29:50 Modern Astronomy
Fall 2002
Lecture 13...October 4, 2002
The Energy Source of the Sun and Stars (cont)

Minimum of Algol Sunday night, 9:45 PM. Find Algol this weekend, then check it on Sunday night.

(1) Last time, discussed total inadequacy of chemical reactions to power the stars.

(2) To find the right answer, consider the deep structure of the Sun. In the core of the Sun

• The density is 200 grams/cubic cm
• The temperature is K.

(3) At those temperatures, nuclear fusion reactions occur. drawing of structure of atom.

(4) An example of a nuclear fusion reaction:

These three reactions are called the proton-proton cycle, and are a way of .

(5) So what? The mass on the left hand side of is 0.7 % more than on the right hand side.

• 4H = kg
• He = kg
• difference kg

(6) The famous Einstein relation says that matter and energy can be converted from one to the other.

(7) Now work out energy yield /kg for the proton-proton cycle.

• 1 kg H 0.993 kg He
• kg
• Joules

It's good we didn't have a fusion balloon popped in class.

(8) Question for the august assembly: Is nuclear fusion satisfactory as an energy source for the Sun and stars? Follow same reasoning as last time with chemical reactions.

(9) When details of star are taken into account, only about 10 percent of the mass (in the core) goes through the proton-proton cycle, so the Main Sequence lifetime of a star like the Sun is years.

(10) Considerations on the nature of stars.

1. The fuel for stars is their own hydrogen matter.
2. Stars are ``magnificent equilibria'' between gravity and gas pressure.
3. Question: Consider a 10 solar mass star. Will it be able to last longer or shorter than a star like the Sun?
4. Table of Main Sequence lifetimes. See Figure 19.9 of book. Spectral class O stars don't last long. If we see spectral class O stars, it means they have formed recently (astronomically speaking) URL of Rosette Nebula.
5. Astrobiology implications Higher life formed on Earth about halfway through the main sequence lifetime of the Sun. Is that early or late compared to other stars?
6. What happens to stars after the Main Sequence? What happens before?

• About this document ...