1. Lab 7 – The HR Diagram

2. Ratio of Flux (brightness) between each magnitude step is 2.512
Magnitude Scale
2.512
2.512
2.512 x x  16
2.512
Dimmer
Brighter
2.512
2.512
Brightest stars in the night sky
2.512 x x x X = 100
Faintest stars visible to naked-eye
2.512 x  6.3
Ratio of Flux (brightness) between each magnitude step is 2.512

3. Absolute Magnitude C Apparent Magnitude B D A Sun 5 pc 10 pc 15 pc 4.2
5.0
6.0
0.0 B
3.3
2.0 D A
2.0
1.3
Sun
-26.5
5 pc
10 pc
15 pc

4. Magnitude Stellar Brightness
Apparent Magnitude (mv) - Brightness as seen from the Earth (Depends on the star's luminosity (wattage) and distance).
Absolute Magnitude (Mv) - Brightness as seen from 10 pc (Depends only on the star's luminosity).

5. O B A F G K M -10 -5 +5 +10 +15 H-R Diagram The Sun+5
+10
+15
Absolute Magnitude
Temperature
H-R Diagram
The Sun
M=+5 G2

7. Stefan-Boltzmann Law E = T4
The Sun E
A Hot Star E R R
Rsun = Rhot star
6,000 K
Temperature
12,000 K ?
1 L
16 L
Stefan-Boltzmann Law E = T4
Luminosity

8. E E Surface Area = 4r2 6,000 K M1 = 5 M2 = 2 1 L 4 L
Star 1 E
Star 2 R1 R2
Surface Area = 4r2
6,000 K
M1 = 5
M2 = 2
Star 2 is more luminous than Star 1:
Star 2 is more luminous than Star 1: Mv1 - Mv2 = = 3 magnitudes
Star 2 is 16 time brighter because it has 16 times the surface area.
Star 2 is more luminous than Star 1: Mv1 - Mv2 = = 3 magnitudes
Star 2 is 16 time brighter because it has 16 times the surface area
M1 – M2 = = 3 magnitudes
Star 2 is 16 times brighter. Why?
Star 2 has 16 times the surface area!
1 L
4 L

9. The Sun’s Spectral and Luminosity Class: G2 V
Size
Ia & Ib
Supergiant II
Bright Giant
III
Giant IV
Sub-giant V
Dwarf
The Sun’s Spectral and Luminosity Class: G2 V

11. Stellar Luminosity
Total amount of radiative energy emitted each second
Surface Area (size of the star)
Temperature (Stefan-Boltzmann Law)

12. The End.