In Lesson 4 we described how we infer several intrinsic properties of stars from observations: their surface temperatures, luminosities, radii, and masses (T_s, L, R, M). By plotting the luminosities vs. surface temperatures (the Hertzsprung-Russell diagram), astronomers found that 90% of the stars fell into a band on the H-R diagram called the main sequence (MS, ranging from blue giants to red dwarfs). The main sequence stars obeyed very well-defined relationships in which any one intrinsic property of the star (say, M) determined all the others: T_s, L, R. They also found that about 10% of the stars do not obey these relationships. These stars could also be classified into groups according to their locations on the H-R diagram. They are called red giants (RG), white dwarfs (WD), and horizontal branch (HB) stars.

This classification exercise (taxonomy) leads us to some big questions:

• Why do most of the stars belong to the main sequence?
• Why do the main sequence stars obey the observed relationships between T_s, L, R, and M?
• Why do 10% of the stars not obey these relationships?
• What accounts for the observed differences among MS, RG, HB, and WD stars?

1. MAIN SEQUENCE STARS:

• Composed of hydrogen and helium throughout
• Energy is generated in the core by fusion of hydrogen into helium
• Properties are the result of three physical principles

1. Pressure balance ("hydrostatic equilibrium") -- heat pressure balances gravitational attraction. This principle tells us how to calculate the interior temperature, T_i, from the mass and radius (M & R --> T_i).
2. Heat transfer -- the theory of how heat escapes through matter by radiation transfer and convection tells us how to calculate the luminosity from the interior temperature and mass and radius (M, R, & T_i --> L).
3. Energy generation -- the theory of how energy is created in the core of a MS star by hydrogen fusion tells us the value of T_i, and also how long the star can survive before the all the hydrogen in its core is converted to helium (see 2. LIFETIMES).

3. EVOLUTION

• After a MS star runs out of hydrogen in its center, it goes through a series of drastic changes in structure, becoming a RG, then HB, then an Asymptotic Giant Branch (AGB) star. These stars have complex interior structures and element composition.

4. CLUSTERS

• We believe that all stars in a cluster of stars were born at nearly the same time.
• The distribution of stars in an H-R diagram for a cluster is determined by the time since the cluster was formed. Therefore, we can determine the age of a star cluster from its H-R diagram.

5. INSTABILITY

• Stars more massive than about 200 Suns cannot exist
• Other kinds of stars pulsate
• Dying stars will eject their outer envelopes, creating beautiful objects called "planetary nebulae"

6. WHITE DWARF STARS

• Are incredibly small and dense
• Are dying stars -- not producing any more energy
• Are not supported by heat pressure, but by atomic pressure
• Have a maximum possible mass of 1.4 Solar masses -- predicted theoretically by S. Chandrasekhar

7. BINARIES

• Roughly half of all MS stars are found in close binary systems
• When stars in a binary system evolve, often one star will transfer mass to the other star
• The mass transfer can change the evolution of the stars considerably and results in wonderful phenomena that we can observe.

(Return to course home page)
Last modified September 27, 2000
Copyright by Richard McCray