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Characteristics of Stars What is a Star? • A sphere of super-hot gases • Mostly Hydrogen and Helium • 1 to 2 percent of a stars mass may consist of heavier elements. The Sun •Our solar system’s star. •Light from the Sun takes 8 minutes to reach Earth. •The Sun is 150 million km away from Earth Distance to Stars • Earth is about 150 million km from the Sun. (93million miles) • That is equal to 1AU (astronomical unit) Distance to Stars • Since Stars are so far away: – A light year is used to express great distances. – Light travels at 300,000 km/s – 1 light year = the distance a ray of light travels in 1 year. 9.5 trillion km. Horsehead Nebula is 1,500 light years away) Proxima Centauri • The next closest star is 40 trillion km from the sun. (more than 260,000AU) • Light takes 4.2 years to reach Earth. • Most stars are much further away. Distance to Stars • Stars appear to be the same distance from Earth in the night sky. • How do we know that they are not? – Parallax • Is the apparent change in position of an object when you look at it from different positions. Distance to Stars • Only works for near by stars. (less than 1,000 light years away. • Astronomers look at the star when Earth is on one side of the sun and again six months later. Distance to Stars • They measure how much the star appears to move against the background of stars. • The less the star appears to move the further away it is. Characteristics ? Classifying Stars • Size (Mass) • Color & Temperature • Brightness Mass (size) • Mass is the total amount of material in a body. • Some stars are five, ten or more times the size of the sun. Others are less massive. Mass (size) • 1. Super Giant – HUGE stars. – Betelgeuse • 2. Giant Stars- very large stars – Rigel • 3. Medium Size Star- Our Sun • 4. White dwarf- smaller than the sun, about the size of Earth. • 5. Neutron Star- very small, 20km Density of a Star • One teaspoon full of material from Sirius is so dense that on Earth it would weigh a ton. Temperature • How can the temperature of a star be determined? – By the color • Red Stars= –Cool • Blue Stars= –Hot Temperatures & Colors 1. Red Stars - 3,000◦C (Betelgeuse) 2. Red-Orange Stars- 5,000 ◦C (Aldebaran) 3. Yellow Stars – 5,500◦C – 6,000 ◦C (Sun) 4. White Stars- 10,000◦C (Sirius) 5. Blue Stars- more than 15,000◦C (Rigel) Brightness (Luminosity) • Depends on size and temperature. Betelgeuse is large but is cool. It’s size makes it appear bright. • Rigel is smaller but is very hot so it shines brightly. • How bright a star appears from Earth depends on far away the star is. Brightness (Luminosity) • Depends on size and temperature. • Betelgeuse is large but is cool. It’s size makes it appear bright. • Rigel is smaller but is very hot so it shines brightly. Brightness (Luminosity) • Stars are far from Earth. • As a result, what problem does this cause? – Cannot accurately determine the actual brightness of a star from earth. • How bright a star appears from Earth depends on far away the star is. Apparent Magnitude • The measure of how bright a star appears to be from earth. – Less bright objects that are closer to you could be seem brighter than objects that are far and actually brighter. • The lower the magnitude the brighter the star. Absolute Magnitude • The brightness a star would appear if it was set at a standard distance from Earth. – Astronomers calculate the stars apparent magnitude and it’s distance from Earth. – Then calculate the brightness if it were a standard distance from Earth. HR- Diagram HR- Diagram • Herzsprung and Russell • A graph used to find out if temperature and brightness are related. • Plots absolute magnitude vs. surface temperature. HR- Diagram • More than 90 % of stars are Main Sequence, form a diagonal band.