Section 1: Characteristics of Stars Preview Objectives Analyzing Starlight The Compositions of Stars The Temperatures of Stars The Sizes and Masses of Stars Stellar Motion Distances to Stars Light-Year Stellar Brightness Absolute and Apparent Motion

Objectives Describe how astronomers determine the compositions and temperature of stars. Explain why stars appear to move in the sky. Describe one way astronomers measure the distances to stars. Explain the difference between absolute magnitude and apparent magnitude.

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.

Analyzing Starlight, continued 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.

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 stars. Small quantities of carbon, oxygen, and nitrogen are also found in stars.

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.

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 have about the same mass as the sun, however, some stars may be more or less massive.

Stellar Motion Apparent Motion The apparent motion of stars is the motion visible to the unaided eye. Apparent motion is caused by the movement of Earth. The rotation of Earth causes the apparent motion of stars to be as though the stars are moving counter-clockwise around the North Star. Earth’s revolution around the sun causes the stars to appear to shift slightly to the west every night.

Stellar Motion, continued Reading Check Why does Polaris appear to remain stationary in the night sky? Polaris is almost exactly above the pole of Earth’s rotational axis, so Polaris moves only slightly around the pole during one rotation of Earth.

Stellar Motion, continued Circumpolar Stars Some stars are always visible in the night sky. These stars never pass below the horizon. In the Northern Hemisphere, the movement of these stars makes them appear to circle the North Star. These circling stars are called circumpolar stars.

Stellar Motion, continued Actual Motion of Stars Most stars have several types of actual motion. Stars rotate on an axis. Some stars may revolve around another star. Stars either move away from or toward our solar system.

Stellar Motion, continued Actual Motion of Stars Doppler effect an observed change in the frequency of a wave when the source or observer is moving The spectrum of a star that is moving toward or away from Earth appears to shift, due to the Doppler effect. Stars moving toward Earth are shifted slightly toward blue, which is called blue shift. Stars moving away from Earth are shifted slightly toward red, which is called red shift.

Stellar Motion, continued The spectrum of a star that is moving toward or away from Earth appears to shift, as shown in the diagram below.

Distances to Stars light-year the distance that light travels in one year. Distances between the stars and Earth are measured in light-years. parallax an apparent shift in the position of an object when viewed from different locations. For relatively close stars, scientists determine a star’s distance by measuring parallax.

Light-Year

Stellar Brightness Apparent magnitude the brightness of a star as seen from the Earth. The apparent magnitude of a star depends on both how much light the star emits and how far the star is from Earth. Absolute magnitude the brightness that a star would have at a distance of 32.6 light-years from Earth The brighter a star is, the lower the number of its absolute magnitude.

Stellar Brightness The lower the number of the star on the scale shown on the diagram below, the brighter the star appears to observers.

Absolute and Apparent Motion