermal pressure: The pressure exerted by gas particles due to their thermal motion, counteracting gravitational forces in a cloud.

Star formation: The process through which dense regions within molecular clouds collapse under their own gravity to form stars.