EARTH & SPACE EARTH & SPACE SCIENCESCIENCE

Chapter 30 Stars, Galaxies, Chapter 30 Stars, Galaxies, and the Universeand the Universe

30.1 Characteristics of 30.1 Characteristics of StarsStars

30.1 Characteristics of 30.1 Characteristics of StarsStarsObjectivesObjectives Describe how astronomers determine the Describe how astronomers determine the

compositions and temperature of stars.compositions and temperature of stars. Explain why stars appear to move in the Explain why stars appear to move in the

sky.sky. Describe one way astronomers measure Describe one way astronomers measure

the distances to stars.the distances to stars. Explain the difference between absolute Explain the difference between absolute

magnitude and apparent magnitude.magnitude and apparent magnitude.

Analyzing StarlightAnalyzing Starlight

A star is a large celestial body that is A star is a large celestial body that is composed of gas and that emits light.composed of gas and that emits light.

Nuclear fusion is the combination of Nuclear fusion is the combination of light atomic nuclei to form heavier light atomic nuclei to form heavier atomic nuclei.atomic nuclei.

Astronomers learn about stars by Astronomers learn about stars by analyzing the light that the stars emit. analyzing the light that the stars emit.

Starlight passing through a Starlight passing through a spectrograph produces a display of spectrograph produces a display of colors and lines called a spectrum.colors and lines called a spectrum.

Analyzing StarlightAnalyzing Starlight

All stars have dark-line spectra, which All stars have dark-line spectra, which are bands of color crossed by dark are bands of color crossed by dark lines where the color is diminished.lines where the color is diminished.

A star’s dark-line spectrum reveals the A star’s dark-line spectrum reveals the star’s composition and temperature.star’s composition and temperature.

Stars are made up of different elements Stars are made up of different elements in the form of gases.in the form of gases.

Because different elements absorb Because different elements absorb different wavelengths of light, scientists different wavelengths of light, scientists can determine the elements that make can determine the elements that make up a star by studying its spectrum.up a star by studying its spectrum.

The Composition of The Composition of StarsStars Scientists have learned that stars are Scientists have learned that stars are

made up of the same elements that made up of the same elements that compose Earth.compose Earth.

The most common element in stars is The most common element in stars is hydrogen.hydrogen.

Helium is the second most common Helium is the second most common element in star.element in star.

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

The Temperature of The Temperature of StarsStars The temperature of most stars ranges The temperature of most stars ranges

from 2,800˚C to 24,000˚C.from 2,800˚C to 24,000˚C. Blue stars have average surface Blue stars have average surface

temperatures of 35,000˚C.temperatures of 35,000˚C. Yellow stars, such as the sun, have Yellow stars, such as the sun, have

surface temperatures of about surface temperatures of about 5,500˚C.5,500˚C.

Red stars have average surface Red stars have average surface temperatures of 3,000˚C.temperatures of 3,000˚C.

The Sizes and Masses The Sizes and Masses of Starsof Stars Stars vary in size and mass.Stars vary in size and mass. Stars such as the sun are considered Stars such as the sun are considered

medium-sized stars. The sun has a medium-sized stars. The sun has a diameter of 1,390,000 km.diameter of 1,390,000 km.

Most stars visible from Earth are Most stars visible from Earth are medium-sized stars. medium-sized stars.

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

Stellar MotionStellar Motion

Apparent MotionApparent Motion– The apparent motion of stars is the motion The apparent motion of stars is the motion

visible to the unaided eye. visible to the unaided eye. – Apparent motion is caused by the Apparent motion is caused by the

movement of Earth. movement of Earth. – The rotation of Earth causes the apparent The rotation of Earth causes the apparent

motion of stars sees as though the stars are motion of stars sees as though the stars are moving counter-clockwise around the North moving counter-clockwise around the North Star.Star.

– Earth’s revolution around the sun causes the Earth’s revolution around the sun causes the stars to appear to shift slightly to the west stars to appear to shift slightly to the west every night.every night.

Stellar MotionStellar Motion

Circumpolar StarsCircumpolar Stars– Some stars are always visible in the night Some stars are always visible in the night

sky. sky. – These stars never pass below the horizon. These stars never pass below the horizon. – In the Northern Hemisphere, the In the Northern Hemisphere, the

movement of these stars makes them movement of these stars makes them appear to circle the North Star. appear to circle the North Star.

– These circling stars are called circumpolar These circling stars are called circumpolar stars.stars.

Stellar MotionStellar Motion

Actual Motion of StarsActual Motion of Stars– Most stars have several types of actual Most stars have several types of actual

motion. motion. – Stars rotate on an axis.Stars rotate on an axis.– Some stars may revolve around another Some stars may revolve around another

star.star.– Stars either move away from or toward our Stars either move away from or toward our

solar system.solar system.

Stellar MotionStellar Motion

Actual Motion of Stars (continued)Actual Motion of Stars (continued)– Doppler effect an observed change in the Doppler effect an observed change in the

frequency of a wave when the source or frequency of a wave when the source or observer is movingobserver is moving

– The spectrum of a star that is moving toward The spectrum of a star that is moving toward or away from Earth appears to shift, due to the or away from Earth appears to shift, due to the Doppler effect. Doppler effect.

– Stars moving toward Earth are shifted slightly Stars moving toward Earth are shifted slightly toward blue, which is called blue shift.toward blue, which is called blue shift.

– Stars moving away from Earth are shifted Stars moving away from Earth are shifted slightly toward red, which is called red shift.slightly toward red, which is called red shift.

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

Distances to StarsDistances to Stars

A light-year is the distance that light A light-year is the distance that light travels in one year.travels in one year.

Distances between the stars and Earth Distances between the stars and Earth are measured in light-years.are measured in light-years.

Parallax is an apparent shift in the Parallax is an apparent shift in the position of an object when viewed position of an object when viewed from different locations.from different locations.

For relatively close stars, scientists For relatively close stars, scientists determine a star’s distance by determine a star’s distance by measuring parallax.measuring parallax.

http://ircamera.as.arizona.edu/NatSci102/NatSci102/text/parallax.htm

Stellar BrightnessStellar Brightness

Apparent magnitude the brightness of a Apparent magnitude the brightness of a star as seen from the Earth.star as seen from the Earth.

The apparent magnitude of a star The apparent magnitude of a star depends on both how much light the star depends on both how much light the star emits and how far the star is from Earth.emits and how far the star is from Earth.

Absolute magnitude the brightness that Absolute magnitude the brightness that a star would have at a distance of 32.6 a star would have at a distance of 32.6 light-years from Earth.light-years from Earth.

The brighter a star is, the lower the The brighter a star is, the lower the number of its absolute magnitude.number of its absolute magnitude.

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