28.2 Stars and Their Characteristics

Early Observations Watching stars is one of the oldest human pursuits Some observations are still used today

Constellations Names given to groups of stars Refer to – Mythical Hero’s Hercules – Monsters Draco (Dragon) – Familiar Objects Lyra (Harp) – Modern Names Telescopium, Microscopium

HERCULES BOÖTES COMA BERENICES CYGNUS CEPHUSLACERTA CASSIOPEIA CANES VENATICI LEO LEO MINOR URSA MAJOR URSA MINOR LYNX DRACO PERSEUS PEGASUS PISCES LYRA CAMELOPARDALIS

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

Circumpolar Constellations

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

Brightest Stars – 1 st Magnitude – Some stars are even brighter and have magnitudes less than 1 Sirius andThe Sun 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

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 1 186,000 mi/sec or 300,000 Km/sec – 5.88 x Miles or 9.46 x Km

Distance in Space Parsec – Parallax second – Light Years – x Km 2 AU

Assignment Lab Calculating the Light Year 28.1 RSG and # 1 – 6 pg 624 (Homework)

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

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 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

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

Absolute Magnitude Bright Dim Luminosity O B A F G K M

Luminosity Actual brightness of a star – Depend on size and temp – Two stars same size, hotter star is brighter

Absolute Magnitude Brightness of a star if all stars were the same distance from earth – 10 parsecs from earth

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

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