1. A105 Stars and Galaxies  Homework due today  Remote observing this weekend Today’s APODAPOD

2. Announcements… Remote Obs Oct. 14 @ 10 PM and Oct. 16 @ 7 AM Take the Swain West elevator to the 3 rd floor and follow the signs to SW 403

3. The HR Diagram Giants and Supergiants White Dwarf Main Sequence

4. Temperature Luminosity An H-R diagram plots the luminosity and temperature of stars

5. Most stars fall somewhere on the main sequence of the H-R diagram WHY WHY WHY ???

6. Mass measurements of main- sequence stars show that the hot, blue stars are much more massive than the cool, red ones High-mass stars Low-mass stars

7. Binary Stars We can determine the masses of stars from their motions in binary star systems

8. A Visual Binary - Mizar Mizar is the middle star in the handle of the Big Dipper 88 light years from Earth Both stars are themselves binaries!!

9. Visual Binary We can directly observe the orbital motions of these stars

10. Eclipsing Binary We can measure periodic eclipses

11. Binary Simulations http://www.astro.ubc.ca/~scharein/a311/Sim.html Playing with Binary Stars!

12. Spectroscopic Binary We determine the orbit by measuring Doppler shifts

13. Main-sequence stars are fusing hydrogen into helium in their cores like the Sun massive main- sequence stars are hot (blue) and luminous Less massive stars are cooler (yellow or red) and fainter

14. The mass of a normal, hydrogen- burning star determines its luminosity and temperature! High-mass stars Low-mass stars

15. Mass & Lifetime Sun’s life expectancy: 10 billion years Life expectancy of 10 M Sun star: 10 times as much fuel, uses it 10 4 times as fast 10 million years ~ 10 billion years x 10 / 10 4 Life expectancy of 0.1 M Sun star: 0.1 times as much fuel, uses it 0.01 times as fast 100 billion years ~ 10 billion years x 0.1 / 0.01

16. Main-Sequence Star Summary High Mass: High Luminosity Short-Lived Large Radius Blue Low Mass: Low Luminosity Long-Lived Small Radius Red

17. Off the Main Sequence Stellar properties depend on both mass and age: those that have finished fusing H to He in their cores are no longer on the main sequence All stars become larger and redder (and cooler) after exhausting their core hydrogen: giants and supergiants Most stars eventually end up small and hot after fusion has ceased: white dwarfs

18. Star Clusters

19. Goals: –Understand how we learn about stellar evolution from the properties of stars in clusters –Understand how we can determine the distances of star clusters –Understand how we can determine the ages of star clusters

20. “Globular" Clusters and “Open" Clusters Globular Clusters 10 4 -10 6 stars old! compact balls of stars high star density Open Clusters 10-10 4 stars generally young loose low star density

21. Properties of Stars in Clusters Formed at the same time Stars are the same age All stars have the same composition The stars are held in a group by their common gravity

22. Constructing an HR Diagram

23. What’s this B-V color? Astronomers measure the brightness of stars in different colors –Brightness measured in blue light is called “B” (for “Blue”) –Brightness measured in yellow light is called “V” (for “Visual) Astronomers quantify the “color” of a star by using the difference in brightness between the brightness in the B and V spectral regions The B-V color is related to the slope of the spectrum

24. The slope of the spectrum is different at different temperatures

25. Cluster HR Diagrams Hotter stars are brighter in blue light than in yellow light, and have low values of B-V color, and are found on the left side of the diagram. Cooler stars are brighter in yellow light than in blue light, have larger values of B-V color, and are found on the right side of the diagram. hotter cooler

26. Distances to Star Clusters The Sun has a “B-V” color of about 0.6. What would the apparent magnitude of the Sun be at the distance of the cluster Messier 6? Stars in Messier 6 with B-V colors of 0.6 have similar mass and luminosity to the Sun hotter cooler Stars like the SUN

27. Ages of Star Clusters The “bluest” stars left on the main sequence of the cluster tell us the cluster’s age. As the cluster ages, the bluest stars run out of hydrogen for fusion and lose their “shine” hotter cooler

28. The HR diagrams of clusters of different ages look very different

29. Main Sequence Turnoffs of Star Clusters Burbidge and Sandage 1958, Astrophysical Journal Here we see a series of HR diagrams for sequentially older star clusters that have been superimposed

30. Ages of Star Clusters ClusterTurnoff ColorAge NGC 7520.351.1 billion years M 670.452.5 billion years Hyades0.15800 million years Pleiades-0.15100 million years M 34-0.10180 million years Jewelbox-0.2516 million years Thinking Question: Why has a cluster with a turnoff color of B-V=1.0 never been discovered?

31.  Stellar Evolution – 60, 61, 62, 64  News Quiz on Tuesday  Homework Due EACH THURS.