14 Star Birth and Star Stuff

Nebulae – Stellar Nurseries

  • Stars form in dark clouds of dusty gas in interstellar space.

  • The gas between the stars is called the INTERSTELLAR MEDIUM.

  • Nebulae are regions that emit visible wavelengths of light.

  • Nebulae can be:

    • Emission nebulae: light emission following atomic excitation caused by absorbing radiation from nearby stars.

    • Reflection nebulae: reflection of light emitted by nearby stars.

    • Dark nebulae: blocking light from behind.

Stages of Star Birth

  • Gravitational contraction of a nebula:

    • Gravitational potential energy decreases and kinetic energy increases.

    • Density and temperature increase.

    • Outward-pointing gas pressure increases.

  • A typical gas cloud (T~ 30 K, n ~ 300 particles/cm^3) must contain at least a few hundred solar masses for gravity to overcome pressure.

  • The build-up of the outward pointing “gas” pressure can be slowed by converting some of the increasing thermal energy into infrared and radio photons that escape from the cloud.

  • Conservation laws during star formation:

    • Conservation of energy: The cloud heats up due to gravitational contraction.

    • Conservation of angular momentum: The rotational speed increases as it collapses, conserving angular momentum.

  • Protostars are very luminous despite their low temperature because they are big.

  • A protostar contracts and heats until the core temperature is high enough to ignite hydrogen fusion.

  • The contraction and warming of a newly formed star continues until hydrostatic balance is established.

Masses of Newborn Stars

  • Very massive stars are rare; low-mass stars are common.

  • Upper Limit to a Star’s Mass:

    • Radiation Pressure: photons exert a slight pressure when they strike matter (p = F/A).

    • Observations seldom found stars more massive than about 150\text{M}_{\text{Sun}}.

  • Lower Limit on a Star’s Mass:

    • Fusion will not begin in a contracting cloud if some sort of force stops gravitational contraction before the core temperature rises above 10^7 K.

    • Degeneracy pressure halts the contraction of objects with mass <0.08\text{M}_{\text{Sun}}.

Failed Stars – Brown Dwarfs

  • Starlike objects that are not massive and hot enough to start fusion are called brown dwarfs.

  • A brown dwarf emits infrared light because of the heat left over from gravitational contraction.

  • The luminosity of a brown dwarf gradually declines with time as it loses thermal energy, eventually becoming a black dwarf.