Astro 1050     Fri. Oct. 11, 2002
   Today:  Chapter 8, Properties of Stars
             Please bring calculators to class
                Astronomy New Articles

How to recognize patterns in data
What patterns matter for people – and how do we recognize them?
Weight and Height are easy to measure
Knowing how they are related gives insight into health
A given  weight tends to go with a given height
Weight either very high or very low compared to trend ARE important
Plot weight vs. height and look for deviations from simple line
Example of cars from the book
Note “main sequence” of cars
Weight plotted backwards
Just make main sequence a line
which goes down rather than up
Points off main sequence are
“unusual” cars

Stars:  Patterns of L, T, R
The Hertzsprung-Russsell (H-R) diagram
Plot L vs. backwards T.      (We can find R given L and T)

How are L, T, and R related?
L = area ´sT4 = 4 p R2 sT4
Stars can be intrinsically bright because of either large R or large T
Use ratio equations to simplify above equation
(Note book’s symbol for Sun is circle with dot inside)
Example:  Assume T is different but size is same
A star is ~ 2 ´ as hot as sun, expect L is 24 = 16 times as bright
M star is ~1/2 as hot as sun, expect L is 2-4 = 1/16 as bright
B star is ~  4 ´ as hot as sun, expect L is 44 = 256 times as bright
Example:  Assume T same but size is different
If you have a G star 4 ´ as large as sun, expect L would be 42=16 times as bright

L, T, R, and the H-R diagram
L = 4 p R2 sT4
The main sequence consists very roughly of similar size stars
The giants, supergiants, and white dwarfs are much larger or smaller

Lines of constant R in the H-R diagram

Slide 7

Different “types” of H-R diagrams

Luminosity Classes

Spectra of Different Luminosity Classes

Spectroscopic “Parallax”

What fundamental property of a star
varies along the main sequence?

Masses of Binary stars

Masses of Binary stars

Masses of Binary stars

Measuring a and P of binaries
Two types of binary stars
Visual binaries: See separate stars
a large, P long
Can’t directly measure component of a along line of sight
Spectroscopic binaries:  See Doppler shifts in spectra
a small, P short
Can’t directly measure component of a in plane of sky
If star is visual and spectroscopic binary get get full set of information and then get M

Masses and the HR Diagram
Main Sequence position:
M:    0.5 MSun
G:        1 MSun
B:       40 Msun
Luminosity Class
Must be controlled by something else

The Mass-Luminosity Relationship
L = M3.5

Eclipsing Binary Stars
System seen “edge-on”
Stars pass in front of each other
Brightness drops when either is hidden
Used to measure:
size of stars (relative to orbit)
relative “surface brightness”
area hidden is same for both eclipses
drop bigger when hotter star hidden
tells us system is edge on
useful for spectroscopic binaries