1. Lecture 21 Stellar Evolution

2. Announcements Homework 11 due now Homework 11 due now Homework 12 – Due Monday April 30 Homework 12 – Due Monday April 30 –Unit 62: RQ1, P3, TY1 –Unit 64: RQ1, 3, TY1 –Unit 65: RQ3, TY1, 2 –Unit 67: RQ5, P2, TY1

3. Pressure vs. Gravity

4. The Pressure- Temperature Thermostat Hotter temperatures: Hotter temperatures: –Atomic nuclei move faster. –They “touch” more frequently. –More fusion reactions each second. –More energy released. –HIGHER PRESSURES PUSH OUTWARD. Cooler temperatures: Cooler temperatures: –Atomic nuclei move more slowly. –Less fusion. –PRESSURE GOES DOWN.

5. The Main Sequence More Mass means: More Mass means: –More gravity, so the star weighs more. –The star needs to create more internal pressure to support its weight. What Creates More Internal Pressure? What Creates More Internal Pressure? –More nuclear fusion! –More fusion = more heat and light! So more massive stars are brighter and hotter. So more massive stars are brighter and hotter.

6. Recall the H-R Diagram -5 -3 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500 Relates temperature vs. luminosity Stars are found only in certain regions of the diagram.

7. The Main Sequence -5 -3 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500 Most (90%) of stars are found in this band that runs diagonally from the upper left to the lower right. This band is called the main sequence.

8. The Main Sequence -5 -3 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500 These stars are brighter than main sequence stars of the same temperature, so they must be larger. They are called giants (or red giants, because many are cool, red stars).

9. The Main Sequence -5 -3 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500 These stars are brighter (and therefore larger) than even giants of the same temperature. They’re called supergiants.

10. The Main Sequence -5 -3 1 3 5 7 9 40,000 20,000 10,000 5,000 2,500 Finally, these stars are hot (white and blue-white), but very dim, so they must be very small. They are called white dwarfs.

11. But What Are These Types of Stars? The Main Sequence are normal stars. The Main Sequence are normal stars. They burn hydrogen to helium in their cores. They burn hydrogen to helium in their cores. They have a relationship between mass, temperature, and luminosity: They have a relationship between mass, temperature, and luminosity: –More massive = bigger (higher R) –More massive = hotter (higher T) –More massive = brighter (higher L)

12. But What Are These Types of Stars? The (Red) Giants are very old stars nearing the end of their life. The (Red) Giants are very old stars nearing the end of their life. They have run out of hydrogen fuel in their cores. They have run out of hydrogen fuel in their cores. They are low and intermediate mass (typically less than 8 times the sun’s mass). They are low and intermediate mass (typically less than 8 times the sun’s mass). They are large (R = 10 to 100) and bright (L = 100 to 10,000) They are large (R = 10 to 100) and bright (L = 100 to 10,000)

13. One Day The Sun Will Become A Red Giant!

14. Supergiants The supergiants are also stars near the end of their lives. The supergiants are also stars near the end of their lives. They have run out of hydrogen fuel in their cores. They have run out of hydrogen fuel in their cores. They are high mass (more than 8 solar masses). They are high mass (more than 8 solar masses). They are very large (R = 100 to 1,000) and extremely bright (L = 10,000 to 1,000,000). They are very large (R = 100 to 1,000) and extremely bright (L = 10,000 to 1,000,000).

15. The White Dwarfs White dwarfs are what low and intermediate mass stars become when they die. White dwarfs are what low and intermediate mass stars become when they die. They are tiny, hot, collapsed stars that have totally run out of nuclear fuel. They are tiny, hot, collapsed stars that have totally run out of nuclear fuel.

16. What Makes The Stars Shine? Stars shine because of NUCLEAR FUSION. Stars shine because of NUCLEAR FUSION. Atomic nuclei all have positive electric charge. Atomic nuclei all have positive electric charge. Two nuclei repel each other with a very powerful force. Two nuclei repel each other with a very powerful force. The bigger the nuclei, the more powerful the repulsive force. The bigger the nuclei, the more powerful the repulsive force.

17. What Makes The Stars Shine? Normally this force keeps nuclei from getting close together. Normally this force keeps nuclei from getting close together. At very high temperatures, the nuclei move so quickly that they can “touch” (sort of) before the repulsive force can push them apart. At very high temperatures, the nuclei move so quickly that they can “touch” (sort of) before the repulsive force can push them apart.

18. What Makes The Stars Shine? The nuclei then “fuse” into a single nucleus. The nuclei then “fuse” into a single nucleus. IF the two nuclei were lighter than iron: IF the two nuclei were lighter than iron: –The new nucleus is just a bit lighter than both old nuclei together. –The “missing” mass is converted into energy: heat and light. IF the two nuclei were as heavy/heavier than iron: IF the two nuclei were as heavy/heavier than iron: –The new nucleus is just a bit heavier than both old nuclei together. –Heat energy is converted into some extra mass.

19. Nuclear Fusion So fusing elements lighter than iron gives off energy. So fusing elements lighter than iron gives off energy. Fusing elements heavier than iron takes in energy. Fusing elements heavier than iron takes in energy.

20. Some Like It Hot… Fusion produces a LOT of energy, but things need to be VERY hot and under very high pressure to get it started. Fusion produces a LOT of energy, but things need to be VERY hot and under very high pressure to get it started. Heavier nuclei than hydrogen require even HIGHER temperatures and pressures to start fusion! Heavier nuclei than hydrogen require even HIGHER temperatures and pressures to start fusion!

21. The Fuel Of The Stars Nearly all stars fuse hydrogen into helium. Nearly all stars fuse hydrogen into helium. BUT at high enough temperatures, a star can fuse heavier elements as fuel: BUT at high enough temperatures, a star can fuse heavier elements as fuel: –Helium into Carbon, Nitrogen, and Oxygen. –Oxygen into Magnesium and Neon. –And others….

22. The Mass-Luminosity Relation There is a definite relationship between a main sequence star’s mass and it’s luminosity: There is a definite relationship between a main sequence star’s mass and it’s luminosity: L = M 3.5 M=0.2 L=0.0036 M=1 L=1 M=3.2 L=58.6

23. The Mass-Luminosity Relation The Mass-Luminosity relation can be used to figure out how long before a star begins to run out of hydrogen fuel. The Mass-Luminosity relation can be used to figure out how long before a star begins to run out of hydrogen fuel. –A star’s mass determines how much hydrogen fuel it has to burn (more matter = more fuel) –A star’s luminosity determines how fast the hydrogen fuel is burned (more luminous = burning fuel faster)

24. Stars = Cars? A low-mass star is like an economy car: A low-mass star is like an economy car: –Small fuel tank –Poor performance (low energy output) –Excellent “gas mileage.” Low mass stars burn their fuel very, very slowly. Low mass stars burn their fuel very, very slowly. So they last a very long time. So they last a very long time.

25. Stars = Cars? A high-mass star is like a sports car: A high-mass star is like a sports car: –Larger fuel tank –High performance (high energy output) –Poor “gas mileage.” High mass stars burn their fuel very, very quickly. High mass stars burn their fuel very, very quickly. So they run out of fuel very quickly. So they run out of fuel very quickly.

26. Live Fast, Die Young Lifetime of a main- sequence star: t = 1/M 2.5 × 10 billion (years) …gives the length of time before the star runs out of hydrogen fuel.

27. Something’s Wrong… But let’s do a quick test… But let’s do a quick test… The sun’s mass is 2×10 30 kg. About 75% of that mass is hydrogen. The sun’s mass is 2×10 30 kg. About 75% of that mass is hydrogen. The sun converts about 6×10 11 kg of hydrogen into helium every second. The sun converts about 6×10 11 kg of hydrogen into helium every second.

28. Something’s Wrong… So the sun has 0.75 × (2 × 10 30 ) = 1.5×10 30 kg of hydrogen available. Every second it uses 6×10 11 kg of that hydrogen. So the sun should be able to fuse hydrogen into helium for: 1.5×10 30 / 6×10 11 = 2.5×10 18 s That’s equal to 80 billion years.

29. What Gives? So why will the sun run out of fuel in only 10 billion years? So why will the sun run out of fuel in only 10 billion years? Has to do with the structure of the sun! Has to do with the structure of the sun!

30. The Sun’s Fuel Tank Remember the sun’s inside is divided into layers… Convection Zone Radiative Zone Core

31. The Sun’s Fuel Tank Only the CORE has temperatures and pressures high enough for hydrogen to helium fusion! So once the core runs out of hydrogen, the sun has effectively run out of fuel!

32. Different On The Inside Here’s the inside of the sun again. Other main sequence stars look different on the inside. Convection Zone – the gas here is “churning” or “boiling” Radiative Zone – the gas doesn’t “flow” here – light and heat radiate through this layer. Core – where fusion is happening

33. Different On The Inside High mass stars have radiative outsides and convective insides. Convection Zone – the gas here is “churning” or “boiling” Radiative Zone – the gas doesn’t “flow” here – light and heat radiate through this layer. Core – where fusion is happening

34. Different On The Inside Very low mass stars are convective all the way through. Convection Zone – the gas here is “churning” or “boiling” Core – where fusion is happening

35. The Life And Times Of A Star Here’s a diagram of a main sequence star near the beginning of its life. Hydrogen is being fused into helium in the star’s core.

36. The Life And Times Of A Star Helium is heavier than hydrogen, so it sinks down to the very center of the star and collects.

37. The Life And Times Of A Star The star’s core isn’t hot enough to fuse helium. So the tiny little knot of helium starts to collapse under its own weight (no fusion to provide internal pressure)

38. The Life And Times Of A Star The little knot of helium heats up a little as it collapses.

39. The Life And Times Of A Star And this extra heat flows out into the core, heating up the core (just a little bit).

40. The Life And Times Of A Star Because the core is just a little bit hotter, it fuses hydrogen to helium just a little bit faster. Two things happen. 1.Helium “ash” is produced a little faster than before. 2.The extra heat from the faster fusion flows out into the rest of the star.

41. The Life And Times Of A Star The extra internal pressure from the extra heat makes the star swell up (just a tiny little bit) and cool off (just a tiny little bit). Why cool off? Remember when you expand a gas (swell up), you cause it to cool off a bit.

42. The Life And Times Of A Star This process is very slow. Over time, the helium core gets bigger. The star also gets bigger and cooler, BUT: The core is hotter, so there is more fusion, so the star is also getting brighter.

43. The Life And Times Of A Star For a very, very long time this process is very, very slow. Over the last 5 BILLION years, the sun has doubled in brightness and increased in size by perhaps 10%.

44. The Life And Times Of A Star Over the next 5 BILLION years the sun will again double in brightness, and continue to slowly grow in size (and get cooler).

45. Main Sequence Evolution So as they age, main sequence stars: So as they age, main sequence stars: –Very slowly get brighter. –Very slowly get bigger. –Very slowly get cooler (a bit redder). Eventually (after about 10 billion years for our sun), the helium core will begin to take up most of the sun’s core. Eventually (after about 10 billion years for our sun), the helium core will begin to take up most of the sun’s core.

46. The Life And Times Of A Star Eventually (after about 10 billion years for our sun), the helium core will begin to take up most of the sun’s core. The sun’s core will be much hotter than when it was first born. Hydrogen is now burning in a thick shell around a “dead” helium core. And it’s burning very, very fast !

47. The Life And Times Of A Star The hydrogen fusion is happening very, very fast now, so a LOT of waste helium is getting produced and dumped onto the dead helium core. The hydrogen fusion is happening very, very fast now, so a LOT of waste helium is getting produced and dumped onto the dead helium core. The dead helium core is now very big and massive, but still isn’t producing energy using nuclear fusion, so it continues to collapse under its own weight. The dead helium core is now very big and massive, but still isn’t producing energy using nuclear fusion, so it continues to collapse under its own weight.

48. The Life And Times Of A Star As the helium core collapses it produces enormous amounts of heat. The fusion becomes even faster in the hydrogen shell. Also areas once too cool for fusion are now hot enough to begin to convert hydrogen into helium.

49. The Life And Times Of A Star The tremendous heat produced floods into the star’s outer layers. The outer layers continue to swell up and cool off. This is like what was happening when the star was main sequence, but it’s happening MUCH, MUCH faster. The star is swelling into a RED GIANT!

50. The End Of The Main Sequence The star’s core is now full of helium ash collapsing under it’s own weight. The star’s core is now full of helium ash collapsing under it’s own weight. The star has run out of hydrogen fuel in its core. The star has run out of hydrogen fuel in its core. The hydrogen fusion is now happening in a thin shell surrounding the “dead” core. The hydrogen fusion is now happening in a thin shell surrounding the “dead” core. All the extra energy has swelled the stars outer layers a great deal, and caused the star to cool off. All the extra energy has swelled the stars outer layers a great deal, and caused the star to cool off. It has left the main sequence and become a red giant. It has left the main sequence and become a red giant.

51. The Red Giant Phase The Power Source: Hydrogen-burning shell slowing burning out from the core. Waste helium is dumped onto core.

52. The Red Giant Phase The Helium Core: Getting heavier (more massive) with time. Slowly shrinking. Heating up as it shrinks. Extra heat is speeding up fusion in the surrounding hydrogen shell.

53. The Red Giant Phase Outer Layers (Envelope): Slowly expanding away from the core. Star is getting larger with time. Getting cooler as they expand. Star is getting redder.

54. The Red Giant Phase So as the red giant ages: So as the red giant ages: –It gets bigger. –It gets brighter. –It gets cooler and redder. Similar to a main sequence star, but it’s happening much, much faster! Similar to a main sequence star, but it’s happening much, much faster!

55. About How Big Will Our Sun Get? Here’s the sun today … Here’s the sun today …

56. About How Big Will Our Sun Get? A few million years after leaving the main sequence, the sun will be much bigger and brighter… A few million years after leaving the main sequence, the sun will be much bigger and brighter… L = 50 T = 3,700 K R = 17

57. About How Big Will Our Sun Get? For the next few million years, the sun continues to swell and become brighter… For the next few million years, the sun continues to swell and become brighter… L = 2,100 T = 2,900 K R = 183

58. How Big Will The Sun Get? This is large enough to swallow Mercury and Venus, but the Earth is spared. This is large enough to swallow Mercury and Venus, but the Earth is spared.

59. Red Giants Lose Mass The sun’s mass hasn’t gone up. So sun’s surface gravity is now over 10,000 times weaker (because the surface is over 100 times farther from the center). The sun’s mass hasn’t gone up. So sun’s surface gravity is now over 10,000 times weaker (because the surface is over 100 times farther from the center). The escape velocity from the sun’s surface is reduced by 10 times. The escape velocity from the sun’s surface is reduced by 10 times. The fastest moving gas atoms at the sun’s surface can now easily escape into space. The fastest moving gas atoms at the sun’s surface can now easily escape into space.

60. Red Giants Lose Mass THE RESULT: The solar wind becomes much more intense. THE RESULT: The solar wind becomes much more intense. The sun begins to quickly lose several percent of its mass every few million years. The sun begins to quickly lose several percent of its mass every few million years. –The sun is “recycling” hydrogen back into space. –As it becomes lighter, it’s gravity weakens. –Mass loss speeds up with time. –The remaining planets drift outward a bit.

61. A Second Birth… In the core: The temperatures eventually reach 100 million K. Helium begins to fuse into Carbon, Nitrogen, and Oxygen. Helium Ignition Has Begun!

62. The Helium Flash When the sun was first born hydrogen to helium fusion began slowly. When the sun was first born hydrogen to helium fusion began slowly. Helium fusion begins very quickly: Helium fusion begins very quickly: –Called the Helium Flash! –Core Temperature rises to over 350 million K! –Energy output spikes dramatically (goes up by millions of times).

63. Reset The Thermostat! The Pressure-Temperature Thermostat kicks back in: The Pressure-Temperature Thermostat kicks back in: –The expansion cools the star’s core off. –Puts out the hydrogen burning shell. –Star’s luminosity and size go down, and the star shrinks in size a little. The star becomes a stable helium- burning giant. The star becomes a stable helium- burning giant.

64. The Sun’s Second Life The sun is now a horizontal branch star (we’ll see why next time). The sun is now a horizontal branch star (we’ll see why next time). L = 40 T = 4,500 K R = 10.5

65. For Next Time Look over Units 64, 65 and 66 Look over Units 64, 65 and 66 We will discuss the final fate of stars including the Sun (and the Earth). We will discuss the final fate of stars including the Sun (and the Earth). –aka, the kaboom lecture