Ch. 27 Stars and GalaxiesCh. 27 Stars and Galaxies

Ch. 27.1 Characteristics of StarsCh. 27.1 Characteristics of Stars

ObjectivesObjectives

How is a star’s surface temperature and How is a star’s surface temperature and composition determined?composition determined?

Explain why, to an observer on earth, stars Explain why, to an observer on earth, stars appear to move.appear to move.

Describe the ways distances from the Describe the ways distances from the earth to the stars are measured.earth to the stars are measured.

Explain the difference between absolute Explain the difference between absolute and apparent magnitude.and apparent magnitude.

StarStar—body of gas that gives off —body of gas that gives off tremendous radiant heat and light energy.tremendous radiant heat and light energy.

The sun is a star.The sun is a star.Stars may have different colors, masses Stars may have different colors, masses

and diameters.and diameters.Other differences include composition, Other differences include composition,

temperature, brightness, and distance temperature, brightness, and distance from earth.from earth.

Composition and TemperatureComposition and Temperature

SpectrometersSpectrometers attached to optical attached to optical telescopes separate light into lines of telescopes separate light into lines of different colors, called a different colors, called a spectrumspectrum..

Each chemical element has a Each chemical element has a characteristic dark-line spectrum.characteristic dark-line spectrum.

The same elements found on earth can be The same elements found on earth can be found in stars, but hydrogen and helium found in stars, but hydrogen and helium are the two most common elements.are the two most common elements.

A stars surface temperature determines its A stars surface temperature determines its color.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.

MotionMotion

Due to Earth’s rotation, stars appear to Due to Earth’s rotation, stars appear to rotate around the motionless North Star.rotate around the motionless North Star.

Because of Earth’s revolution around the Because of Earth’s revolution around the sun, some stars are not at visible all times sun, some stars are not at visible all times during the year.during the year.

The stars that are visible year-round and The stars that are visible year-round and circle the North Star are the circle the North Star are the circumpolarcircumpolar stars.stars.

Due to the Doppler effect, stars moving Due to the Doppler effect, stars moving toward the earth have their spectrum toward the earth have their spectrum shifted to the blue wavelength (blue shift).shifted to the blue wavelength (blue shift).

Stars moving away from the earth Stars moving away from the earth experience red shift.experience red shift.

Most distant galaxies and star groups are Most distant galaxies and star groups are red-shifted…moving away from us.red-shifted…moving away from us.

Distance to the StarsDistance to the Stars

Very large distances are measured in Very large distances are measured in light light yearsyears…the distance light travels in one …the distance light travels in one year.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.ParallaxParallax is used to measure the distance is used to measure the distance

to nearby stars.to nearby stars.

Nearby stars apparent position in relation Nearby stars apparent position in relation to more distant stars changes as earth to more distant stars changes as earth moves in its orbit from one side of the sun moves in its orbit from one side of the sun to the other.to the other.

Limited to measuring the distance to stars Limited to measuring the distance to stars within 1000 light years of earth.within 1000 light years of earth.

Distance to more distant stars can be Distance to more distant stars can be determined by comparing a stars true determined by comparing a stars true brightness (deduced from its spectrum) to brightness (deduced from its spectrum) to its apparent brightness.its apparent brightness.

Special Special Cepheid Variable Cepheid Variable stars can be stars can be used to determine the distance to very used to determine the distance to very distant galaxies.distant galaxies.

Stellar MagnitudesStellar Magnitudes

Visibility of a star depends on two things; Visibility of a star depends on two things; its brightness, and its distance from earth.its brightness, and its distance from earth.

Apparent MagnitudeApparent Magnitude—the brightness of a —the brightness of a star as it appears from earth. Assigned a star as it appears from earth. Assigned a number on a scale. Small and negative number on a scale. Small and negative numbers are brighter. Larger numbers are numbers are brighter. Larger numbers are dimmer.dimmer.

Absolute MagnitudeAbsolute Magnitude

The true brightness of a star.The true brightness of a star.How the star would appear if it was seen How the star would appear if it was seen

from a distance of 32.6 light-years away.from a distance of 32.6 light-years away. If the sun was 32.6 light years away, it If the sun was 32.6 light years away, it

would be a +5 on the apparent magnitude would be a +5 on the apparent magnitude scale, so its absolute magnitude is +5.scale, so its absolute magnitude is +5.

Most stars have an absolute magnitude of Most stars have an absolute magnitude of between -5 and +15.between -5 and +15.

Classification of StarsClassification of Stars

When the stars surface temperatures are When the stars surface temperatures are plotted against their absolute magnitude plotted against their absolute magnitude on a graph, a pattern develops.on a graph, a pattern develops.

This is the This is the Hertzsprung-Russell DiagramHertzsprung-Russell Diagram (H-R Diagram).(H-R Diagram).

Most stars (including the sun) fall along a Most stars (including the sun) fall along a line from cool, dim stars on the lower right, line from cool, dim stars on the lower right, to hot, bright stars on the upper left. to hot, bright stars on the upper left. These are the These are the main sequencemain sequence stars. stars.

A group of cool, but large, and therefore A group of cool, but large, and therefore very bright stars are in the upper right side very bright stars are in the upper right side of the H-R Diagram (the Giants and of the H-R Diagram (the Giants and Supergiants).Supergiants).

A group of hot, but small, and therefore A group of hot, but small, and therefore very dim stars are in the lower left of the very dim stars are in the lower left of the diagram (White Dwarfs).diagram (White Dwarfs).