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Protostars and the Nebula

This animation shows how a star begins to form out of a nebula.

    A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%) and helium (3%). Within a nebula, there are varying regions when gravity causes this dust and gas

    Pictures from NASA - Protostar in the Eagle Nebula
    We just know that nebulae (plural for nebula) are the birth place of stars. The Hubble Space Telescope has increased our knowledge about this with some great photos from space which clearly show stars in different stages of development within a nebula.

    to “clump” together. As these “clumps” gather more atoms (mass), their gravitational attraction to other atoms increases, pulling more atoms into the “clump.”

    What causes these “gravitational centers” to form in these huge clouds?
    If you knew that, you’d have a Nobel Prize!

    Adding atoms to the center of a protostar is a process astronomers call accretion. Because numerous reactions occur within the mass of forming star material, a protostar is not very stable.
    In order to achieve life as a star, the protostar will need to achieve and maintain equilibrium. What is equilibrium? It is a balance, in this case a balance between gravity pulling atoms toward the center and gas pressure pushing heat and light away from the center. Achieving and keeping this balance is tough to do. When a star can no longer maintain equilibrium, it dies.

Equilibrium: How it Works!

Equilibrium is a battle between gravity and gas pressure. It works like this:

1. Gravity pulls gas and dust inward toward the core.
2. Inside the core, temperature increases as gas atom collisions increase.
3. Density of the core increases as more atoms try to share the same space.
4. Gas pressure increases as atomic collisions and density (atoms/space) increase.
5. The protostar’s gas pressure RESISTS the collapse of the nebula.
6. When gas pressure = gravity, the protostar has reached equilibrium and accretion stops
    Equilibrium for a protostar occurs when gas pressure equals gravity. Gravity remains constant, so what changes the gas pressure in a protostar? Gas pressure depends upon two things to maintain it: a very hot temperature (keep those atoms colliding!) and density (lots of atoms in a small space).

    There are two options for a protostar at this point:

    • Option 1: If a critical temperature in the core of a protostar is not reached, it ends up a brown dwarf. This mass never makes “star status.”
    • Option 2: If a critical temperature in the core of a protostar is reached, then nuclear fusion begins. We identify the birth of a star as the moment that it begins fusing hydrogen in the core into helium.
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