Chapter 10 The Nature of Stars

What do you think?

• What color are stars?

• Are most stars isolated from other stars, like the Sun?

A star’s color reveals its surface temperature

Photometry• Technique used to accurately measure the color

of stars using UBV filters– U - ultraviolet wavelengths– B - blue wavelengths– V - visible wavelength

• astronomers compare combinations of U minus B (U-B) and B minus V (B-V)

• these combinations are called color indices

Stars are classified by their spectra as O, B, A, G, K, and M spectral types

Stars are classified by their spectra as O, B, A, G, K, and M spectral types

• O B A F G K M

• hottest to coolest

• bluish to reddish

• An important sequence to remember:– Our Best Astronomers Feel Good Knowing

More– Oh Boy, An F Grade Kills Me– Oh Be a Fine Guy (or Girl), Kiss Me

The Hertzsprung-Russell diagram

identifies a definite relationship between

temperature and absolute magnitude

HR DIAGRAM

Absolute magnitude vs temperature

or

luminosity vs spectral type

Determining the Sizes of Stars from an HR Diagram

• Main sequence stars are found in a band from the upper left to the lower right

• Giant and supergiant stars are found in the upper right corner

• Tiny white dwarf stars are found in the lower left corner of the HR diagram

Luminosity classes

• Class I includes all the supergiants

• Class V includes the main sequence stars

• e.g., the Sun is a G2 V

Monitoring how binary stars move provide information about stellar masses

Monitoring how binary stars move provide information about stellar masses

There is a relationship

between mass and luminosity

for main-sequence stars

Bigger is brighter!

There is a relationship

between mass and luminosity

for main-sequence stars

Bigger is brighter!

The orbital motion of binary stars affects the wavelengths of their

spectral lines

Radial-Velocity Curve showing the positions of the stars and their spectra at four instants in an orbital period.

Some binary stars eclipse each other

Mass transfer in close binary systems can produce unusual double stars

• Close binary systems are where only a few stellar diameters, or less, separate the stars

• Mass can be dramatically transferred between the stars– detached binary (no mass transfer)– semidetached binary(material can flow across

along a path called the Roche lobe)– contact binary (the two stars share a common

envelope of material)

What did you think?

• What color are stars?Stars are found in a wide range of colors, from

red through violet as well as white.

• Are most stars isolated from other stars, like the Sun?In the vicinity of the Sun, two-thirds of the stars

are found in pairs or larger groups.

Self-Check1: Describe the observations necessary for the determination of basic physical properties

of stars such as mass, temperature, luminosity, chemical composition, sizes, and motions.

2: Explain the classification of stars according to their spectra and their luminosity.

3: Draw and describe the H-R diagram and its value for displaying physical properties of stars and in expanding our knowledge about stellar sizes and distances.

4: List the classes of binary stars and state the criteria for determining membership in each.

5: Draw, label, and describe a graph of the mass-luminosity relation and specify limitations on its applicability.

6: Explain the characteristics of the radial-velocity curves for binary stars.

7: Describe the differences between the light curves for total and partial eclipsing binary stars.