1. The Birth & Death of Stars Part 1 Are Stars similar to our Sun? How far away are they? What do they do? Astronomy 12

2. The Birth & Death of Stars Part 1 Learning Outcomes (Students will…): Describe and apply classification systems and nomenclature used in the classification of starsDescribe and apply classification systems and nomenclature used in the classification of stars Use parallax to determine distanceUse parallax to determine distance Calculate luminosityCalculate luminosity Describe nuclear fusion (what stars “do”)Describe nuclear fusion (what stars “do”)

3. Stars – What are they? Stars are great balls of hot gas held together by their own gravity. If stars were not so hot, the gas would collapse into a small dense body.

4. Life on Earth depends on the Sun. Nearly all of our energy comes from the Sun, even coal and oil. This could be true for other extrasolar planets! Stars – Why are they important?

5. Near the center of the Sun nuclear fusion generates energy and this energy flows outward towards the surface where it is radiated into space as heat and light and other forms of electromagnetic radiation. Stars – What do they do?

6. Nuclear Fusion and the Sun Stars create energy through nuclear fusion. In this process, 2 nuclei (mostly hydrogen) combine to produce a single, larger nucleus which creates energy. In large enough quantities, nuclear fusion can be used as an energy source. To get large enough quantities, atoms must be heated to 1.5 x 10 7 K (the heat of the Sun)! 1.5 x 10 7 K = 15,000,000 K OR 14,999,726°C

7. Nuclear Fusion The Sun radiates energy at the rate of 3.9x10 26 W (watts) and has been doing so for several billion years. The fusion reaction in the Sun is a multistep process in which (typically) hydrogen is burned into helium. Hydrogen is the “fuel” Helium is the “product”

8. Nuclear Fusion 2 protons ( 1 H) combine to form a deuteron ( 2 H), a positron (e + ) and a neutrino (v). 1 H + 1 H  2 H + e + + v The positron quickly reacts with an electron (e - ) and both particles annihilate, their mass energy appearing as two gamma-ray photons (γ). e + + e -  γ + γ

9. Nuclear Fusion The deuteron ( 2 H) collides with another proton ( 1 H) and forms a 3 He nucleus and a gamma ray (γ). 2 H + 1 H  3 He + γ Two 3 He nuclei may eventually (within ten thousand years) find each other. 3 He + 3 He  4 He + 1 H + 1 H

11. The Sun’s Nuclear Energy – When will it run out??? The Sun has been creating nuclear energy for about 5 billion years. There is enough Hydrogen left to continue this process for another 5 billion years.

12. What about nuclear fusion that creates other atoms??? If the core temperature increases to about 1 x 10 8 K then energy can be produced by burning helium to make carbon. As a star evolves and becomes hotter than 1 x 10 8 K, other elements can be formed by other fusion reactions. This only works for elements up to the iron-56. At this point, more energy would be consumed than produced.

13. Nuclear Fusion Review Questions 1) What is the fuel for nuclear fusions (in most cases)? 2) What is the product? 3) What is the largest atom that can be used in nuclear fusion? Why? 4) What is the overall equation for nuclear fusion? 5) How long will the Sun’s life be in total?

14. 1) Hydrogen 2) Helium 3) Iron- 56 because at this point, more energy would be consumed than produced. 4) ** See slide 10 5) 10 billion years

15. There are lots of stars in the universe…and a lot of different stars!

16. Panorama view of the sky

18. Classifying Stars: The Four Basic Characteristics of a Star Luminosity Luminosity Size Size Mass Mass Surface Temperature Surface Temperature parallax! To discover these characteristics, distance to a star must be determined…use parallax!

19. The Geometry of Parallax d (in Parsecs) = 1 (AU) p (in arcseconds) p 1 parsec (pc) = 3.26 ly

20. http://www.astronomynotes.com/starpr op/s2.htm

21. Parallax from a Different Planet If we lived on Mars, orbiting 1.5 times farther away from the Sun, the parallax would be 1. the same as from Earth 2. 1.5 times smaller than from Earth 3. 1.5 times bigger than from Earth

22. Journal Formula Symbols d = distance in parsecs p = angle in arcseconds Units pc = 1 parsec = 3.26 light years (ly) 1 light year = the distance that light travels in a vacuum in one Julian year (365.25 days) = 9,460,730,472,580.8 km

23. Suppose a star has a parallax of 0.01 arc seconds. How many parsecs away is it? distance (in parsecs) = 1 / p Answer: 100 parsecs Journal: Example 1 d = 1 (AU) 0.01

24. Suppose a star has a distance of 12 parsecs. How many arcseconds is its angle? Answer: 0.083333 arcseconds Journal: Example 2

25. Category 1 LUMINOSITY

26. Apparent or Visual Brightness (m)…Brighter objects have smaller magnitudes than fainter objects. Apparent or Visual Brightness (m)…Brighter objects have smaller magnitudes than fainter objects.

27. Luminosity Luminosity (absolute brightness) depends on distance and apparent brightness Luminosity is the amount of electromagnetic energy a body radiates per unit of time.

29. Luminosity Luminosity depends on: 1) size of the star (bigger = more luminous!) 2) distance to the star (closer = more luminous!) 3) intervening matter (dust and gas can absorb light – reduces luminosity and increases redness)

30. Calculating Luminosity If we know the distance to the star we can measure luminosity: L = 4πfd 2 where… the distance d to the star (m), the Flux f of the star (W/m 2 ) where flux measures light intensity the luminosity L of the star (Watts) 1 Parsec = 3.08568025 × 10 16 meters

31. Example 1: Finding Luminosity What is the luminosity of our Sun which has a flux of 1360 W/m 2 and a distance of 4.84813×10 -6 parsecs? Step 1: Change distance to m

32. Step 2: L = 4πfd 2 L = 4π (1360)(1.495977899 x 10 11 ) 2 L = 3.82 x 10 26 Watts

33. Luminosity By definition (using more accurate measurements): Lsun = 3.9 x 10 26 W However, we can measure astronomical luminosity in Solar luminosity units, where Lsun = 1 Solar luminosity unit Solar luminosity units = L© (this should be a circle with a dot!)

34. Example 2: Changing into Solar Luminosity Units If a star has a luminosity of 1.3 x 10 10 Watts, what is its luminosity in Solar luminosity units?

35. There is a Big Range of Stellar Luminosities Out there! Star Luminosity (in units of solar Luminosity) Sun1 Proxima Centauri 0.0006 Rigel (Orion) 70,000 Deneb (Cygnus) 170,000

36. Example 3: Finding Distance from Luminosity You have a 100 W lightbulb in your laboratory. Standing at a distance of d from the lightbulb, you measure flux of the lightbulb to be 0.1 W/m 2. How can you use this information to determine the distance from you to the lightbulb?

37. 1) Rearrange 2) Sub in values 3) Solve for d

38. Example 4: Finding Flux Find the flux of the star Arcturus if it is 11.25 pc away from Earth and its Solar luminosity is 114. Step 1: Change luminosity into Watts 114 Solar luminosity units x ( 3.9x10 26 Watts ) 1 Solar luminosity unit 4.446 x 10 28 Step 2: Change distance into metres 11.25 pc x (3.0856802 x 10 16 m) 1 pc 3.47 x 10 17 m

39. Example 4: Finding Flux Find the flux of the star Arcturus if it is 11.25 pc away from Earth and its Solar luminosity is 114.

40. Step 3: Rearrange equation, sub in values, solve for f

41. Luminosity The luminosity of stars ranges from 0.01 Lo to 1 x 10 6 Lo. The Sun is not nearly as bright as the most luminous stars but is brighter than most stars. The Sun is gradually becoming more luminous (about 10% every 1 billion years). The Sun used to be fainter in the past, which is possibly the reason life on Earth has only existed for about 1 billion years on land.

42. Category 2 SIZE!

43. Stellar Size Stars are very spherical so we characterize a star’s size by its radius. Stars are very spherical so we characterize a star’s size by its radius. We always compare to our Sun! We always compare to our Sun! R Stellar Radii vary in size from ~1500 R Sun for a large Red Giant to 0.008 R Sun for a White Dwarf.

44. Size Equation L = 4πR 2 s T 4, Where L is the luminosity in Watts R is the radius in meters s is the Stefan-Boltzmann constant T is the star's surface temperature in Kelvin NOTE: R is often given in Solar radii (radius of Sun)

45. Example 1 Find the Luminosity of Zeta Puppis which has a temperature of about 38,000 K and a radius of 18 Ro. Then change this into Solar luminosity units. Step 1: Change units R = 18 x (6.955 x 10 8 m) = 1.2519 x 10 10 m Step 2: Plug into equation L = 4πR 2 s T 4 L = 4π(1.2519 x 10 10 ) 2 (5.67 x 10 -8 ) (25000) 4 L = 2.328 x 10 32 W L = 597,038 Lo

46. Example 2 Find the radius of a new star, SBUTLER, that has a luminosity of 5Lo and a temperature of 25,000K. Step 1: Change units 5Lo x (3.9 x 10 26 Watts/1Lo) = 1.95 x 10 27 Step 2: Rearrange equation L = 4πR 2 s T 4

47. Category 3 MASS!

48. Mass Stars fall into a narrow mass range because below 0.08 Solar masses, nuclear reactions cannot be sustained AND greater than 100 Solar masses stars are unstable. High mass stars burn their fuel at higher rates and live shorter lives than low-mass stars.

49. How do you weigh a star? By observing the star and anything that orbits it (maybe even another star) By observing the star and anything that orbits it (maybe even another star) Use Kepler’s Laws of Planetary Motion and Newton’s Law of Gravitation to determine mass Use Kepler’s Laws of Planetary Motion and Newton’s Law of Gravitation to determine mass

50. Formula – A “metric” version of Kepler’s Law! Where: M = mass of star in kg N = mass of planet in kg a =distance from star to planet in m (average) G = gravitational constant (6.673 × 10 -11 m 3 kg -1 s -2 ) P = period of the orbit in seconds

51. What do I have to know? How to change from a mass in kg to a mass in Solar mass units Change 15 Mo into kg. Change 1.52 x 10 35 kg into Mo.

52. Category 4 SURFACE TEMPERATURE

53. The Sun Although nuclear fusion needs heat of 1.5 x 10 7 K (core), the surface temperature of the Sun is about 5,700 K. The temperature of stars are between 3,000K to 30,000 K.

54. How do you measure the surface temperature of a star? Every element (when heated) will emit lines that lie along the visible light spectrum a.k.a. R-O-Y-G-B-I-V Every element (when heated) will emit lines that lie along the visible light spectrum a.k.a. R-O-Y-G-B-I-V Stars are mostly composed of hydrogen (and some helium)

55. Spectral Types of Stars Spectral types are defined by the: Spectral types are defined by the: existence of spectral lines belonging to various elements, ions, & molecules in a star’s spectrum existence of spectral lines belonging to various elements, ions, & molecules in a star’s spectrum the relative strengths of these lines the relative strengths of these lines However, spectral type is not determined by a star’s composition. However, spectral type is not determined by a star’s composition. all stars are made primarily of Hydrogen & Helium all stars are made primarily of Hydrogen & Helium

56. Reason for Spectral Types Spectral type is determined by a star’s surface temperature. Spectral type is determined by a star’s surface temperature. Astronomers use Kelvin (K) as temperature unit… 272 K = 0 o C 298 K = 30 o C 1000 K = 727 ºC

57. Spectral Type Classification System O B A F G K M Oh Be A Fine Girl/Guy, Kiss Me! 50,000 K 3,000 K Temperature

58. Stellar Classification

59. Analogy Make a plot that shows the general relationship between height and weight for humans. - now add to your plot the population of basketball players who are very tall and very thin. - now add the population of obese children The plot would show a cluster of people that would have similar “middle-of-the-road” height/weight ratios It would also show a smaller cluster of “very tall and very thin” AND a smaller cluster of “very short and very fat”

60. The plot would show a cluster of people that would have similar “middle-of-the-road” height/weight ratios It would also show a smaller cluster of “very tall and very thin” AND a smaller cluster of “very short and very fat” Weight Height Most people in the world Very Tall, Very Thin Very Short, Very Fat ? ?

61. How can we classify stars 1)Collect information on a large sample of stars. 2)Measure their luminosities 3)Measure their surface temperatures (need their spectra)

62. The Hertzsprung-Russell Diagram

63. HOTCOOL BRIGHT FAINT

64. The Hertzsprung-Russell Diagram The Main Sequence ~90% of all stars are in the main sequence (MS) ~90% of all MS stars are cooler spectral types than the Sun (i.e., at the lower MS) All MS stars fuse H into He in their cores.

65. The Hertzsprung-Russell Diagram Mass-Luminosity Relation: L  M 3.5 For example, if the mass of a star is doubled, its luminosity increases by a factor 2 3.5 ~ 11. Mass of MS Star L  M 3.5 The relation is for main sequence stars only!

66. The Hertzsprung-Russell Diagram Red Giants - Red Giant stars are very large, cool and quite bright. e.g., Betelgeuse is 100,000 times more luminous than the Sun but is only 3,500K on the surface. It’s radius is 1,000 times that of the Sun.

67. The Hertzsprung-Russell Diagram Supergiants Very bright Very hot Size of Star:

68. The Hertzsprung-Russell Diagram White Dwarfs - White Dwarfs are hot, but since they are so small, they are not very luminous.

69. Main Sequence Lifetime All M-S stars have temperatures sufficient to fuse H into He in their cores. All M-S stars have temperatures sufficient to fuse H into He in their cores. Luminosity depends directly on mass: Luminosity depends directly on mass: more mass = more pressure from upper layers more mass = more pressure from upper layers fusion rates must be high to maintain equilibrium fusion rates must be high to maintain equilibrium Lifetime  (Amt of Fuel)/(Rate of Burning)  M / L  M / M 3.5  1 / M 2.5 Lifetime  (Amt of Fuel)/(Rate of Burning)  M / L  M / M 3.5  1 / M 2.5 Higher mass stars have shorter lives! Higher mass stars have shorter lives!

70. The Hertzsprung-Russell Diagram Lifetime of Star Shorter Longer More mass, more fuel, very fast burning. Less mass, less fuel, slow, steady burning. Think SUV vs Honda Civic

71. Mass (M Sun ) Luminosity (L Sun ) Surface Temperature (K) Radius ( R Sun ) Main sequence lifespan (yrs) 0.10 3×10 -3 2,9000.16 2×10 12 0.500.033,8000.6 2×10 11 0.750.35,0000.8 3×10 10 1.016,0001.0 1×10 10 1.557,0001.4 2×10 9 36011,0002.5 2×10 8 560017,0003.8 7×10 7 1010,00022,0005.6 2×10 7 1517,00028,0006.8 1×10 7 2580,00035,0008.7 7×10 6 60790,00044,50015 3.4×10 6 As mass of a M-S star increases, luminosity, surface temperature and radius (size) increaseAs mass of a M-S star increases, luminosity, surface temperature and radius (size) increase As mass of a M-S star increases, life span on M-S decreasesAs mass of a M-S star increases, life span on M-S decreases

72. Review Questions: The H-R Diagram 1. Where are most stars? 2. What is the common characteristics of MS stars? 3. What determines the location of a star in the MS? 4. Where do you find the largest stars? 5. The smallest? 6. The most massive one? 7. The coolest stars? 8. How do we know the age of a star? 1. MS, 2. H  He, 3. M, 4. upperright, 5. lowerleft, 6. upperleft, 7. lowerright, 8. normally we don’t