1. When you wish upon a star...

2. Luminosity and all that
shows what a dweeb you really are ..!
Luminosity and all that
Luminosity
Inverse square law
Magnitudes
Distance, temperature, composition ..
H-R diagram

3. Getting our bearings

4. Luminosity Observing apparent brightness.
Brightness is the amount of energy striking per unit area of the human eye or a detector.
The amount we receive is affected by distance according to the inverse square law.
Apparent brightness (energy flux)  Luminosity/distance2

5. Luminosity and magnitudes
Apparent brightness.
Absolute brightness.
Apparent magnitude
Absolute magnitude
To compare intrinsic or absolute properties of stars, use a standard distance of 10 pc.

6. Lets make this difficult (actually the ancient Greeks are to blame)
Around second century B.C.E., Hipparchus scaled naked eye stars into a ranking of 1 to 6 ( brightest to least bright).
1 – 6 range spans a factor of 100 in apparent brightness. ( a 1st magnitude star is 100 X brighter than a 6th magnitude star).
The physiology of the human eye dictates that each magnitude change of 1 corresponds to a change of 2.5 in apparent brightness.
Combining both concepts: 2.55  100
A 1 st magnitude star is approximately 100 X brighter than a 6 th magnitude star

7. But what does it mean? Well, lets look at the 10 pc thing:
Earth
Apparent Mag. > Absolute Mag.
Apparent Mag. < Absolute Mag.
Apparent brightness vs. absolute brightness?

8. Oh ! Brightness decreases this way ! Graph of apparent
magnitudes of some
common things in the
sky .
Brightness increases this way !

9. Luminosity and magnitude
We know from
Apparent brightness  luminosity/distance2
And 2.55  100 -> 1001/5  2.5.
So for every magnitude change we see with our eyes the brightness changes 10X.
IDEA! We can build a chart to relate luminosity to magnitudes:
luminosity
magnitude

10. Recipe: brightness to luminosity
To determine a star’s luminosity:
1. Determine apparent brightness (use a chart or for a new star, measure amount of energy detected per unit time).
2. Measure the star’s distance (parallax method for nearby stars).
3. Use: apparent brightness ~ luminosity/ d2

11. Using our recipe for more stuff
Let m = apparent brightness
Use our recipe: luminosity = d2 m.
Star A: {d = pc, m = 1}, Star B:{ d = 2.12 pc, m = 1}.
Find luminositys for Star A and Star B

12. More luminosity & magnitude stuff
Making things simpler
Scale luminosities to solar luminosity – this way we won’t have to deal with units
Let m – apparent magnitude, M – absolute magnitude.
Throw in the inverse square relationship and some math and….

13. Tah Dah!!!
D = 10 pc x 10(m –M)/5
We have another formula for distance, D!
Do we believe it! Lets look at an example.
(alot like More Precisely ex., page 447)

14. Luminosity, temperature, size ….
We know relationship between luminosity and magnitude (Table previous slide).
Using Wien’s Law: (peak emission)  1/temperature
And Stefan’s law: total energy emitted  temperture4
Wien’s law: the hotter the object the bluer is its emission.
Stefan’s law: energy emitted per unit area increases as the 4th power of the temperature…..
Luminosity  radius2 * temperture4
Stellar size!

15. More tools from what we know
Knowledge of color/temperature relationship and now, luminosity/radius/tem-perature relationship combined with emission/absorption spectrum we get from certain stars, lets us classify our spectra (OBAFGKM) according to temperature.

16. H-R Diagram Sizes, Temperature, Luminosities And: Stellar Lifetime:
Star life time ~
1/(star mass)3

17. Features: 1. 2. 3. 4.

18. H-R Diagram stellar mass determines lifetime behavior of star
stellar mass determines lifetime behavior of star
With regards to mass, you may want to note
size/masses of stars that spend:
all their lives on the main sequence
some of their lives on the main sequence
leave main sequence early