Ch. 27 Stars and Galaxies Ch Characteristics of Stars

Objectives How is a star’s surface temperature and composition determined? How is a star’s surface temperature and composition determined? Explain why, to an observer on earth, stars appear to move. Explain why, to an observer on earth, stars appear to move. Describe the ways distances from the earth to the stars are measured. Describe the ways distances from the earth to the stars are measured. Explain the difference between absolute and apparent magnitude. Explain the difference between absolute and apparent magnitude.

Star—body of gas that gives off tremendous radiant heat and light energy. Star—body of gas that gives off tremendous radiant heat and light energy. The sun is a star. The sun is a star. Stars may have different colors, masses and diameters. Stars may have different colors, masses and diameters. Other differences include composition, temperature, brightness, and distance from earth. Other differences include composition, temperature, brightness, and distance from earth.

Composition and Temperature Spectrometers attached to optical telescopes separate light into lines of different colors, called a spectrum. Spectrometers attached to optical telescopes separate light into lines of different colors, called a spectrum. Each chemical element has a characteristic dark-line spectrum. Each chemical element has a characteristic dark-line spectrum. The same elements found on earth can be found in stars, but hydrogen and helium are the two most common elements. The same elements found on earth can be found in stars, but hydrogen and helium are the two most common elements.

A stars surface temperature determines its color. A stars surface temperature determines its color. The hottest blue stars may be 50,000 C. The hottest blue stars may be 50,000 C. Cool red stars average 3,000 C. Cool red stars average 3,000 C. Yellow stars (the sun) are about 5,500 C. Yellow stars (the sun) are about 5,500 C.

Motion Due to Earth’s rotation, stars appear to rotate around the motionless North Star. Due to Earth’s rotation, stars appear to rotate around the motionless North Star. Because of Earth’s revolution around the sun, some stars are not at visible all times during the year. Because of Earth’s revolution around the sun, some stars are not at visible all times during the year. The stars that are visible year-round and circle the North Star are the circumpolar stars. The stars that are visible year-round and circle the North Star are the circumpolar stars.

Due to the Doppler effect, stars moving toward the earth have their spectrum shifted to the blue wavelength (blue shift). Due to the Doppler effect, stars moving toward the earth have their spectrum shifted to the blue wavelength (blue shift). Stars moving away from the earth experience red shift. Stars moving away from the earth experience red shift. Most distant galaxies and star groups are red-shifted…moving away from us. Most distant galaxies and star groups are red-shifted…moving away from us.

Distance to the Stars Very large distances are measured in light years…the distance light travels in one year. Very large distances are measured in light years…the distance light travels in one year. Alpha Centauri is 4.2 light years away. Alpha Centauri is 4.2 light years away. The North Star is 700 light years away. The North Star is 700 light years away. Parallax is used to measure the distance to nearby stars. Parallax is used to measure the distance to nearby stars.

Nearby stars apparent position in relation to more distant stars changes as earth moves in its orbit from one side of the sun to the other. Nearby stars apparent position in relation to more distant stars changes as earth moves in its orbit from one side of the sun to the other. Limited to measuring the distance to stars within 1000 light years of earth. Limited to measuring the distance to stars within 1000 light years of earth.

Distance to more distant stars can be determined by comparing a stars true brightness (deduced from its spectrum) to its apparent brightness. Distance to more distant stars can be determined by comparing a stars true brightness (deduced from its spectrum) to its apparent brightness. Special Cepheid Variable stars can be used to determine the distance to very distant galaxies. Special Cepheid Variable stars can be used to determine the distance to very distant galaxies.

Stellar Magnitudes Visibility of a star depends on two things; its brightness, and its distance from earth. Visibility of a star depends on two things; its brightness, and its distance from earth. Apparent Magnitude—the brightness of a star as it appears from earth. Assigned a number on a scale. Small and negative numbers are brighter. Larger numbers are dimmer. Apparent Magnitude—the brightness of a star as it appears from earth. Assigned a number on a scale. Small and negative numbers are brighter. Larger numbers are dimmer.

Absolute Magnitude The true brightness of a star. The true brightness of a star. How the star would appear if it was seen from a distance of 32.6 light-years away. How the star would appear if it was seen from a distance of 32.6 light-years away. If the sun was 32.6 light years away, it would be a +5 on the apparent magnitude scale, so its absolute magnitude is +5. If the sun was 32.6 light years away, it would be a +5 on the apparent magnitude scale, so its absolute magnitude is +5. Most stars have an absolute magnitude of between -5 and +15. Most stars have an absolute magnitude of between -5 and +15.

Classification of Stars When the stars surface temperatures are plotted against their absolute magnitude on a graph, a pattern develops. When the stars surface temperatures are plotted against their absolute magnitude on a graph, a pattern develops. This is the Hertzsprung-Russell Diagram (H-R Diagram). This is the Hertzsprung-Russell Diagram (H-R Diagram). Most stars (including the sun) fall along a line from cool, dim stars on the lower right, to hot, bright stars on the upper left. These are the main sequence stars. Most stars (including the sun) fall along a line from cool, dim stars on the lower right, to hot, bright stars on the upper left. These are the main sequence stars.

A group of cool, but large, and therefore very bright stars are in the upper right side of the H-R Diagram (the Giants and Supergiants). A group of cool, but large, and therefore very bright stars are in the upper right side of the H-R Diagram (the Giants and Supergiants). A group of hot, but small, and therefore very dim stars are in the lower left of the diagram (White Dwarfs). A group of hot, but small, and therefore very dim stars are in the lower left of the diagram (White Dwarfs).