Onset of Collapse: Contributing Factors

• The interstellar cloud: Tens of pc across, usually a cold molecular cloud.
• Helping factors: The main one is gravity; Collapse can possibly be triggered by shock waves from events like supernova explosions (or other large objects nearby), which can start a chain reaction.
• Opposing factors: The main ones are heat and rotation; Nearby massive stars can also prevent star formation by heating and stirring the interstellar matter.
• 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 because of its large surface; It emits IR and protostellar wind.
• Newborn star: T above 10 MK in the core ignites fusion of H into He and halts collapse; A sunlike star arrives on the Main Sequence after 40-50 Myr or so.
• 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 but brown dwarfs, which may actually be very common) to about 100 solar masses (more massive objects would be unstable, the heat and pressure of the forming stars ejecting any extra material).
• Young stars: Most are surrounded by a disk of leftover matter, flattened by rotation and swept by a strong stellar wind; Sometimes we see jets and blobs of matter, and evidence of planet formation.

 Main Sequence Lifetime

• What holds stars up? Equilibrium between thermal pressure and gravity (and radiation pressure in massive stars); lasts for 90% of their life: 10 Gyr for the Sun, more for smaller stars.
• How is the energy produced? Between the time when T > 10 million K and when they run out of H, He production in the core, by the proton-proton chain (T < 20 million K) or CNO cycle (dominant at T > 20 million K).
• How does the energy get out? Radiation and convection, like in the Sun; May take a million years to reach the surface.

Evolution of Low-Mass Stars (M < 10 solar masses)

• H shell burning: H is depleted in the core, He core shrinks; T rises around the core, energy production by H fusion continues at a faster rate in a shell, and the star becomes brighter.
• Star growth: Envelope expands, so it cools down, while the core shrinks and heats up; The star becomes a red giant. (Very low-mass stars end their lives at this stage as He white dwarfs.
• Sunlike stars: When T in the core reaches 10⁸ K, He burning to C starts with a flash; The core expands and the luminosity decreases, as the star moves to the horizontal branch of the HR diagram.
• He shell burning: Eventually He is depleted in the core, and the C core shrinks; T rises, the envelope expands > Larger red giant phase.

  What Does a Sun-Like Star Eventually Become?

• Planetary nebula: He shell flashes and ejection of the star's envelope, ionized by the star's UV radiation; The ejected gas may look round but most have bipolar lobes, probably because of magnetic fields, and possibly due to orbiting companions.
• White dwarf: By this time, the star has shed almost half its mass, and no more matter remains around it either, including nearby planets it may have had; The hot remnant of the C core cools down; Earth-sized but with the Sun's mass, stabilized by electron degeneracy pressure.
• Examples: The first one discovered was Sirius B (small but very hot); We know other ones, but they are hard to see unless they are in binary systems...