1. Discuss how a star forms and name some of the types of forming stars.

2. Discuss how the luminosity of a forming star is generated.

3. Describe the condition of hydrostatic equilibrium and how this condition initially marks the beginning of the main sequence phase of stellar evolution.

4. Place the position of forming stars and main sequence stars on the HR diagram.

5. Explain three mechanisms of nuclear fusion which lead to stellar energy generation: the proton-proton cycle, the CNO cycle, and the triple alpha process.

6. Describe which mechanisms of stellar energy generation dominate in main sequence stars and how this depends on the mass of the main sequence star.

7. Discuss the generation of solar neutrinos and results from solar neutrino experiments.

¶ early generation stars (Population II stars)

¶ later generation stars (Population I stars)

¶ nuclear energy generation

¶ protostars

¶ T Tauri stars

¶ Herbig-Haro objects

¶ hydrostatic equilibrium

¶ main sequence phase

¶ conservation of angular momentum

¶ Hertzsprung-Russell (HR) diagram

¶ O and B Association

¶ thermonuclear reaction

¶ E = mc²

¶ proton-proton chain

¶ CNO tri-cycle

¶ triple alpha process

¶ neutrino

¶ solar neutrino experiment

¶ Stars form from contracting clouds of gas and dust. Most of the matter in these clouds is hydrogen (about 75% by mass).

¶ The early generations of stars (called Population II stars) formed from material that was not very enriched with heavy elements, while later generations of stars (called Population I stars) formed from material that was enriched with heavy elements by supernovae explosions in massive, fast evolving stars. The heavy elements ejected into interstellar gas and dust clouds by supernovae were synthesized in the interiors of these massive, fast evolving stars during the nuclear energy generation process.

¶ Astronomers can observe protostars (stars in the process of formation) in the Milky Way Galaxy by looking for visually luminous objects in regions filled with dense clouds of gas and dust. These objects occupy the low temperature side (right hand side) of the Hertzsprung-Russell (HR) diagram.

¶ A number of types of protostars have been identified. One of the well-studied types are the T Tauri stars, named after the prototype star T Tauri. T Tauri stars vary their luminosity irregularly by up to a factor of 10. They emit broad, very intense emission lines, and sometimes x-ray and radio emission. The emission lines are broad because of Doppler shifts, indicating rapid motions of the mass of the outer parts of the star. Herbig-Haro objects result from gas ejected from T Tauri stars.

¶ The luminosity of a forming star comes from the energy generated by the star’s gravitational collapse.

¶ Initially a cloud of a few hundred solar masses or more starts to collapse due to the inward force of gravity.

¶ This cloud eventually fragments into smaller pieces, forming a group or groups of stars of lower mass.

¶ As the protostar collapses due to the inward force of gravity, its temperature and pressure rise.

¶ Once the temperature and pressure in the center (core) of the protostar increase enough, nuclear reactions will start to occur in its core. The nuclear reactions generate energy which cause the temperature and pressure to increase further.

¶ Interior pressures cause outward forces which make protostars and stars expand.

¶ Eventually the inward forces of gravity are balanced by the outward pressure forces and a stable star forms (it ceases its collapse). This is called hydrostatic equilibrium and the star reaches its main sequence phase of evolution (the prime of its life).

¶ During the process of star formation and collapse, the protostar begins to rotate faster and faster (just like skaters spin faster and faster when they bring their arms closer to their bodies).

¶ The faster rotation is the result of conservation of angular momentum, but the angular momentum must eventually be lost by the protostar. If it was not, stars would rotate much faster than they do.

¶ Stars will lose angular momentum by forming planetary systems. Some have speculated that stars that don’t form planetary systems, form binary star systems. Probably about half of the stellar systems are binaries.

¶ The star formation process tends to form clusters of stars which have a range of masses. Type OBAFGKM main sequence stars are eventually formed which typically range from 50 solar masses (O type) to 0.2 solar masses (M type).

¶ Low mass stars are much more common than high mass stars.

¶ The O type stars have the hottest surface temperatures, largest diameters and greatest luminosities, while the M type stars have the coolest surface temperatures, smallest diameters and lowest luminosities.

¶ The stars which form the fastest are the most massive stars, like the O and B types. They can form in about 10,000 years. Young groups of stars at the top of the main sequence in the HR diagram are called O and B Associations.

¶ A star of one solar mass would take 10 million years to form, while the least massive stars must have taken hundreds of millions of years to form.

¶ The core of a collapsing gas and dust cloud with mass less than 0.2 solar masses will not reach high enough temperature and pressure to begin thermonuclear reactions. In stars with masses larger than this, thermonuclear reactions will occur.

¶ Nuclear reactions generate gamma rays. Gamma rays are very high energy electromagnetic radiation. This energy is created because, in fusion reactions, mass is converted into energy (E = mc²). The gamma rays are responsible for heating a star’s core.

¶ Three main thermonuclear fusion reactions generate energy in stellar cores:

¶ A proton is simply a hydrogen nucleus, while an alpha particle is a helium nucleus (2 protons plus 2 neutrons).

¶ A positron is a form of antimatter, opposite an electron. When positrons are created they immediately meet electrons and annihilate each other, creating gamma rays.

¶ Neutrinos are almost invisible particles that carry a tiny amount of energy and angular momentum. Neutrinos escape the interior of the Sun immediately (2 sec) after they are created, because the Sun’s matter is transparent to neutrinos.

¶ Our understanding of stellar interiors is primarily based on theoretical calculations. One way to check these calculations is to observe the number of neutrinos coming from the core of the Sun.

¶ An experiment to observe the neutrinos has been set up 1.5 km underground in the Homesteak gold mine in South Dakota. 100,000 gallons of liquid cleaning fluid (C₂C1₄) have been placed in a tank in this mine. Theoretically, in a tank this size, a neutrino should interact with a chlorine isotope to form a radioactive argon atom once each day.

¶ However, only about one radioactive argon atom is produced every three days.

¶ This has led to a number of suggestions about how particle theory or experimental techniques must be wrong.

¶ New, more sensitive, neutrino detection experiments will be performed in the coming years.