1. Stars

2. Composed of ~98% H and He
Fusion in the core supports the star
Full spectrum of masses

3. Key Properties Apparent Brightness Luminosity Temperature / Color Mass
Evolutionary State

4. Brightness Absolute brightness Apparent brightness Luminosity
Power emitted by star into space
Only depends on star
Lsun = 4 X 1026 Watts
Apparent brightness
How bright star appears in the night sky
Power per unit area
Depends on star’s brightness and distance

5. Inverse square law for light
Apparent brightness measured in watts per square meter
Drops off as square of distance

6. Measuring Distance Stellar Parallax
Caused by motion of Earth in its yearly orbit
d = 1/p
where p is in arcsecs and d is in parsecs
1 parsec = 3.26 lyrs
All stars have very small paralax angles (<1 arcsec), which is why ancient Greeks were never able to measure it

7. Magnitudes Logarithmic Large values are dim objects
Small values are bright objects

8. Magnitudes Absolute Magnitudes Apparent Magnitudes
A bright a star would appear if it were 10 pc away
Does not depend on distance
How objects appear from here on Earth
Depends on distance
We can only see objects with m≤6

9. Color and Temperature
Color is the difference between intensity in two filters
B-V color is a good proxy for temperature
Color is independent of distance

10. Spectral Type
Spectral types are subdivided for intermediate temperatures
Values run from 0-9
Smaller numbers are hotter
Larger numbers are cooler
Eg. B1 is hotter than B7

11. Spectral Types
Order was alphabetical depending on strength of Hydrogen line
Williamina Flemming
Revised to follow a more natural order
Annie Cannon

12. Measuring Stellar Masses
Using Binary Systems

13. Visual Binaries

14. Eclipsing Binaries

16. Spectroscopic Binaries

17. HR Diagram
Main Sequence
Giants
Supergiants
White Dwarfs

18. HR Diagram Luminosity class gives size and luminosity information
With spectral type and luminosity class, we can completely classify a star

19. Main Sequence Mass is the most important property for a star on the MS
Stars spend 90% of their lives here, burning H in their cores
MS lifetime depends on mass

20. Main Sequence More massive stars live much shorter lives
Burn fuel very quickly to support such a large star
Less massive stars live longer
Less fuel, but burn it more slowly
A 10M(sun) has L of about 10,000L(sun)
10 times the H, burned 10,000 times as quickly, so the star only lives for 1/1,000 the length of the sun

21. Life After the Main Sequence
When stars run out of H in their cores, they evolve off the MS
Giants and Supergiants expand to extremely large sizes
Temperatures are very low
Luminosity is very high
White dwarfs are small and hot
Have no nuclear fusion
Heated by collapse of gas

22. Star Clusters
All stars in the cluster formed about the same distance from Earth
All stars in the cluster formed at about the same time
Very useful in understanding stellar formation and evolution
Can use them as clocks
Most of what we know about stars comes from studying clusters

23. Open Clusters Only a few million years old
Contain lots of luminous blue stars
Contain several thousand stars
~30 lyrs across
Picture is of M36

24. Globular Clusters Often several billion years old
Some of the oldest objects in the galaxy
Contains mostly smaller stars
Around stars concentrated in a relatively small volume
lyrs across
Picture is of M15

25. Age of Cluster
Main Sequence Turnoff (MSTO) – more massive stars have evolved off of the Main Sequence
MSTO gives age of cluster
Lifetime of cluster same as MS lifetime of stars at the MSTO
MSTO

26. Young clusters still have their massive stars on the MS
Old clusters are missing the massive blue stars on the MS