Characteristics of Stars

Analyze how stars are classified based on their physical

characteristics.

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.

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

Color Surface Temperature (°C)

Blue Above 25,000

Blue-white 10,000-25,000

White 7,500-10,000

Yellow-white 6,000-7,500

Yellow 5,000-6,000

Orange 3,500-5,000

Red Below 3,500

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.

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.

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.

• 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.

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.

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.

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.

• 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.

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.

• To understand the difference between apparent magnitude and absolute magnitude look at the table below:

Magnitudes of Selected Stars

Star Distance from Earth

Apparent Magnitude

Absolute Magnitude

Sun 8.3 light-minutes -26.8 +4.8

Sirius 8.6 light-years -1.46 +1.4

Betelgeuse 640 light-years +0.45 -5.6

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.

• 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.