1. CHARACTERISTICS OF STARS

2. LEARNING GOALS  I will be able to explain the colour, mass, and temperature of a star.  I will be able to distinguish between absolute and apparent magnitude.

3. STAR BRIGHTNESS  Luminosity: amount of energy produced each second by a star.  Luminosity is measured by comparing a star to the luminosity of our sun (luminosity = 1)  The brightest star in the night sky, Sirius, has a luminosity = 22. Sirius

4.  Luminosity is also affected by the distance each source is from the observer.  Sirius is 22 times brighter than the sun and yet, the sun appears brighter. Why?  Sirius is 9 ly away and the sun is 1.6 x 10- 5 ly away  When placed at equal distances from the observer, the light with higher luminosity will appear brighter.

5. APPARENT & ABSOLUTE MAGNITUDE  Apparent Magnitude: The brightness of stars in the night sky as they appear from Earth.  Lower magnitude = brighter celestial objects.  i.e Sun >> Apparent magnitude = -27; vs. Polaris >> Apparent magnitude = 1.97  Absolute Magnitude: The brightness of stars as if they were all located 33 ly from Earth.  I.e: Sun >> Absolute magnitude = 4.8 vs. Polaris >> Absolute magnitude = -3.63

6.  Therefore, if Polaris and the Sun were side by side, Polaris would appear much brighter.  Which of these five stars is the brightest from Earth? From an equal distance away?  From Earth: The Sun, apparent magnitude -26.8  From equal distance: Deneb, abs. magnitude -7.5 StarApparent MagnitudeAbsolute Magnitude Sun-26.84.83 Sirius-1.451.5 Vega0.040.5 Betelgeuse0.41-5.6 Deneb1.99-7.5

7. STAR COLOR & TEMPERATURE  Color of a star gives scientists an indication of the surface temperature.  Hottest stars appear bluish-white whereas cooler stars appear reddish  Our sun, with a temperature of 6000 0 C and a yellowish-white appearance falls midway in the table. Blue star Rigel

8. COLOR AND TEMPERATURE RANGES OF SOME STARS ColorTemperature Range ( 0 C) Example(s) Bluish25000 – 50000Zeta Orionis Bluish-white11000 – 25000Rigel Whitish7500 – 11000Sirius Yellowish-white6000-7500Polaris Yellowish5000-6000Sun Orangish3500-5000Arcturus Reddish2000-3500Betelgeuse Zeta Orionis in Horsehead Nebula

9. THE MASS OF A STAR  The mass of our sun is 2 x 10 30 kg and is referred to as one solar mass.  Scientists use this measurement to compare masses of other stars.  These masses can range from 0.1 to 120 solar masses.  i.e. AI (a massive star) has a solar mass of 118!

10. LEARNING GOALS REVISISTED  I will be able to explain the colour, mass, and temperature of a star.  I will be able to distinguish between absolute and apparent magnitude.

11. YOUR TURN …  Please complete p373 #7, 8, 10, 11