4.1 Molecular clouds and star formation

Molecular clouds are the cosmic nurseries where stars are born. These vast, cold regions of space are packed with gas and dust, providing the raw materials for star formation. Their unique properties and complex structures set the stage for the gravitational dance that leads to the birth of new stars.

The interplay of gravity, turbulence, and magnetic fields within molecular clouds shapes the star formation process. These forces work together to create dense cores that can collapse and form protostars, while also influencing the final masses and characteristics of the newly formed stars.

Molecular cloud properties and structure

Physical characteristics and composition

Top images from around the web for Physical characteristics and composition

• 

  The Orion A molecular cloud from VISTA | This image from the… | Flickr View original

  Is this image relevant?

• 

  Star Formation | Astronomy View original

  Is this image relevant?

• 

  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

  Is this image relevant?

• 

  The Orion A molecular cloud from VISTA | This image from the… | Flickr View original

  Is this image relevant?

• 

  Star Formation | Astronomy View original

  Is this image relevant?

1 of 3

Top images from around the web for Physical characteristics and composition

• 

  The Orion A molecular cloud from VISTA | This image from the… | Flickr View original

  Is this image relevant?

• 

  Star Formation | Astronomy View original

  Is this image relevant?

• 

  ESA Science & Technology: Herschel's view of the Taurus molecular cloud - annotated View original

  Is this image relevant?

• 

  The Orion A molecular cloud from VISTA | This image from the… | Flickr View original

  Is this image relevant?

• 

  Star Formation | Astronomy View original

  Is this image relevant?

1 of 3

• Molecular clouds are vast, cold, dense regions of the interstellar medium composed primarily of molecular hydrogen (H2) and trace amounts of other molecules such as CO, NH3, and H2O
• Typical temperatures within molecular clouds range from 10-20 Kelvin, with densities between 102 to 106 particles per cm3, significantly higher than the surrounding interstellar medium
• The chemical composition of molecular clouds is dominated by H2, but also includes a variety of complex organic molecules, which can be observed through their rotational and vibrational transitions in the radio and infrared spectrum (e.g., formaldehyde, methanol, and amino acids)

Hierarchical structure and mass distribution

• Molecular clouds exhibit a hierarchical structure, with smaller, denser clumps and cores embedded within larger, more diffuse clouds. This structure is often described as fractal or self-similar across various scales (e.g., from giant molecular clouds to individual star-forming cores)
• The mass of molecular clouds can range from a few solar masses to several million solar masses, with sizes spanning from a few parsecs to hundreds of parsecs (e.g., the Orion Molecular Cloud Complex, which spans over 100 parsecs)
• Molecular clouds are often sites of active star formation, as the dense cores within these clouds can gravitationally collapse to form protostars and eventually main-sequence stars
• The mass distribution of molecular clouds follows a power-law relationship, with more low-mass clouds than high-mass clouds. This mass distribution is thought to be related to the turbulent fragmentation processes within the interstellar medium

Gravity and turbulence in star formation

Gravitational collapse and Jeans instability

• Gravity plays a crucial role in star formation by causing the dense cores within molecular clouds to collapse, leading to the formation of protostars
• The Jeans instability describes the condition under which a molecular cloud or a region within it will collapse under its own gravity. This occurs when the gravitational potential energy of the cloud exceeds its internal thermal energy
• The Jeans mass and Jeans length are critical parameters that determine the minimum mass and size of a cloud fragment that can collapse under its own gravity (e.g., a typical Jeans mass in a molecular cloud is around 1 solar mass)

Turbulence and its impact on star formation

• Turbulence within molecular clouds can both hinder and promote star formation processes. On large scales, turbulence can provide support against gravitational collapse, preventing the formation of stars
• On smaller scales, turbulence can create local density enhancements or clumps within the molecular cloud. These high-density regions can become gravitationally unstable and collapse to form protostars (e.g., turbulent compression can create dense cores with masses ranging from 0.1 to 10 solar masses)
• The interplay between gravity and turbulence leads to a hierarchical and clustered pattern of star formation within molecular clouds, with stars forming in groups or associations
• Supersonic turbulence within molecular clouds can also contribute to the observed power-law distribution of stellar masses, known as the initial mass function (IMF)

Magnetic fields in cloud collapse

Magnetic support and mass-to-flux ratio

• Magnetic fields can significantly influence the star formation process within molecular clouds by providing additional support against gravitational collapse
• The magnetic pressure and tension forces can counteract the effects of gravity, slowing down or even preventing the collapse of molecular cloud cores
• The ratio of mass to magnetic flux, known as the mass-to-flux ratio, determines the relative importance of magnetic fields in the collapse process. Clouds with a high mass-to-flux ratio are more likely to collapse under their own gravity (e.g., a mass-to-flux ratio greater than unity indicates that the cloud is supercritical and can collapse)

Ambipolar diffusion and magnetic field effects

• Ambipolar diffusion, the process by which neutral particles drift relative to charged particles and the magnetic field, can gradually reduce the magnetic support in molecular clouds over time, allowing gravitational collapse to proceed
• Magnetic fields can also influence the formation and orientation of protostellar disks and outflows, as the field lines can guide the flow of matter and angular momentum (e.g., bipolar outflows are often aligned with the local magnetic field direction)
• The presence of magnetic fields can lead to the formation of magnetohydrodynamic (MHD) turbulence within molecular clouds, which can affect the fragmentation process and the resulting stellar mass distribution
• Magnetic fields can also contribute to the formation of filamentary structures within molecular clouds, as the field lines can guide the flow of matter along preferred directions

Stages of star formation

Protostellar collapse and early stages

• The first stage of star formation is the gravitational collapse of a dense, gravitationally bound core within a molecular cloud. This leads to the formation of a protostar, a central object that continues to accrete matter from its surroundings
• As the protostar accretes matter, it becomes obscured by a dense cocoon of gas and dust. This stage is known as the Class 0 phase, characterized by strong submillimeter and far-infrared emission from the cold dust envelope (e.g., L1544 in the Taurus molecular cloud)
• As the protostar continues to accrete matter and evolve, it enters the Class I phase. During this stage, the protostar becomes visible in the near-infrared as the surrounding envelope becomes less opaque. Bipolar outflows and jets are often observed during this phase (e.g., HH 30 in the Taurus molecular cloud)

Late stages and disk evolution

• In the Class II phase, also known as the classical T Tauri stage for low-mass stars, the protostar has accreted most of its final mass, and the surrounding envelope has dissipated. The star is now visible in the optical, and is surrounded by a circumstellar disk (e.g., TW Hydrae)
  • Planetesimals and eventually planets can form within the circumstellar disk during this stage, as dust grains settle to the midplane and coagulate into larger bodies
• The final stage of star formation is the Class III phase, or the weak-lined T Tauri stage. By this point, the circumstellar disk has largely dissipated, and the star has contracted to the point where it begins fusion of deuterium in its core (e.g., V830 Tauri)
• Once hydrogen fusion begins in the core, the star reaches the main sequence and is considered a fully-formed star. The time taken to reach the main sequence depends on the star's mass, with more massive stars evolving more rapidly (e.g., a 1 solar mass star takes around 30 million years to reach the main sequence, while a 10 solar mass star takes only around 1 million years)