Sources of Instability

What are the sources of instability that cause portions of a molecular cloud to begin to collapse? At least five are suspected to have an influence:

  • Shock waves generated by supernova explosions.
  • Density waves in spiral galaxies
  • Shock waves generated by the ignition of massive hot stars
  • The collision of molecular clouds
  • Density perturbations generated by collisions and interactions of galaxies

    • Relative Importance
    Although the first four may all play a role, it is likely that the first two are the primary sources of new stars in normal galaxies. The last one can lead to riotous star formation in interacting galaxies. Note that it need not be completely independent of the first four, since the interaction of two galaxies can certainly lead to phenomena like supernova explosions, ignition of hot stars, and collisions of molecular clouds.

    Shocking the Tarantula
    The image adjacent right shows a portion of the Tarantula Nebula, which is located in the satellite galaxy of the Milky Way called the Large Magellanic Cloud. The region at the lower right is a group of brilliant stars called Hodge 301.

    Many massive stars in Hodge 301 have already become supernovae. The material that they have blasted into the surrounding nebula is shocking and compressing the gas and producing the sheets and filaments seen in the rest of the image. There are at least 3 red supergiants in Hodge 301 that are likely to become supernovae soon, so the modification of this portion of the Tarantula Nebula by supernova remnants will continue (Ref).

    Spiral Density Waves
    The adjacent image shows the spiral galaxy M100. The strong blue color of the spiral arms is because star formation is taking place there and many hot, young, blue stars are concentrated in these regions that have be destabilized by spiral density waves, leading to gravitational collapse and star formation.

    Colliding Galaxies
    The following image shows prolific star formation in the collision of two galaxies. The blue regions correspond to at least 1000 bright new star clusters that have been created in the collision.



    Shocks Generated by Massive Stars
    The adjacent image shows a shell that has been expelled from a type of hot massive star called a Wolf-Rayet star. The star is generating shock waves in the nebula, and lies near a thick molecular cloud. We may expect that the ignition and subsequent strong solar winds from massive stars can destabilize nearby molecular clouds.