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

3. 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

4. Temperature example
A star radiates most strongly at 600 nm. How hot is the star’s surface?
T = 3,000,000/wavelength
T = 3,000,000/600
T = 5000 K

5. Luminosity
Luminosity: how bright is a star
Given by
Inverse Square Law
Magnitude Scale

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

7. Luminosity Inverse Square Law
The farther away a star is, the dimmer it is.
Proportional to 1/distance2

8. = 1/25th as bright as the closer star
Luminosity
Example:
Two identical stars. One is 5 times further from the earth than the other. How much dimmer does the more distant star appear to be?
= 1/distance2 = 1/52
= 1/25th as bright as the closer star

9. 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 to emphasize how bright a star looks to an observer

10. 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

12. Magnitude
Astronomers use absolute magnitude to measure a star’s luminosity
The absolute magnitude of a star is the apparent magnitude that same star would have at 10 parsecs (32.6 light years)
A comparison of absolute magnitudes is now a comparison of luminosities, no distance dependence
An absolute magnitude of 0 approximately equates to a luminosity of 100 x The Sun

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

16. Parallax Parallax is a way of measuring distance to the stars.
Only accurate to 100 parsecs
How? (fingers and eyes)
So, each of your eyes is like when the earth is on each side of the earth (we must wait until January and June) and the star we are looking at seems to change positions
Then, we can measure the parallax angle and solve for the distance using the formula:
Distance = 1/parallax angle
Distance (parsecs)
Parallax angle (arc seconds)

18. Parallax Problem Solve: Distance = 1.26 parsecs
Solve: Parallax = .758 arc seconds

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