Announcements• Wednesday night star parties begin this

week, 8:45 pm, weather permitting. Attend one for 4 points extra credit! (Staff signature required.)

• 10:00 office hour cut short today• Majors Fest at Union Building Gallery

Nuclear Reactions in Stars (part 2)

1 November 2006

Today:

• What makes the stars shine?• How long do they last?• Then what happens?

The Sun’s Interior

Thermonuclear energy zone

Radiative zone

Convective zone

The Sun’s Interior

Nuclear reactions only take place in the innermost 30% of the sun’s radius.

The central density is 150 times that of water; the central temperature is 15 million kelvin.

In summary…

The sun is a mass of incandescent gas,

A giant nuclear furnace,

Where hydrogen is built into helium

At a temperature of millions of degrees.

-- Zim and Baker, A Golden Guide to Stars, 1951; borrowed by They Might Be Giants, 1993

Can we test any of this theory?Yes! Look for the neutrinos…

Fusion of Hydrogen into Helium

4 1H (protons) 4He

This reaction powers all main-sequence stars.

The more massive the star, the more pressure at its center and therefore the faster the reaction occurs.

Masses of Stars

Sizes of Main-Sequence Stars

Should be white, not green!

Hottest stars are actually somewhat larger

Reds are greatly exaggerated!

Main Sequence Lifetimes(predicted)

Mass (suns)

Surface temp (K)

Luminosity (suns)

Lifetime (years)

25 35,000 80,000 3 million15 30,000 10,000 15 million3 11,000 60 500 million

1.5 7,000 5 3 billion1.0 6,000 1 10 billion0.75 5,000 0.5 15 billion0.50 4,000 0.03 200 billion

What happens when the core of a star runs out of hydrogen?

• With no energy source, the core of the star resumes its collapse…

• As it collapses, gravitational energy is again converted to thermal energy…

• This heat allows fusion to occur in a shell of material surrounding the core…

• Due to the higher central temperature, the star’s luminosity is greater than before…

• This increased energy production causes the outer part of the star to expand and cool (counterintuitive!)…

• We now have a very large, cool, luminous star: a “red giant”!