Onset of Collapse: Contributing Factors

• The interstellar cloud: 10's of pc across, usually a molecular cloud in which H₂ and other molecules form [CO, H₂O, ...] as well as dust, with T = 10-100 K, about 1000 atoms/cc.
• Helping factors: The main one is gravity; Collapse can possibly be triggered by shock waves from events like supernova explosions (or galaxy collisions, and possibly large "objects" such as globular clusters passing by), which can start a chain reaction.
• Opposing factors: The main ones are heat and rotation [and magnetism]; Nearby massive stars can also prevent star formation by heating and stirring the interstellar matter; Turbulence can compress insterstellar matter in some locations, but its overall effect is to oppose collapse.
• Rates: A few stars are probably born each year on average in our galaxy; The number is much higher in active galaxies.

Early Stages and Newborn Stars

• Initial collapse: The cloud gets warmer but energy escapes as (infrared) radiation.
• Fragmentation: The cloud splits into fragments, 0.01 pc wide or so, each of which will produce a star, or a multiple system, depending on its rotation.
• Protostar: Dense enough not to be transparent; Becomes hotter, and is initially very bright; Has a surface, emits IR and protostellar wind.
• Newborn star: T above 10 MK in the core ignites fusion of H into He and halts collapse.
• Arrival on the Main Sequence: After a total of 40-50 Myr for the Sun; (more than 100 Myr for small stars, less than 1 Myr for massive ones).
• Range of sizes: Masses range from 0.08 solar masses (below that value objects do not have enough mass for H fusion to start and are not considered stars) to about 100 solar masses (more massive objects would be unstable, the heat and pressure of the forming stars ejecting any extra material).
• Large stars: Some very large stars exist; the Pistol Star has 100 solar masses and LBV 1806-20, 150 (it is 40 million times brighter than the Sun); Even more massive stars may have formed in the early universe, when the abundance of heavy elements was very low.

  Behavior of Young Stars

• Features of young stars: Most are surrounded by a protostellar disk of leftover matter, flattened by rotation and swept by a strong stellar wind; Sometimes we see jets and blobs of matter.
• Herbig-Haro objects: Young stars emitting high-velocity gas [near 300 km/s], which collides with a surrounding nebula of interstellar material, heating it to sufficiently high temperatures to make it glow and produce X-rays.
• [Other special type: T Tauri stars, erratically variable pre-main sequence stars.]
• Fate of disk: Forms planets (evidence accumulates!), or is blown away by stellar wind and jets.

Result of Star Formation

• Size distribution: The star formation process leads to star associations and clusters, with a few bright stars and many small ones; A good example is the Orion Nebula nursery. Stars above 100 solar masses don't form (they would be unstable) and Objects below 0.08 solar masses (about 80 Jupiters) are not stars; they are hard to see but may be common.
• Speed of evolution: The same difference in the rate of evolution before the main sequence applies to later stages – the more massive the star, the faster it evolves.
• Fate of clusters: The more massive stars explode before clusters can change significantly; Smaller stars go their own way later or are kicked out by a gravitational encounter (possibly what happened to the Sun), and open clusters dissolve over billions of years.