Stars

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 • 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

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

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

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

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

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

24. Globular Clusters • Often several billion years old • Some of the oldest objects in the galaxy • Contains mostly smaller stars • Around 105-106 stars concentrated in a relatively small volume • 50-150 lyrs across

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