1. Apparent Magnitude Astrophysics Lesson 7

2. Learning Objectives  Define luminosity & intensity.  Place astronomical objects with a range of intensities on a magnitude scale.  Recall and use the equation m = -2.5 lg I + constant, where m is the apparent magnitude and I is the intensity.  Calculate the ratio in intensities given a difference in magnitude.  Define apparent magnitude

3. Luminosity The luminosity of a star id the total energy emitted per second (units of Watts). The Sun’s luminosity is about 4 x 10 26 W. The most luminous stars have a luminosity of about million times that of the Sun!

4. Stars have a range of L

5. Relative Sizes

8. Brightness The intensity, I of an object is the power received from it per unit area at Earth. This is the effective brightness of an object. It can be calculated using the equation:-

9. Apparent Magnitude The Greek astronomer Hipparchus classified stars according to their apparent brightness to the naked eye, about 2000 years ago. Its scale was 1 for the brightest star to 6 for the dimmest star. It is still used today and is called the apparent magnitude scale.

10. Apparent Magnitude Apparent magnitude, m is based on how bright things appear from Earth. It is related to intensity using the following equation:- m = -2.5 log I + constant Back to front and logarithmic (base 10!). Enjoy!

11. Pogson’s Law In the 19 th Century the scale was redefined using a strict logarithmic scale: A magnitude 1 star has an intensity 100 times greater than a magnitude 6 star. Expressed mathematically this is:-

12. Apparent Magnitude By logging both sides by 10, this can be re- written:- Where m is the apparent magnitude And I is the intensity.

13. Apparent Magnitude Scale Have a go!

14. Apparent Magnitude Scale

15. The apparent magnitude is given the code m. Magnitude 1 stars are about 100 times brighter than magnitude 6 stars. A change in 1 magnitude is a change of 2.512 (100 1/5 = 2.512). The scale is logarithmic because each step corresponds to multiplying by a constant factor.