1. To help understand the HR Diagram 
More Equations
To help understand the HR Diagram 

3. Is everything appears to what it is?

4. Temperature - Wein’s Law
SURFACE Temperature
Based on color
Measures spectrum of a star
Strongest wavelength tells you the surface temperature using Wein’s Law
T= 3 x 106
wavelength

5. Temperature example

6. Only Boys and Fearless Girls Kill Mice
Spectral Classes
Only Boys and Fearless Girls Kill Mice

7. Spectral Classes Spectra of Stars Can Tell Us: Temperature Motion
Composition
Rotation Speed
Luminosity

8. Luminosity
Luminosity: how bright is a star based on absolute magnitude

9. Luminosity Depends on 2 factors Temperature Radius Hotter = brighter
Bigger = brighter
Distance can make bright stars seem dim and dim stars appear brighter

10. Scenarios
1) If both stars are 7500 K but star A is 5 x times bigger than star B, which is more luminous? Why?
2) If both stars 7500 K but star A is 5 parsecs away and star B is 15 parsecs away, which is more luminous? Why?
3) If star A is 10,000 K and is 20 parsecs away but star B is 3,000 K and is 10 parsecs away, which is more luminous? Why?

11. Magnitude
About 150 B.C., the Greek astronomer Hipparchus measured apparent brightness of stars using units called magnitudes
Brightest stars had magnitude 1 and dimmest had magnitude 6
The system is still used today and units of measurement are called apparent magnitudes - how bright a star appears to look to an observer

12. Magnitude
A star’s apparent magnitude depends on the star’s luminosity and distance – a star may appear dim because it is very far away or it does not emit much energy
The apparent magnitude can be confusing
Scale runs “backward”: high magnitude = low brightness
Modern calibrations of the scale create negative magnitudes
Magnitude differences equate to brightness ratios:
A difference of 5 magnitudes = a brightness ratio of 100
1 magnitude difference = brightness ratio of 1001/5=2.512

13. Magnitude
Astronomers use absolute magnitude to measure a star’s luminosity
The absolute magnitude of a star is how bright a star is at 10 parsecs away(32.6 light years)
A comparison of absolute magnitudes is now a comparison of luminosities, distance variable is removed!
An absolute magnitude of 0 approximately equates to a luminosity of 100 x The Sun

15. Practice Order the stars based on luminosity and temperature.
Temperature Luminosity  
Blue Giant
Blue Super Giant
Neutron Star
Red Dwarf
Red Giant
Red Super Giant
White Dwarf
Yellow Dwarf