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ASTRONOMY

ASTRONOMY. Chapter 25 Stellar Distances and Motions. Apparent Brightness. In the night sky, some stars are brighter than others. What can cause this? One star can shine brighter (put out more energy) than another One star can be closer to us than the other. Apparent Magnitude Scale.

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ASTRONOMY

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  1. ASTRONOMY Chapter 25 Stellar Distances and Motions

  2. Apparent Brightness • In the night sky, some stars are brighter than others. • What can cause this? • One star can shine brighter (put out more energy) than another • One star can be closer to us than the other.

  3. Apparent Magnitude Scale • Stars were classified by the time of Ptolemy using a scale of brightness. • Brightest stars were assigned a magnitude of 1. • 6th magnitude were the stars just visible with the naked eye. • This difference of 5 magnitudes between bright and dim stars is used in our modern classification system.

  4. Apparent Magnitude Scale • Since the brightness of a star can now be measured by electronic means, today’s system is much more precise. • Some stars are brighter than the old 1st magnitude stars. • So a 0th and -1st magnitudes have been added. • The sun has a magnitude of -26.8 • Dimmer stars (higher magnitudes) ar also now included.

  5. Note!!! • The BRIGHTER the star, the LOWER the magnitude number.

  6. Calculations with Magnitudes • A magnitude difference of 5 means a difference in brightness of exactly 100 times. • A magnitude 3 star is exactly 100 times brighter than a magnitude 8 star. • A magnitude difference of 1 is a brightness difference of 2.5 (actually 2.512).

  7. Stellar Distances • Parallax can be used to measure the distance to some stars. • Increasingly better instruments allows us to measure smaller angles (= greater distances).

  8. Parsecs • A parsec is the distance from the Earth that a star would be if it subtends an angle of parallax of one arc second using a baseline of 1 AU. • 1 parsec = 3.26 light years • Both of these are units of distance • One light year is about 6 trillion miles.

  9. Hipparcos • In 1989 the European space agency launched a satellite to measure the parallax of thousands of stars. • Accuracy was 1-2 milliarcseconds. • Result is distances accurate to within 10% for stars within 300 ly.

  10. Absolute Magnitude • Remember this? • What can cause this? • One star can shine brighter (put out more energy) than another • One star can be closer to us than the other. • So how can we compare the brightness of stars? • Remove the distance factor.

  11. Inverse Square Law • As the distance from a light source increases, it is spread out further making it dimmer. • At n times the distance, it is spread out n2 times. This makes it times as bright.

  12. Absolute Magnitude • Absolute Magnitude is the brightness a star would have if it were at a distance of 10 parsecs (33 ly). • Absolute Magnitude is indicated by M • Apparent Magnitude is indicated by m • If both are know, the distance can be calculated: d=actual distance(ly); D=standard distance(ly)

  13. Hertzsprung-Russell Diagram • When the absolute magnitude of stars is graphed vs. the spectral type, an interesting pattern is seen. • Hottest stars are to the left. • Brightest stars are near the top. • Several groups are apparent.

  14. Hertzsprung-Russell Diagram http://astronomyonline.org/OurGalaxy/Images/hrdiagram.jpg

  15. Hertzsprung-Russell Diagram • The majority of stars fall along a diagonal from top left to bottom right called the main sequence or dwarf stars. • Two groups above the main sequence are the giants and supergiants. • The white dwarfs are below and to the left of the main sequence stars.

  16. The Doppler Effect • The Doppler Effect is an apparent change in the frequency (color) of light waves due to the motion of the observer or the source. • Doppler effect is a result of radial motion – motion towards or away from each other.

  17. The Doppler Effect

  18. Stellar Motion • To find the space velocity of a star, you need to know the distance to the star, the proper motion of the star, and the radial velocity of the star. • The space velocity can be calculated with simple trigonometry.

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