Star Formation Forces

The form and internal processes that occur in the condensed object are determined by

• The amount of material in the cloud
• The force of gravity which accretes material from the cloud onto the forming body and contracts the body
• Atomic forces which resist gravitational contraction
• The energy released by atomic collisions, radioactive decay and atomic fusion
• The transfer of energy by radiation, convection and conduction

Stages of Star Formation

1. Gravitational contraction

   Star formation begins when gravity condenses a cloud of gas and dust into a rotating disk. As gravity packs the gas atoms closer in a central core, heat and pressure build up. Radiation released from atom collisions in the gas eventually causes the protostar to glow brightly at the red end of the spectrum, mostly in infrared. The core contraction phase of a sun-sized star lasts for about 100 million years.

   Multiple stars often form from the contracting disk. Planets, moons, asteroids and comets also form as part of the condensation process.

2. Fusion ignition

   If the pressure builds high enough, fusion of hydrogen atoms commences, forming helium atoms and releasing lots of energy. The atoms at the core of the star, heated by the energy released by fusion, are set in motion with sufficient force to counteract the force of gravity and halt further contraction.

3. Hydrogen fusion to produce helium

   Once fusion of hydrogen commences, the star then maintains a relatively stable size for as long as the fusion continues - for around ten billion years for sun-sized stars. The density of matter at the core of a sun-sized star is about 60 grams per cubic centimetre - about ten times more dense than lead. The atoms, however, are fluid and the material is gaseous. The electrons have been stripped from the atomic nucleii, so the gas is called a plasma.

4. Core contraction and ignition of helium fusion

   When all fusible hydrogen is exhausted, hydrogen fusion ceases and the core temperature drops. A new episode of gravitational contraction begins. Temperature and pressure rise as the core is crushed smaller. Eventually the pressure and temperature become high enough for helium fusion to commence.

5. Helium fusion to produce carbon and oxygen

   Helium fusion produces carbon and oxygen and releases energy. The energy released keeps the atoms in the core moving with sufficient force to halt gravitational contraction once again. The higher core temperature causes the outer layers of the star to swell. It becomes a red giant. The star then maintains a relatively stable size for as long as helium fusion continues - for around 100,000 years in the case of sun-sized stars.

6. White Dwarf and Planetary Nebula formation

   For stars up to about 8 solar masses, as helium is depleted, the core contracts and temperatures rise. The tenuous outer atmosphere of the star is transported into surrounding space by radiation pressure and forms a planetary nebula. Temperatures and pressures do not reach high enough levels to ignite further fusion reactions. Fusion ceases altogether. An extraordinarily hot, very dense core of carbon and oxygen atoms coated with a thin layer of helium atoms is all that remains. The body is about the size of Earth but has the mass of the Sun. A cubic centimetre of material from the core would weigh several tonne. The atoms are crushed together by gravity. Inter-atomic electron degeneracy forces prevent further collapse. The body can no longer generate energy and simply cools for the rest of time.

7. Fusion of heavier atoms to produce elements up to iron

   For stars above about 8 solar masses, fusion proceeds beyond helium. Discrete episodes of fusion consume progressively heavier elements. Each round of fusion releases additional energy, raising core temperature and pressure until a new balance with gravity is reached. Eventually the core consists mostly of iron. Instead of releasing energy, iron soaks up energy when it fuses. Once all fusion releasable energy has been depleted, fusion ceases.

8. Core collapse and fusion of atoms, in a supernova explosion, to produce the heaviest elements

   When fusion ceases in a massive star, the force of gravity is strong enough to overpower electron degeneracy in the core. Electrons combine with protons to produce neutrons. The collapse of atoms causes a core collapse. The shock waves that rebound through the core provide energy to fuse iron and create all heavier elements. A massive supernova explosion occurs and scatters the outer layers of the core into surrounding space. The central core becomes one massive ball of neutrons - a neutron star, about 20 km across but with the mass of the Sun.

   In the most massive stars, core collapse proceeds even further. Neutron degeneracy cannot withstand gravity. During the collapse, heavy elements are synthesised and a supernova explosion scatters material widely. The central core collapses to a black hole.

References

Inglis, Mike, Observer's Guide to Stellar Evolution, The Birth, Life and Death of Stars, Springer, 2003.

Kaler, James, B, Stars and their Spectra, an Introduction to the Spectral Sequence, Cambridge University Press, reprinted 1997 with corrections.

Kaler, James, B., Stars, Scientific American Library, 1992.
"It seems the only time we can say that we understand the stars is if we do not look too closely. The wealth of detail they exhibit is overwhelming, and the closer we look, the more we see to amaze us."

Zeilick, Gregory and Smith, Introductory Astrophysics, Third Edition, Saunders College Publsihing, 1992