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Published byErnest Nelson Modified over 8 years ago
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28.2 Stars and Their Characteristics
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Early Observations Watching stars is one of the oldest human pursuits Some observations are still used today
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Constellations Names given to groups of stars Refer to – Mythical Hero’s Hercules – Monsters Draco (Dragon) – Familiar Objects Lyra (Harp) – Modern Names Telescopium, Microscopium
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HERCULES BOÖTES COMA BERENICES CYGNUS CEPHUSLACERTA CASSIOPEIA CANES VENATICI LEO LEO MINOR URSA MAJOR URSA MINOR LYNX DRACO PERSEUS PEGASUS PISCES LYRA CAMELOPARDALIS
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Constellations and Movement 88 Seen from N and S Hemisphere Big Dipper (Ursa Major – Great Bear) – Can Be used to find Ursa Minor and Cassiopeia Apparent motion East to west – Change position with the seasons Directly over head Polaris – stationary – Circumpolar constellations Move counter clockwise around Polaris – Can be seen year round Ex - Ursa Minor, Ursa Major and Cassiopeia
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Circumpolar Constellations
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Apparent Magnitude How bright a star appears as seen from Earth 120 B.C.E Hipparchus devised brightness scale for approx 850 stars – Scale from 1 to 6 with 1 being the brightest and 6 dimmest
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Brightest Stars – 1 st Magnitude – Some stars are even brighter and have magnitudes less than 1 Sirius -1.45 andThe Sun -26.7 Faintest Stars – 6 th Magnitude Modern Magnitude scale – Increase by factor of 2.5 – 1 st is 100 x’s brighter than 6 th – Telescopes have identified 20 th magnitude stars Betelgeuse Rigel
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Distances to Stars Regular Units used on Earth ineffective in Space due to it’s immense size AU or Astronomical Unit – Distance from Earth to the Sun – Used within Solar system 93,000,000 miles or 150,000,000 Km Light Year - Distance Light travels in 1 year @ 186,000 mi/sec or 300,000 Km/sec – 5.88 x 10 12 Miles or 9.46 x 10 12 Km
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Distance in Space Parsec – Parallax second – 3.258 Light Years – 3.086 x 10 13 Km 2 AU
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Assignment Lab Calculating the Light Year 28.1 RSG and # 1 – 6 pg 624 (Homework)
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Elements in Stars Mostly Hydrogen and Helium 1 to 2 % Heavier elements – Like C, N 2, Na, or O 2 Spectral Analysis – Determines Composition and Temperature No Two stars alike – Each Element has Unique Wavelengths like a fingerprint of a person
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Mass, Size and Temperature Mass is an estimate – can not measure directly – Determined by gravity and inertia Expressed as masses of the sun – Solar Mass – 2 x 10 30 Kg – Range 1/10 Solar Mass to 20 Solar Masses Size - Varies more than mass – Largest star 2000 x’s the diameter of the sun Density – Differs even more than size – Betelgeuse 1/10,000,000 That of the sun – Star near Sirius 1 teaspoon is about 1 ton on earth
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Temperature and Color Stars grouped by Temp and color Harvard Spectral Classification Scheme “Oh Be A Fine Girl (or Guy), Kiss Me”. Only Bored Astronomers Find Gratification Knowing Mnemonics
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Absolute Magnitude Bright Dim Luminosity O B A F G K M
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Luminosity Actual brightness of a star – Depend on size and temp – Two stars same size, hotter star is brighter
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Absolute Magnitude Brightness of a star if all stars were the same distance from earth – 10 parsecs from earth
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Variable Stars Pulsating - Stars that change brightness due to expansion and contraction – Cepheid variables 1 to 50 day cycles most 5 days Comparing absolute and apparent magnitudes we can calculate distances to galaxies containing them
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Variable Stars Non Pulsating – most stars are parts of a system of 2 or more stars – Eclipsing Binaries When 1 star passes in front of another, it blocks light from second star thus decreasing brightness
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