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ASTRONOMY Stars

OBJECTIVES By the end of this presentation, students will be able to explain the role of nuclear fusion in the sun’s core to release energy; communicate scientific ideas about the way stars, over their life cycle, produce elements; illustrate the life span of the sun.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… The exact path of development depends on the mass of the initial interstellar gas cloud from which the star forms.

Stellar Formation and Life Cycle Photosphere Convection Zone Radiation Zone Core

H-R DIAGRAM A pattern of groups of stars emerge when plotting the Abs. Mag. of a star as a function of its temperature. 30K 10K 7.5K 6K 5K 3K TEMPERATURE (ºK) ABSOLUTE MAGNITUDE White Dwarfs Red Giants Super Giants Main Sequence Red Dwarfs Wolf-Rayet Stars Hypergiants Red Giants Super Giants Hypergiants White Dwarfs Red Dwarfs

H-R DIAGRAM A pattern of groups of stars emerge when plotting the Abs. Mag. of a star as a function of its temperature. RUSSELL-VOGT THEOREM – the equilibrium structure of an ordinary star is determined uniquely by its mass and chemical composition. 30K 10K 7.5K 6K 5K 3K TEMPERATURE (ºK) ABSOLUTE MAGNITUDE

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 1. Protostar Stage – the gravitational collapse of a gaseous cloud mass. The collapse may be triggered by the passing of, the eruption of, or the explosion of a near-by star. Energy production is described by the “Helmholtz Contraction”, where the heat build up by the gravitational collapse.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 1. Protostar Stage Temperatures increase from -441°F to 1340°F. Out-lying gasses enshroud the new protostar.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 1. Protostar Stage Large clouds subdivide into enormous chunks while collapsing, forming a protocluster. The protoclusters fragment into individual protostars.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 2. Pre-Main Sequence Stage – temperatures and pressures in the center continue to rise, slowing the rate of collapse. Energy production is described by the “Helmholtz Contraction”.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 2. Pre-Main Sequence Stage Convection currents transport large amounts of energy from the interior, making the protostar visible and bright for short periods at a time. Temperatures and pressures in the core rise enough to initiate nuclear reactions for energy production.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 2. Pre-Main Sequence Stage Brightness fluctuates (T-Tauri star), until…. Out-lying gasses (the COCOON NEBULA) are blown away by the solar wind of the new-burning star.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 3. Main Sequence Stage – a stable state where pressures from energy production in the core balance the pressures from the gravitational collapse of the shell. The more massive the star, the faster it moves through this stage.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 3. Main Sequence Stage Energy production is described by the Proton-Proton Chain. 2 1 H → 2 H + e + + neutrino 2 H + 1 H → 3 He + photon 3 He + 1 H → 4 He H + photon The more massive the star, the faster it moves through this stage.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 3. Main Sequence Stage Energy production in larger stars is described by the C-N-O Cycle. The more massive stars have greater pressures and temperatures in the core to produce heavier elements. the more massive the star, the faster it moves through this stage.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 3. Main Sequence Stage (Larger Stars) The C-N-O Cycle: 12 C + 1 H → 13 N + photon 13 N → 13 C + e + + neutrino 13 C + 1 H → 14 N + photon 14 N + 1 H → 15 O + photon 15 O → 15 N + e + + neutrino 15 N + 1 H → 12 C + 4 He + photon

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 4. Giant Stage – The initial hydrogen fuel in the stellar core lessens. Helium fission intensifies and the shell expands suddenly (Helium flash). A new position of stability is reached after other periods of expansion and contraction.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 4. Giant Stage – Larger, more massive stars may get so hot and expand so rapidly, the outer layers of the convection zone are blown off as the inner layers over expand, forming supergiants.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 4. Giant Stage – Energy production in the core is described by the Triple Alpha Process: 4 He + 4 He → 8 Be + photon 8 Be + 4 He → 12 C + photon

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 4. Giant Stage – The final phases of the Giant stage are marked by still higher temperatures and pressures in the core to synthesize larger elements. Super and hyper giants can produce all of the natural elements by nuclear S-Process and R-Process reactions.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 5. Variable Stage – ionized helium collecting on the newly expanded surface of the convection zone absorbs outgoing energy. Pressure builds. The outer surface expands. Energy is released. Surface subsides. Process repeats.

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 5. Variable Stage Some 28 different types if variable stars. Long period variable stars Short period variable stars Cephied variable stars Irregular variable stars

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 6. Terminal Stage - depending on the initial mass of the star, it may either 1) fizzle out, forming a brown dwarf; 2) explode, blowing off the outer photosphere and forming a smaller, hotter white dwarf star;

Stellar Formation and Life Cycle Stars are theorized to evolve through six stages of development… 6. Terminal Stage – 3) explode and leave behind a dense core, which is called a neutron star. 4) explode and leave behind a dense core that collapses in on itself, which is called a black star.

ASTRONOMY Stars