Light Years and Parsecs

1. Light Years and Parsecs Measures of interstellar distances

2. The Light Year • The distance that light travels in 1 year is a light year 1 light year = 9.46 x 1015 metres.

3. The parsec Radius of Earth orbit (1 a.u.) This angle is equal to 1 second of arc (1/36000) x The distance x is one parsec i.e. the parsec is the distance from the sun at which the radius of the Earth orbit subtends an angle of one second of arc. The name is an abbreviation of the term “parallax second” 1 parsec = 3.09 x 1016m or 3.26 light years

4. Absolute Magnitude

5. The apparent brightness of stars conveys no information about their distance from us. Some of the brightest stars here are more distant than the faintest

6. Apparent and Absolute Magnitude • The apparent magnitude gives us information about how bright a star appears to be from Earth. • It gives us no information about the how bright the star actually is! • We need another idea to compare the actual brightness of the stars. • This is what the idea of absolute magnitude does.

7. Absolute Magnitude • If the stars were equally distant then their apparent magnitude would give us a true comparison of their brightness. • Absolutemagnitude gives us the value of a star’s brightness ata standard distance of 10 parsecs

8. 1 5 2 3 4 6 Absolute Magnitude Formula • We know that the magnitude scale is a logarithmic scale x 2.512 x 2.512 x 2.512 x 2.512 x 2.512 brighter than 3 brighter than 6 brighter than 2 brighter than 4 brighter than 5

9. From This table we can see it can be determined that the relationship between the two quantities is Taking logs of both sides

10. The Absolute Magnitude Formula • Now where M is the apparent magnitude of the star brought to a distance of 10 parsecs and I the intensity of light received from the star at that distance and m and i are the original values

11. From the inverse square law: Where D is the standard distance of 10 parsecs Combining this equation with Finally

12. Example • Capella is a bright nearby star. Its apparent magnitude is +0.05 and its distance is 14 parsecs. What is its absolute magnitude. • Compare this value to the absolute magnitude of the Sun(+4.8). How many magnitudes is Capella brighter than the Sun and therefore calculate the how many times more power is emitted by Capella than our Sun.

13. Answer Capella is 5.5 magnitudes brighter than the Sun How much more powerful than the Sun? So the intensity of radiation reaching Earth from Cappella at a distance of 10 parsecs is about 160 times more than the Sun at the same distance. So Capella emits around 160 times the power of the Sun.