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Characteristics of Stars What is a Star? • A star is a large collection of matter that emits huge amounts of energy • The sun is the most important star to us as it is at the centre of our solar system Distances in Space • The closest star to our solar system is Alpha Centauri • It is 40 trillion km away • A light year is the distance light travels in one year (9.46 trillion km) • Alpha Centauri is 4.3 light years away Life cycles of Stars How are these stars different? Classifying Stars • Stars are classified based on: – – – – Colour Temperature Size Brightness Colour/Temperature • The colours of stars tell us something about their temperature • A relatively COOL star glows RED • A very HOT star glows BLUISH-WHITE or BLUE Colour Temperature Range (°C) Example Blue 25 000 – 50 000 Zeta Orionis Bluish-White 11 000 – 25 000 Rigel White 7500 – 11 000 Vega or Sirius Yellowish- White 6000 – 7500 Polaris Yellow 5000 – 6000 Sun or Alpha Centauri Orange 3500 – 5000 Arcturus Red 2000 – 3500 Betelgeuse Size • Stars also range in size from being SUPERGIANT to DWARF. Brightness of Stars • Just over 2000 years ago, a Greek astronomer Hipparchus decided to classify stars based on their brightness: – Brightest First Magnitude – Faintest Sixth Magnitude Two ways to look at magnitude: • APPARENT magnitude is the brightness as it APPEARS to us • ABSOLUTE magnitude is the actual amount of light given off by a star at a standard distance Plotting the Properties of Stars • Astronomers plot the properties of stars on a diagram that compares brightness with surface temperature and colour. Constellations • A constellation is a group of stars that form a pattern • We recognize 88 constellations today • Most familiar constellations are named after characters in Greek mythology The Big Dipper • A part of the constellation Ursa Major (big bear) • Resembles an old fashioned plow and the shape of a ladle The Little Dipper • Part of the constellation of Ursa Minor (little bear) • Tip of the little dipper’s handle is the brightest star, Polaris (the north star) Can you see it? Bootes • Has the unmistakable shape of a kite • Located by following the handle of the Big Dipper • The brightest star is Arcturus • Arcturus is 100x brighter than the sun Can you see it? Cassiopiea • One of the most recognizable shapes among constellations • A distinct W, formed by five main stars Can you see it? Orion • One of the most distinct features is his belt made of three stars • Two key stars are Betelgeuse and Rigel Can you see it? Canis Major • Represents one of the dogs that accompanied Orion the Hunter. • Dominated by the lead star Sirius (popularly known as the dog star) Can you see it? A Map of the Sky