Protostar Evolution

We have already talked about the basics of star formation. The fundamental ideas are:

• the material in the ISM is not spread uniformly, it is clumped.
• the clumps which eventually form stars are known as Giant Molecular Clouds
• the clouds are usually in hydrostatic equilibrium and there needs to be a trigger to initiate the star formation process:
  • passage through spiral arms
  • supernova explosions
  • winds and radiation from massive stars as observed by the HST
  • ... , ??
• the initial collapse occurs on the free-fall time scale because the clouds are not efficient at trapping the heat generated by the compression of the gas===>clouds remain cold and at low pressure
• after the clouds become dense enough, they can start to trap the heat generated by the compression more efficiently and they start to heat up ===>pressure increases and the collapse slows
• the clouds next slowly contract on an energy loss time scale (thermal time scale--Why a thermal time scale?)
• the contraction continues until the temperature and density in the core of the star becomes high enough to cause the ignition of hydrogen burning ===>cloud can replace the energy it loses to radiation from its surface and so can settle into hydrostatic equilibrium.

  At this point, the cloud becomes a star and assumes its place on the Main Sequence

To make the ideas concrete, I will talk about the formation of stars of mass, say, less than around 2 x the mass of the Sun. Such protostars are referred to as T Tauri stars. Note, however, that the basic ideas are the same for all protostars. It turns out that it is primarily how protostars appear and where they form in the clouds (and therefore presumably how they are triggered) which differs according to their masses.

• T Tauri stars may form in smaller Giant Molecular Clouds (not well-established) where T less than ~ 10 K, e.g., Taurus Complex; also may form throughout such clouds
• Massive Protostars appear to form in the larger Giant Molecular Clouds where T greater than 20 K, e. g., Orion; appear to form at the edges of the clouds (more secure)