Sources of Instability in Molecular Clouds

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 exploding stars (supernova explosions).
  • Density waves (slowly moving regions of higher density) in spiral galaxies
  • Shock waves generated by the birth of massive hot stars
  • The collision of molecular clouds
  • Density perturbations generated by collisions and interactions of galaxies

    • We give a brief description of these mechanisms below, but most will be discussed in considerably more detail in later chapters.

    Relative Importance
    Although the first four instabilities listed above may all play a role, it is likely that the first two are the primary sources of new stars in normal (non-colliding) 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 three 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. In Chapter 21, we shall consider in more detail the role of supernova explosions in modifying the surrounding medium.

    Spiral Density Waves
    The adjacent image shows the spiral galaxy M100 (the designation Sc is a spiral galaxy classification that will be introduced in Chapter 24). The strong blue color of the spiral arms is because star formation is taking place there. Many hot, young, blue stars are concentrated in these regions that have been destabilized by spiral density waves, leading to gravitational collapse and star formation. The role of spiral density waves in triggering new star formation will be discussed in more depth in Chapter 23.