Properties of Stars

1. Properties of Stars

2. Chapter 30 Analyzing Starlight Star-a large celestial body that is composed of gas and that emits light • Nuclear fusion is the combination of light atomic nuclei to form heavier atomic nuclei • Astronomers learn about stars by analyzing the light that the stars emit • Starlight passing through a spectrograph produces a display of colors and lines called a spectrum

3. Chapter 30 Analyzing Starlight • All stars have dark-line spectra, which are bands of color crossed by dark lines where the color is diminished • A star’s dark-line spectrum reveals the star’s composition and temperature • Stars are made up of different elements in the form of gases • Because different elements absorb different wavelengths of light, scientists can determine the elements that make up a star by studying its spectrum

4. Chapter 30 The Compositions of Stars • Scientists have learned that stars are made up of the same elements that compose Earth • The most common element in stars is hydrogen • Helium is the second most common element in star • Small quantities of carbon, oxygen, and nitrogen are also found in stars

5. Chapter 30 The Temperatures of Stars • The temperature of most stars ranges from 2,800˚C to 24,000˚C. • Blue stars have average surface temperatures of 35,000˚C. • Yellow stars, such as the sun, have surface temperatures of about 5,500˚C. • Red stars have average surface temperatures of 3,000˚C.

6. Chapter 30 The Sizes and Masses of Stars • Stars vary in size and mass • Stars such as the sun are considered medium-sized stars • The sun has a diameter of 1,390,000 km • Most stars visible from Earth are medium-sized stars • Many stars also have about the same mass as the sun, however some stars may be more or less massive

7. THE HERTZSPRUNG-RUSSELL DIAGRAM • In the early 20th century E. Hertzsprung (Danish) and H.N. Russell (American) independently came up with an extremely useful way to categorize stars. • They plotted their COLORS (blue to red using Spectral Classes) or TEMPERATURES (T) on the X-axis and they plotted their absolute MAGNITUDES or LUMINOSITIES on the Y-axis. • These H-R diagrams are therefore sometimes called Color-Magnitude diagrams.

8. Chapter 30 Classifying Stars • Main sequence the location on the H-R diagram where most stars lie • It has a diagonal pattern from the lower right to the upper left • One way scientists classify stars is by plotting the surface temperatures of stars against their luminosity • The H-R diagram is the graph that illustrates the resulting pattern • Astronomers use the H-R diagram to describe the life cycles of stars • Most stars fall within a band that runs diagonally through the middle of the H-R diagram • These stars are main sequence stars.

9. Chapter 30 Classifying Stars