Star Formation

    Stars do not last forever. They have finite lives and so they are born and they die. How are stars formed? First, we must start with the correct ingredients. Huge volumes of space with the correct collection of atoms and molecules at the correct density are called molecular clouds. One of the most famous molecular clouds is the nearby Orion Nebula. The life of a star is determined from the very beginning by the amount of mass used to make the star. From Hubble Space Telescope images, we know that star formation is going on right now in the Orion Nebula. We see what have been called proplyds. The lowest possible mass for a star seems to be about 0.08 solar masses. Any mass smaller than that becomes what is called a brown dwarf. Whereas the most massive stars might have as much as 150 times the mass of the Sun.

    Seven Stages of Star Formation

  1. Hydrogen Cloud
        Also known as Molecular Clouds, These objects are sometimes seen as dark "holes" in a field of stars, but are more likely associated with the nice red glow of hydrogen alpha emission.

  2. A Collapsing Cloud Fragment
        A huge interstellar cloud will collapse to form perhaps hundreds of individual stars. It is thought that a nearby supernova might create a shock wave which would initiate collapse. There may also be other mechanisms of star collapse.

  3. Fragmentation Ceases
        Each cloud fragment is now about the size of our solar system surrounded by a cocoon nebula and the core is heating up due to Helmholtz contraction

  4. Protostar
        If we could see to the center of the surrounding nebula, we would see something that might look like a red giant star, at least superficially.

  5. Protostar Evolution
        The protostar will continue to shrink and its core will approach the magic temperature. We can plot the changing characteristics of a protostar as paths called Hayashi tracks across an H-R diagram.

  6. Newborn Star
        Once the core reaches 10 million K, nuclear reactions begin and the star reaches the T Tauri stage. In the T Tauri stage, a star has an accretion disk which contains enough dust to obscure the central star, at lest occassionally. Hundreds of T Tauri type stars are known and cataloged. There are also stars which do not show the variability of classic T Tauri stars, yet are suspected of being young objects. These have been called 'naked T Tauri stars'.

  7. Main Sequence at last
        Finally, a happy and contented Zero Age Main Sequence star, churning out energy by means of the proton-proton chain. The mass of the star will determine what kind of ZAMS star it will be.


    How long does all of this take? It depends. More massive stars evolve faster than low mass stars.

    Here is another nice summary of this process.

    And another.

     Now, just because a star has formed, does that automatically mean that there will be planets orbiting it? That is a matter for the Planetary Astronomy course!

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Updated April 7, 2011