A Layperson's Guide to Stars

Recall that the dominant hypothesis for the formation of this and other solar systems (developed by Pierre-Simon Laplace) is the Solar Nebula Hypothesis - that the Sun and planets formed from a collapsing cloud of interstellar gas and dust. The Solar System is part of the Milky Way galaxy, a vast collection of stars and other material roughly 100,000 - 150,000 light years across. The image at right shows the central regions of the Milky Way. It's shape is difficult to discern because we must look at it from inside. Looking into the galaxy, we see evidence of various phases of this process in the form of:

What would make a giant molecular cloud collapse to form a solar system?

Jeans mass: Sir James Jeans (1877 - 1946) determined that collapse is a function of of two variables: For a given pair of values for density and temperature, there is a mass, the Jeans mass at which it will spontaneously begin to contract.
  • More exotic processes like: These prime the pump by creating local areas of increased density.

    The stages of cloud collapse:

    Protoplanetary disk of HL Tauri in infrared from Wikipedia.
    Dark circles represent the orbits of coalescing exoplanets.
    Protostars: Within 100 Ma, gravitational release and friction of collisions at the centers of collapsing cloud fragments begins generating considerable heat. At this point we refer to the center of the dense, hot cloud as a protostar.

    Stupid protostar tricks: Protostars, by definition, are not yet experiencing nuclear fusion at their cores. Instead, their heat comes from gravitational release and friction. That heat can be significant, however.

    We assume all of this would have happened in our early Solar System

    Mature Stars

    The Sun's life cycle from Wikipedia

    For our purposes, stars differ in two ways:

    The T Tauri stage ends when the pressure in the protostar's core reaches the threshold at which hydrogen nuclei are fused into helium. It becomes a proper star.

    The Hertzsprung-Russell Diagram from Wikipedia
    But first, an astronomical convention:

    The Hertzsprung-Russell Diagram: Astronomers compare stars and chart their life cycles by plotting their absolute luminosity (y-axis) and color (x-axis) on a standard plot developed by Ejnar Hertzsprung and Henry Norris Russell in 1910. The H-R Diagram encompasses stars of differing mass and at different life-stages. You will note three major groups of stars. During its life, a star will inhabit each of these:

    Solar evolution from Wikipedia
    We start with the example of a star of one solar mass. It follows the indicated path from right to left on the H-R Diagram.

    Spectral Classification of stars based on their mass from Wikipedia
    Variations: Other stars follow the same general sequence but with variations depending on the star's mass. A main-sequence star's color and luminosity vary with mass. Small stars are red dwarfs - dim and red, giant ones are blue giants - blue and bright:

    The Crab Nebula pulsar: Visible wavelengths - red, X-rays - blue (!) from Wikipedia
    Strange remnants: The astrophysics of massive stellar remnants challenge the mind.

    Final food for thought: So far we have only discussed solitary stars. What about binaries? Many interesting variations are possible. For example in a binary containing a red giant and white dwarf, the red giant might expand into the white dwarf's Roche limit, allowing the dense massive dwarf to pull material off of the red giant's outer layers. This forms an accumulation of hydrogen on its surface that eventually undergoes nuclear fusion, exploding as a nova. Of course, if enough hydrogen manages to pile on, its pressure might reignite fusion in the dwarf, or even push it over the Chandrasekhar limit, with amusing results.

    Key concepts and vocabulary: