1. Characteristics of Stars Analyze how stars are classified based on their physical characteristics.

2. Color Astronomers classify stars according to their physical characteristics which include; color, temperature, size, composition, and brightness. Looking up at the night sky you will notice that some stars look reddish, while others are yellow or blue-white. Differences in the colors of stars are due to differences in their surface temperature. The same is true of all objects that glow.

3. Color and Surface Temperatures A star’s color reveals its surface temperature. The coolest stars—with a surface temperature of about 3,200 °C—appear red. Our yellow sun has a surface temperature of about 5,500 °C. The hottest stars, with surface temperatures of over 20,000 °C, appear bluish. Color and Surface Temperatures of Stars ColorSurface Temperature (°C) BlueAbove 25,000 Blue-white10,000-25,000 White7,500-10,000 Yellow-white6,000-7,500 Yellow5,000-6,000 Orange3,500-5,000 RedBelow 3,500

4. Size Looking at stars in the sky, they all appear to be points of light of the same size. Many stars are actually about the size of the sun. However, some stars are much larger than the sun. Most stars are smaller than the sun. Astronomers use the size of the sun to describe the size of other stars.

5. Using Solar Radii Astronomers have indirectly measured the dimensions of the sun. The Sun’s radius is approximately 695,000 km, or about 109 times the radius of Earth. So the sun would equal 1 solar radius. In comparison white dwarfs are about the same size as Earth and would equal 0.01 solar radius. Supergiants can have sizes up to 1,000 solar radii.

6. Chemical Composition Stars very in their chemical composition. The chemical composition of most stars is about 73 % hydrogen, 25 %, and 2 % other elements by mass. Astronomers use spectrographs to determine the elements found in stars. A spectrograph is a device that breaks light into colors and produces an image of the resulting spectrum.

7. The gases in a star’s atmosphere absorb some wavelengths of light produced within a star. When the star’s light is seen through a spectrograph, each absorbed wavelength is shown as a dark line on a spectrum. Each chemical element absorbs light at particular wavelengths. Just like fingerprints, each element has a unique set of spectra lines.

8. Brightness of Stars Stars differ in brightness, the amount of light they give off. The brightness of a star depends on both its size and temperature. A larger star tends to be brighter than a smaller star. A hotter star tends to be brighter than a cooler star. How bright a star appears depends on both its distance from Earth and how bright the star truly is. Because of these two factors, the brightness of a star is described in two ways: apparent brightness and absolute brightness.

9. Apparent Brightness Apparent brightness or apparent magnitude is a star’s brightness as seen from Earth. Astronomers cannot determine how much light a star gives off from it’s apparent brightness. A star closer to Earth may appear to give off more light than others star’s, but it looks so bright simply because its closer.

10. Magnitude Using only their eyes, ancient astronomers described star brightness by magnitude. The brightest stars they could see were called first magnitude and the faintest stars they could see were called sixth magnitude. Using telescopes, astronomers were able to see new stars to dim to see with the naked eye. Instead of replacing the old 1-6 scale, they added to it.

11. Today the brightest stars have a magnitude of about -2 and the faintest stars +30. The magnitude scale may seem backwards. Faint stars have positive (larger) numbers; bright stars have a negative (smaller) numbers.

12. Absolute Brightness Luminosity means actual brightness of a star. To measure a star’s luminosity, astronomers use an absolute brightness scale called absolute magnitude. Absolute magnitude is a measure of how bright a star would be if the star were located at a measured distance. It is like comparing stars luminosity if they were all lined up equal distances from the Earth.

13. To understand the difference between apparent magnitude and absolute magnitude look at the table below: Magnitudes of Selected Stars StarDistance from Earth Apparent Magnitude Absolute Magnitude Sun8.3 light-minutes-26.8+4.8 Sirius8.6 light-years-1.46+1.4 Betelgeuse640 light-years+0.45-5.6

14. H-R Diagram The Hertzsprung-Russell Diagram shows the relationship between surface temperature and absolute brightness of stars. The surface temperatures of stars are plotted on the x-axis and their absolute brightness on the y-axis. Astronomers use H-R diagram to classify stars and to understand how stars change over time.

15. Most of the stars in the H- R diagram form a diagonal area called the main sequence. 90% of all stars are main sequence stars. Within the main sequence, the surface temperature increases as absolute brightness increases. Hot-bluish stars are located at the left and cooler reddish stars on the right. Bright stars near the top and dim stars near the bottom.