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Chapter 27 Stars and Galaxies Section 1 Characteristics of Stars Notes 27-2 Distance to Stars Light years: used to measure distance to stars and galaxies – Distance that light travels in one year – 300,000 km/s = speed of light – 9.5 trillion km in one year – Sun in 8 light minutes from Earth – Proxima Centauri: closest star to Earth (other than the sun) is 4.2 light years away Sirius (brightest star): 9 ly Polaris: 700 ly Distance to Stars Parallax: method used to determine the distance to stars – Because the earth is moving the star appears to shift in the sky depending on the time of year – The closer the star is to the earth the greater the shift – Astronomers use a photo to determine the shift (photographed twice in a 6 month time) Then calculate the distance to within 1000 ly Parallax Distance to Stars Astronomers compare the actual brightness and apparent brightness – Determines the distance from the earth Cepheid (Sef-EE-id) variable star – Star’s brightness varies at a constant pattern Brightens and fades in a cycle that can be used to determine how far away it is – Astronomers can then determine the distance to the galaxy it is in Stellar Magnitudes 3 billion stars can be seen through telescopes on the surface – 6000 can be seen with the unaided eye Over a trillion stars can be seen with the Hubble Space Telescope Apparent Magnitude: brightness as it appears from Earth Absolute Magnitude: brightness as it appears 32.6 ly away Apparent Magnitude The measurement of brightness is assigned a number on a scale – Brightest stars have lowest numbers – Dimmest stars have highest numbers Most powerful telescopes can detect an apparent magnitude of 29 Apparent Magnitude Unaided eye can detect a 6 – Called a sixth magnitude star – First magnitude is one of the brightest in the sky If the number is negative it is brighter than the first magnitude star – Sun (-26.8), Moon (-12.5), Venus (-4.6), Jupiter (-2.7), Sirius (-1.46) Absolute Magnitude Brightness from 32.6 ly Ex: If the sun was 32.6 ly away, it would be a fifth magnitude star. – Its absolute magnitude = +5 Most stars are between a -5 and +15 Absolute Magnitude Each star has two magnitudes – Two depend on the distance the star is from Earth Stars with a low apparent mag. and higher absolute mag. appear brighter than if they were 32.6 ly away – Like the Sun Stars with a high apparent mag. and low absolute mag. – Stars are too far away to look bright Absolute Magnitude How far away is a star with apparent mag. of +7 and absolute mag. of +7? 32.6 light years HR Diagram Graph plotting the surface temperatures versus the absolute magnitudes of stars HR diagram (Hertzsprung-Russell) – Ejnar Hertzsprung – Henry Russell – Both discovered the pattern Brightness increases as the surface temperature increases HR Diagram Main sequence stars – Found in the band running down through the diagram – Sun Giants: – Large and cool – Betelgeuse Supergiants: – Very large and cool – Antares White Dwarfs – Hot and small – About the size of Earth Homework Sticky Notes Key Terms Ch. 27 Due: Wednesday, Dec. 9