Hour 2: Stars, Stellar Evolution, and Creation of Chemical Elements The Sun & Nuclear Fusion - Energy for Life The Sun Compared With Other Stars Stellar.

Hour 2: Stars, Stellar Evolution, andCreation 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

HydrostaticEquilibrium = 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 1H 4He + energy4 protons have

0.048*10-27 kg (= 0.7 %) more mass than 4He.

Energy gain = m*c2

= 0.43*10-11 J

per reaction.

Need large proton speed ( high temperature) to overcome

Coulomb barrier (electromagnetic repulsion between protons).

Sun needs 1038 reactions, transforming 5 million tons of mass into energy every second, to resist its own

gravity.

T ≥ 107 K = 10 million K

The Sun ComparedWith 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

Rigel Betelgeuse

Sun

Polaris

Stellar Evolution -Limits on Star Lifetimes

Slight Detour -DeterminingStar 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 m

asses

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 Msun: ~ 30 million yr.

0.1 Msun: ~ 3 trillion yr.

L ~ M3.5

tlife ~ M-2.5

Table 9-2, p.193

Fig. 10-3, p.203

Habitable (Liquid Water) ZonesAround 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. 10-17, 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