Stellar Evolution after the Main Sequence

1. Stellar Evolutionafter the Main Sequence Low Mass Stars

2. 1000 100 10 1 .1 .01 The Path to the Main Sequence O B A F G K M

3. Life on the Main Sequence • Once a low mass star, such as the Sun, settles down on the Main Sequence, it is in balance • In the core, Hydrogen is being converted to Helium, the resulting energy, in the form of heat and radiation, works its way to the surface • While the force of gravity has not been stopped, the gravitational collapse has been halted • The star will remain in this state for several billion years or more, depending on its mass

4. Nuclear Fusion • In order to supply today's Solar Luminosity, the Sun must convert 600 million tons of Hydrogen to Helium every second. • Simultaneously about 4 million tons of matter is being converted to energy. • Energy in the form of radiation makes its way out of the sun. • But what happens to the Helium?

5. Forming a He Core • Helium is 4 times heavier than hydrogen, so the inert helium will begin to slowly collect at the center of the star • At first there is no problem, but as the amount of the He becomes substantial, an inert core forms

6. Helium Core • The Helium "ash" continues to grow. • The Hydrogen is burning in a shell surrounding the He • Once there is enough He, gravity starts to compress the helium core • It begins to get hot from the compression, the heat causes the H  He reaction to increase causing more He, increasing the core mass which increases the gravitational force • Things start getting out of hand

7. Degeneracy • The He has had a lot of time to pack together; The electrons have formed what is called a degenerate electron gas • At usual stellar densities, the electrons in a gas act as though they were ordinary molecules and obey the usual gas laws • As the electrons are squeezed into tighter and tighter spaces, they begin to encroach upon each others 'territory' - They are not free to move as particles in an ideal gas, but are constrained to move only when other electrons move. It is as if the entire mass of electrons are geared together.

8. This Star has a Problem Let's summarize the situation, The star is converting H to He furiously in a shell about a contracting He core. The core is resisting the pressure because of the electron degeneracy pressure, but that doesn't stop the heat from increasing causing the H to go to He more and more rapidly. Eventually the excess heat, radiation and pressure overwhelm the force of gravity on the hydrogen. Balance is lost and the star begins to expand. (Of course, the He core is still trying to compress)

9. Red Giant As the star expands, the gas cools. This has the following effect, • The color of the star changes gradually to red showing the cooling gas temperature • The surface area becomes larger and larger, causing the brightness to increase despite the cooling temperature. • The He core is still getting hotter causing the 'burning' shell to produce more and more • This star begins to travel to the Red Giant region of the HR Diagram

10. 1000 100 10 1 .1 .01 Leaving the Main Sequence O B A F G K M

11. 4 4 4  + He He He + 2 2 2 4 6 4  + + 12 8 8 Be Be C Helium Flash • At this point the core is still contracting against the electron pressure – It's like a pressure cooker with the lid on tight. • Finally the temperature exceeds 100 million degrees Kelvin • At this temperature, the He nuclei have enough energy to begin to react • The Triple-Alpha process begins to convert helium into carbon:

12. Triple-Alpha Process

13. Helium Flash • One property of the degenerate electron gas is that it conducts temperature very well, so as soon as the energy is released in one part of the core, it is transmitted throughout the core in seconds, producing a rapid heating of all of the He there. • The He burning accelerates like an explosion – the He Flash. • The new energy expands the core rapidly which in turn cools things abruptly reversing the growth of the red giant. Without the overwhelming heat and pressure, the outer atmosphere begins to contract again; the triple-alpha process ceases once the temperature has dropped less than 100 million degrees. • The giant reduces its size (at the cost of heating up and shifting color toward the blue once again) • This moves the star down and to the left once again

14. 1000 100 10 1 .1 .01 After the He Flash O B A F G K M

15. And next… • The pressure has been released, the star has reduced it size (and consequently gotten a hotter surface changing color again) • Now it begins all over, Hydrogen is begin converted to Helium in a shell about the remaining Helium. The star once again collects He in its core, and everything happens all over again --- back to the Red Giant stage as the He compresses heating the Hydrogen shell Only this time there is a difference…

16. Red Giant again • This time there isn't enough time to form the electron gas – these changes have occurred over tens or hundreds of millions of years, not billions • This time when the core temperature reaches 100 million degrees and the He  C, there won't be a He Flash instead the star will be converting H to He and He to C simultaneously.

17. Late stage evolution Thin, cool atmosphere Hydrogen burning shell Helium What's THIS?? It's a core forming – Carbon 'ash'

18. The final stages Our star is now creating a carbon core, as that becomes substantial, gravity begins compressing it, making it denser and hotter (Sound familiar?) The heat from the compressing carbon gas causes the helium shell to burn furiously; that in turn increases the rate of burn for the hydrogen shell. Making the star larger and hotter (moving it left on the HR) The pressure finally overcomes gravity in the helium and hydrogen and the outer layers begin to expand and lift off into space

19. Solar Life-Cycle

20. 1 A.U. 1 A.U. Sun Earth The Red Giant Sun Sun: Main Sequence Sun: Red Supergiant

21. The Fate of the Earth Three possibilities: • The earth enters the supergiant sun. • The Earth will vaporize. • The Earth will melt into a cinder, but remain • The earth remains just outside the supergiant sun. • The Earth will melt into a cinder, but remain But what happens next to the Sun?

22. Planetary Nebulae As the outer layers lift off they form one of the most beautiful sights in space. Emitting mostly in blue and red the gas above the core moves into space. The ring is an illusion as the gas is spherical about the core. At one time we thought that this was a gentle, graceful process. The Hubble telescope has changed our mind M57 – The Ring Nebula

23. Planetary Nebulae Ejected atmosphere Exposed core

24. Planetary Nebulae In the Cat's Eye Nebula, we can see the complex jets and interactions of the expanding gas Hubble's eyesight has shown us that the stars do not "gently go into the night" NGC 2440

25. Planetary Nebulae In the 'Twin Jet Nebula', the gas is a bipolar flow moving at 200 miles/second. The left-over core of this star has a surface temperature of 200,000 ºK

26. White Dwarf What is left is the carbon core of the original star. It is very small, very hot About the size of the Earth, and many times hotter than the Sun. This is a White Dwarf. It is held apart by the degenerate electron pressure. It will slowly cool over billions of years to become a burned out carbon core – a black dwarf. This will be the fate of our Sun.

27. White Dwarf • Remember what's left at this point is a ' carbon core' – It's outer atmosphere has lifted away leaving a very dense, very hot core. • The core's intense gravitational field is balanced by the pressure of the degenerate electron gas

28. White Dwarf • Most of the mass of a solar sized star is concentrated in a core about the size of the Earth • This means it is very dense: A sugar cube’s worth of material at the Earth’s surface could weigh up to 200 tons

29. 0.4 kg 0.8 kg There is an inverse relationship between the mass and the radius --- the more massive, the smaller the white dwarf Chocolate cakes grow larger when their mass increases 0.8 M 0.4 M White dwarfs grow smaller as their mass increases. (More gravity, but same pressure)

30. White Dwarf • The hot core now slowly cools without losing pressure support • The cooling process takes billions (perhaps trillions) of years • When it becomes cool enough, it can crystallize • At some point, when it is cool enough, we declare it to be a black dwarf

31. Solar (low mass star) Evolution 1 .1 .01 O B A F G K M

32. White Dwarf It becomes natural to ask, "What if the star has more mass than the electron gas can balance?" In order to become a white dwarf, a star cannot have more mass than Chandrasekhar's Limit, Mstar < 1.4 Msun

33. Exceeding Chandrasekhar's Limit If a star starts out with more than 1.4 M it cannot become a white dwarf so its evolutionary path must be different (which we will discuss in the next lecture) Is there any other way to exceed Chandrasekhar's Limit?

34. Multiple Star Systems • Let's digress for a moment and consider multiple star systems. • Until now, we have been considering only 'solitares' – stars isolated in space. But this is actually the rarity. Most stars are found with one or more companions. • Their spacing is anywhere from about 2000 AU down to 'almost touching' • For simplicity, let's consider only binary star systems

35. Algol – The Demon Star Algol, Beta Persei, was seen to be the blinking eye in Medusa's head. It fades and brightens in just under 3 days. A very frightening sight Algol is an 'eclipsing binary' Two stars in close orbit oriented so that one passes in front of the other as seen from Earth

36. Binary Star Systems There are other types of variable stars (we will discuss some later). For now let's take a closer look at that image of the Algol system The dotted line about Algol A represents its Roche Limit; Notice that Algol B is deformed. Material from Algol B is being pulled into Algol A.

37. Binary Systems • Suppose one of the companion stars is a white dwarf • As its partner reaches the red giant stage, its atmosphere may impact on the white dwarf's Roche Limit. • Material from the companion will be accreted onto the surface of the white dwarf Dana Berry

38. Nova • If the layer of slowly-accreting hydrogen is heated to the appropriate temperature, it may explode – vastly, but temporarily, increasing the lumenosity of the system. • This is a Nova, or "New Star" • This, generally, does not do lasting damage to the star and, in fact, may be re-occurring – after burning off the accumulated Hydrogen, the capturing process begins again

39. Supernova What if the hydrogen layer is deposited more quickly and so that it doesn’t have the time to heat up enough to 'flash' into a nova, but instead just adds mass to the white dwarf? Once Chandrasekhar's Limit is exceeded, the star reacts by undergoing a cataclysmic explosion.

40. Supernova This explosion totally destroys the star. It is a Type Ia Supernova It has an Absolute Magnitude of -19.3 or about 5 billion times brighter than the Sun

41. SN Ia LightCurves Notice that the light curves from the various supernova are nearly all the same. This implies that the mechanism is very similar (and we will be able to use them as distance indicators From P. Hultzsch, et al

42. Very distant supernovae Since they are so bright, they are used to measure the expansion of the Universe