Hour 2: Stars, Stellar Evolution, and Creation of Chemical Elements The Sun & Nuclear Fusion - Energy for Life The Sun Compared With Other Stars Stellar Evolution - Limits on Star Lifetimes Habitable (Liquid Water) Zones Around Stars Nucleosynthesis - Making The Atoms of Life –The Slow Way –The Fast Way (Bang!)

Take-aways: The sun and other “main sequence” stars make energy by fusing hydrogen into helium in their cores Only stars of certain types exist stably for the length of time complex life needed to develop on Earth Near the end of their lives stars have more complicated nuclear reactions and make elements from Carbon (#6) up to Iron (#26) At the end of their lives, stars expel and “recycle” material, including some of elements made earlier Elements important to life on Earth such as carbon, nitrogen & oxygen are common in the universe

The Sun & Nuclear Fusion- Energy for Life

07CO, p.122

Celestial Profile, p.123

Fig. 9-11, p.188 Hydrostatic Equilibrium = Pressure Balance in the Sun’s Interior

Fig. 9-14, p.190

Fig. 9-13, p.189

Energy generation in the Sun: The Proton-Proton Chain Basic reaction: 4 1 H  4 He + energy 4 protons have 0.048* kg (= 0.7 %) more mass than 4 He.  Energy gain =  m*c 2 = 0.43* J per reaction. Need large proton speed (  high temperature) to overcome Coulomb barrier (electromagnetic repulsion between protons). Sun needs reactions, transforming 5 million tons of mass into energy every second, to resist its own gravity. T ≥ 10 7 K = 10 million K

The Sun Compared With Other Stars

Fig. 8-3, p.147

Fig. 8-5, p.152

The Hertzsprung Russell Diagram Most stars are found along the main sequence

Radii of Stars in the Hertzsprung-Russell Diagram 10,000 times the sun’s radius 100 times the sun’s radius As large as the sun 100 times smaller than the sun RigelBetelgeuse Sun Polaris

Stellar Evolution - Limits on Star Lifetimes

Slight Detour - Determining Star Masses Observing Binary Stars In the best cases, both stars can be seen directly, and their separation and relative motion can be followed directly.

Masses of Stars in the Hertzsprung- Russell Diagram Masses in units of solar masses Low masses High masses Mass The higher a star’s mass, the more luminous (brighter) it is: High-mass stars have much shorter lives than low-mass stars: Sun: ~ 10 billion yr. 10 M sun : ~ 30 million yr. 0.1 M sun : ~ 3 trillion yr. L ~ M 3.5 t life ~ M -2.5

Table 9-2, p.193

Fig. 10-3, p.203

Habitable (Liquid Water) Zones Around Stars

Habitable Zone = HZ Zone around a star in which a planet would have temperature allowing liquid water Different locations for stars of different luminosity

Continuously Habitable Zone = CHZ Zone around a star in which a planet can STAY the right temperature for liquid water for as long as it took complex life to evolve on Earth (3 billion years?) Only F5 … F9, G0 … G9, K0 … K5 stars Abundant M stars ruled out by tidal locking effect ?

Nucleosynthesis: Making the Atoms of Life In the Cores of Stars

Fig. 10-3, p.203

p.212b

Fig , p.221

Take-aways: The sun and other “main sequence” stars make energy by fusing hydrogen into helium in their cores Only stars of certain types exist stably for the length of time complex life needed to develop on Earth Near the end of their lives stars have more complicated nuclear reactions and make elements from Carbon (#6) up to Iron (#26) At the end of their lives, stars expel and “recycle” material, including some of elements made earlier Elements important to life on Earth such as carbon, nitrogen, & oxygen are common in the universe