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