ASTR100 (Spring 2008) Introduction to Astronomy Classifying Stars

1. ASTR100 (Spring 2008) Introduction to AstronomyClassifying Stars Prof. D.C. Richardson Sections 0101-0106

2. What is a Hertzsprung-Russell Diagram?

3. An H-R diagram plots the luminosities and temperatures of stars. Luminosity Temperature

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

6. Large Radius Stars with lower T and higher L must have larger radius R: giants and supergiants. L = 4R2T4

7. Stars with higher T and lower L must have smaller radius R: white dwarfs. L = 4R2T4 Small Radius

8. Giants and Supergiants White Dwarfs

10. Add luminosity class to spectral class: I - supergiant II - bright giant III - giant IV - subgiant V - main sequence Examples: Sun – G2 V Sirius – A1 V Proxima Centauri – M5.5 V Betelgeuse – M2 I

11. H-R diagram depicts: Temperature Color Spectral Type Luminosity Radius Luminosity Temperature

12. C B Which star is the hottest? D Luminosity A Temperature

13. C B Which star is the hottest? A D Luminosity A Temperature

14. C B Which star is the most luminous? D Luminosity A Temperature

15. C B Which star is the most luminous? C D Luminosity A Temperature

16. C B Which star is a main-sequence star? D Luminosity A Temperature

17. C B Which star is a main-sequence star? D D Luminosity A Temperature

18. C B Which star has the largest radius? D Luminosity A Temperature

19. C B Which star has the largest radius? C D Luminosity A Temperature

20. What is the significance of the main sequence?

21. Main-sequence stars are fusing hydrogen into helium in their cores, like the Sun. Luminous main-sequence stars are hot (blue). Less luminous ones are cooler (yellow or red).

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

23. High Mass Low Mass The mass of a normal, hydrogen-burning star determines its luminosity and spectral type!

24. The core pressure and temperature of a higher-mass star need to be higher in order to balance gravity. A higher core temperature boosts the fusion rate, leading to higher luminosity.

25. Mass & Lifetime Until core hydrogen (10% of total) is used up. Sun’s life expectancy: 10 billion years. Life expectancy of 10 MSun star: 10 times as much fuel, uses it 104 times as fast.  10 million years.

26. Mass & Lifetime Until core hydrogen (10% of total) is used up. Sun’s life expectancy: 10 billion years. Life expectancy of 10 MSun star: 10 times as much fuel, uses it 104 times as fast.  10 million years. Life expectancy of 0.1 MSun star: 0.1 times as much fuel, uses it 0.01 times as fast.  100 billion years.

27. Main-sequence Star Summary • High mass: • High luminosity • Short-lived • Large radius • Blue • Low mass: • Low luminosity • Long-lived • Small radius • Red

28. Concept Check • Two stars have the same surface temperature but different luminosities. How can that be? • Answer: one is bigger than the other! • Why? • Thermal radiation law: objects at a given temperature emit a certain luminosity per unit surface area. • Hence the more luminous star has a larger surface area, and so a larger radius.

29. What are giants, supergiants, and white dwarfs?

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

32. Main-sequence stars (to scale) Giants, supergiants, white dwarfs

33. A Which star is most like our Sun? D Luminosity B C Temperature

34. A Which star is most like our Sun? D B Luminosity B C Temperature

35. A Which star will have changed the least 10 billion years from now? D Luminosity B C Temperature

36. A Which star will have changed the least 10 billion years from now? D Luminosity B C C Temperature

37. A Which star can be no more than 10 million years old? D Luminosity B C Temperature

38. A Which star can be no more than 10 million years old? D Luminosity A B C Temperature

39. What are the two types of star clusters?

40. Open cluster: A few thousand loosely packed stars.

41. Globular cluster: Up to a million or more stars in a dense ball bound together by gravity.

42. How do we measure the age of a star cluster?

43. Massive blue stars die first, followed by white, yellow, orange, and red stars.

44. The Pleiades cluster now has no stars with life expectancy less than around 100 million years. Main-sequence turnoff

45. The main-sequence turnoff point of a cluster tells us its age.

46. To determine accurate ages, we compare models of stellar evolution to the cluster data.

47. Detailed modeling of the oldest globular clusters reveals that they are about 13 billion years old…

48. Surprise Quiz!! (10 points) • Take out a piece of paper, print your name and section number on it. • Sketch an H-R diagram… • Label the temperature & luminosity axes. • Sketch the main sequence. • Plot a point representing the Sun. • Plot a main-sequence B star. • Plot a main-sequence M star. • Indicate where giants & supergiants are found. • Indicate where white dwarfs are found.

49. B Star Giant and Supergiants Here’s what your sketch should look like! Sun (G2 V) White Dwarfs M Star