1. Making a Color-Magnitude Diagram for Globular Cluster Omega Centauri Jay Anderson, STScI 1

2. This is the Early-Release Image of Omega Centauri, taken by WFC3/UVIS on board the Hubble Space Telescope (HST) http://hubblesite.org/gallery/wallpaper/pr2009025q/ 2

3. A close up of the central region. 3

4. This image was made by combining separate red, green, and blue images. 4

5. The red image is from filter F814W, which sees only very red light. 5

6. The green image is from filter F336W, which sees only blue light. 6

7. The blue image is from filter F225W, which sees only ultra-violet light. 7

8. The combined image again. 8

9. The colors are so extreme because… 9

10. … the red stars have almost no blue light, and the blue stars have almost no red light. 10

11. The composite image again. 11

12. Astronomers like to study the colors of stars in a quantitative way. 12

13. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13a

14. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13b

15. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13c

16. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13d

17. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13e

18. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13f

19. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13g

20. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13h

21. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13i

22. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13j

23. They like to sort the stars by color, putting the blue stars on the left and the red stars on the right. 13k

24. Note that there are very few extreme stars; most stars are white, meaning they have a balanced spectrum. 14

25. Astronomers also like to characterize the stars in terms of brightness. 14

26. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15a

27. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15b

28. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15c

29. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15d

30. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15e

31. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15f

32. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15g

33. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15h

34. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15i

35. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15j

36. They like to sort the stars, putting the bright stars on top, and the faint stars on the bottom. 15k

37. This is called a Color-Magnitude Diagram (CMD). 16

38. When Astronomers first plotted stars this way, they realized that stars don’t fall just anywhere in the diagram. 17

39. Stars tend to lie along a few well-defined sequences. 18

40. The unmistakable order in diagrams like this led astronomers to develop theories to explain stellar evolution. 19

41. The vast majority of stars lie along the Main Sequence (MS). 20a MS

42. Stars don’t move along this sequence; rather they sit at the same place for a long time fusing their hydrogen into helium. 20b MS

43. This is a sequence in mass. Stars at the bright end are massive, and those at the faint, red end have very low masses. 20c MS

44. The more massive stars consume their hydrogen fuel much faster than the lower-mass stars. 20d MS

45. When fuel becomes sparse in the stellar core, stars readjust their internal structure and move red-ward along the Sub-Giant Branch (SGB). 21 MS SGB

46. They start to burn the hydrogen in a shell around the core and become big and bloated as they move up the Red Giant Branch (RGB). 22a MS SGB RGB

47. As the shell-burning continues, more and more helium gets deposited onto the core. 22b MS SGB RGB

48. When the core has enough mass, it is finally able to ignite helium into carbon. 22c MS SGB RGB

49. The star readjusts its structure again and finds itself on the Horizontal Branch (HB). 23a MS SGB RGB HB

50. The helium fuel is not as potent as the hydrogen, so it runs out quickly. 23b MS SGB RGB HB

51. That is why there are so few stars on the HB compared to the MS. Stars do not spend much time on the HB. 23c MS SGB RGB HB

52. When the helium is gone, the star has no more fuel. With nothing left to burn, it fades away into blue darkness as a White Dwarf (WD). 24 MS SGB RGB HB WD

53. Since a star’s color and brightness tell us its evolutionary phase, we can easily identify stars by phase in the image. 25 MS SGB RGB HB WD