Tuesday, November 15, 2005

Formation of Stars

by Sam Wass (Class of '05)

Stars are the fundamental components of space. While scientists have proved that stars are “self gravitating balls of mostly hydrogen gas that act as thermonuclear furnaces to convert hydrogen into heavier elements of the periodic table” (1, p1), they have been unable to formulate a strong model regarding star formation. For the much of the last 50 years, observation of the star forming process has been severely impeded by the fact that most stars form in dark clouds that optically block the process. Infrared and millimeter-wave technology has significantly expanded our knowledge how stars develop.

The dark clouds that make up the band which splits the Milky Way (when observed on clear sky) are the locations of star formation. These clouds are primarily made up of very cold hydrogen gas. The actual creation process occurs in the dense centers of these clouds. If an otherwise stable dense core is sufficiently unbalanced or loses internal support, the core’s dusty material collapses and thus starts the creation process. Any disturbances, including supernovae and spiral density waves have the potential to offset cloud equilibrium. Scientists have calculated that the molecular core must collapse to a size nearly ten orders of magnitude smaller than the original dimensions of the core in order to be dense enough to develop into a protostar, the first stage of star development. As particles contract closer to the core, their gravitational energies decrease. This results in an increase in temperature through the conversion of gravitational energy to thermal kinetic energy.

Once the protostar is born, the forming star enters the accretion phase of stellar evolution. During this time the star’s core slowly gains mass through the accretion of material being pulled into the core. Once the embryonic core achieves a mass of roughly .2 to .3 solar masses, the core’s temperature is sufficiently high enough to begin deuterium burning nuclear reactions. Once the core achieves a temperature of 10^6 Kelvins, hydrogen fusion commences and for a low mass star, the core commences the main sequence of stellar evolution.

The conditions that determine the main sequence evolution of a future star are the protostar’s mass, radius, and luminosity at the point where accretion stops. Once all of the material floating around a protostar is accreted, the protostar becomes visible to the telescope. The continued hydrogen burning within the star increases the star’s internal pressure to balance gravity. The star will begin a long phase of equilibrium along the main sequence of the stellar life cycle.

This discussion outlines the current model of star formation, but can only be applied to fairly unique star forming conditions. The model cannot represent stars that form in clusters or massive stars larger than eight solar masses. Solving these problems requires learning more about the mysterious formation of dark clouds and thus will require more critical observations and modeling. The study of star formation continues to be a major concentration of astronomical study. Fully understanding this complex phenomenon will hopefully expand our overall understanding of the formation of the universe.

Works Cited (active November 15, 2005):
  1. Star Formation in the Galaxy, An Observational Overview
  2. Formation of Stars and Planets
  3. Wikipedia: Star

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