Properties of Stars: Properties of Stars: Brightness (37)Brightness (37)

Remember…Remember… We have learned how to determine a We have learned how to determine a

star’s:star’s:– Chemical Composition.Chemical Composition.– Direction of MotionDirection of Motion– Speed of MotionSpeed of Motion

But to fully understand a star we also But to fully understand a star we also need to know:need to know:– How much energy does it emit? How much energy does it emit?

(luminosity)(luminosity)– How big is it? (volume)How big is it? (volume)– How much matter does it contain? (mass)How much matter does it contain? (mass)

Ways to Describe Ways to Describe BrightnessBrightness Intrinsic Brightness (Good)Intrinsic Brightness (Good): total : total

amount of light (energy) a star emits.amount of light (energy) a star emits.– Measures amount of energy we receive Measures amount of energy we receive

on Earth.on Earth. FluxFlux: How many Joules per second hitting : How many Joules per second hitting

a square meter.a square meter.– 1 J/s = 1 Watt (W).1 J/s = 1 Watt (W).

– ProblemProblem: Amount of flux/brightness : Amount of flux/brightness received decreases with a star’s received decreases with a star’s distance.distance.

Ways to Describe Ways to Describe Brightness (cont.)Brightness (cont.)

Absolute Visual Magnitude (Better): Absolute Visual Magnitude (Better): how bright a star is if it were seen 32.62 how bright a star is if it were seen 32.62 ly away.ly away.– Easy to find using Easy to find using apparent magnitude apparent magnitude and and

distancedistance from earth. from earth. – ProblemProblem: Based : Based onlyonly on visible light. on visible light.– Sun is 4.78.Sun is 4.78.

Would be no brighter than the Little Dipper’s Would be no brighter than the Little Dipper’s faintest stars.faintest stars.

Ways to Describe Ways to Describe Brightness (cont.)Brightness (cont.)

Luminosity (Best):Luminosity (Best):– Measures Measures TOTAL energy TOTAL energy a star is emitting per a star is emitting per

second.second.– Absolute magnitude only measures Absolute magnitude only measures visible visible

lightlight..– Hot stars emit more UV light, cooler stars emit Hot stars emit more UV light, cooler stars emit

more infrared light.more infrared light.– The sun’s luminosity = The sun’s luminosity = 4 X 104 X 102626 W W..

This is called This is called 1 Solar Luminosity1 Solar Luminosity..

Star Brightness Ad Star Brightness Ad (36)(36)

Create an advertisement for one of the Create an advertisement for one of the science concepts we just discussed.science concepts we just discussed.

Your ad should incorporate ideas about Your ad should incorporate ideas about the concept to convince people why the concept to convince people why they should “buy” it.they should “buy” it.

Be sure to include an eye-catching Be sure to include an eye-catching illustration that relates to the concept.illustration that relates to the concept.

Write 4-6 sentences explaining how Write 4-6 sentences explaining how your your ad and illustrationad and illustration relate to the relate to the concept you have chosen. concept you have chosen.