4. A theoretical model of the Sun shows how energy gets from its center to its surface
Hydrogen fusion takes place in a core extending from the Sun’s center to about 0.25 solar radius
The core is surrounded by a radiative zone extending to about 0.71 solar radius
In this zone, energy travels outward through radiative diffusion
The radiative zone is surrounded by a rather opaque convective zone of gas at relatively low temperature and pressure
In this zone, energy travels outward primarily through convection

8. The spectra of stars reveal their chemical compositions as well as surface temperatures
Stars are classified into spectral types (subdivisions of the spectral classes O, B, A, F, G, K, and M), based on the major patterns of spectral lines in their spectra

9. The spectral class and type of a star is directly related to its surface temperature: O stars are the hottest and M stars are the coolest

10. Most brown dwarfs are in even cooler spectral classes called L and T
Unlike true stars, brown dwarfs are too small to sustain thermonuclear fusion

11. Stars come in a wide variety of sizes
Relationship between a star’s luminosity, radius, and surface temperature
Stars come in a wide variety of sizes

12. Hertzsprung-Russell (H-R) diagrams reveal the different kinds of stars
The H-R diagram is a graph plotting the absolute magnitudes of stars against their spectral types—or, equivalently, their luminosities against surface temperatures
The positions on the H-R diagram of most stars are along the main sequence, a band that extends from high luminosity and high surface temperature to low luminosity and low surface temperature

13. On the H-R diagram, giant and supergiant stars lie above the main sequence, while white dwarfs are below the main sequence

15. A star’s lifetime on the main sequence is proportional to its mass divided by its luminosity
The duration of a star’s main sequence lifetime depends on the amount of hydrogen in the star’s core and the rate at which the hydrogen is consumed
The more massive a star, the shorter is its main-sequence lifetime

16. During a star’s main-sequence lifetime, the star expands somewhat and undergoes a modest increase in luminosity

17. When core hydrogen fusion ceases, a main-sequence star becomes a red giant

18. Red Giants
Core hydrogen fusion ceases when the hydrogen has been exhausted in the core of a main-sequence star
This leaves a core of nearly pure helium surrounded by a shell through which hydrogen fusion works its way outward in the star
The core shrinks and becomes hotter, while the star’s outer layers expand and cool
The result is a red giant star

19. As stars age and become giant stars, they expand tremendously and shed matter into space

20. Fusion of helium into carbon and oxygen begins at the center of a red giant
When the central temperature of a red giant reaches about 100 million K, helium fusion begins in the core
This process, also called the triple alpha process, converts helium to carbon and oxygen