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Star Birth Overview

• Stars are formed in dark clouds of dusty gas, known as star-forming clouds, within the interstellar medium.

Composition of Clouds

• Interstellar gas composition derived from absorption lines in star spectra:

  • 70% Hydrogen (H)

  • 28% Helium (He)

  • 2% Heavier elements

• Most matter in star-forming clouds is molecular (H2, CO).

Molecular Clouds

• Molecular clouds have:

  • Temperature: 10–30 K

  • Density: ~300 molecules/cm³

• Observations of carbon monoxide (CO) provide knowledge about these clouds.

Interstellar Dust

• Tiny solid particles block visibility of stars behind clouds.

• Size: < 1 micrometer, composed of C, O, Si, Fe.

Interstellar Reddening

• Stars appear redder through cloud edges due to dust blocking blue light more than red light.

• Long-wavelength infrared light passes through more easily, revealing stars behind clouds.

Observing Newborn Stars

• Newborn stars often hidden in dark gas clouds, detectable via infrared observations.

• Infrared light from heated dust grains in star-forming regions is significant.

Star Formation Process

1. Initital Stage: Contraction of gas clouds leads to denser regions due to random motions, overcoming thermal pressure.

2. Gravity vs. Pressure: Must overcome thermal pressure for stars to form.

3. Cloud Fragmentation: Dense regions fragment, each capable of forming new stars.

4. Resistance to Gravity: Additional forces like magnetic fields can oppose gravity, requiring more mass for collapse.

Formation of Stars

• Larger clouds can form clusters of stars; smaller clouds may lead to single stars.

• First stars formed from primordial clouds must have been more massive due to higher temperatures (100K).

Role of Rotation

• The rotation speed increases as the cloud contracts; similar to figure skaters pulling in arms.

• Collisions in the cloud cause flattening into a disk shape, leading to the formation of jets.

Protostar Formation

• Protostars develop as contraction slows and dust and gas accumulate.

• Eventually, thermal energy from contraction must be replaced by energy from nuclear fusion as temperatures rise.

Types of Pressure in Stars

• Thermal Pressure: Relies on heat content, dominant in most stars.

• Degeneracy Pressure: Arises when particles cannot occupy the same state, significant in low-mass stars (brown dwarfs).

Star Characteristics and Lifetimes

• Models show that stars like the Sun take ~30 million years to transition from protostar to main sequence.

• Maximum star mass is limited by radiation pressure, estimated around 150 Solar masses.

• Star demographics indicate more low-mass stars are formed than high-mass stars.