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2. Chapter 20 Stellar Evolution

3. 20.1 Leaving the Main Sequence We cannot observe a single star going through its whole life cycle; even short-lived stars (millions of years) live too long for that. But stars do not remain in the same category for their whole life. Stars that are in the Main Sequence can evolve. Observation of stars in star clusters gives us a look at stars in all stages of evolution; this allows us to construct a complete picture.

4. 20.1 Leaving the Main Sequence During its stay on the main sequence, any fluctuations in a star’s condition are quickly restored; the star is in hydrostatic equilibrium: If gravity goes up pressure goes down, if temp goes up, star expands, etc.

5. 20.1 Leaving the Main Sequence Stage 7 Core hydrogen burning – process of nuclear fusion where H is turned to He. Stars spend billions of years as main sequence, but eventually, as hydrogen in the core is consumed, the star begins to leave the main sequence. Its evolution from then on depends very much on the mass of the star: Low-mass stars go quietly (our Sun). High-mass stars go out with a bang!

6. How can astronomers “see” stars evolve?

7. 20.2 Stage 8 Sub-Giant Branch Our Sun has hardly changed in temperature or size over 5 billion years. However, its luminosity has increased by about 30%. The composition of a star’s core is what changes the most. As the core gets more full of helium, less burning occurs.

8. 20.2 Evolution of a Sun-like Star As the fuel in the core is used up, the core contracts and leaves hydrostatic equilibrium; it then begins to collapse on itself. For our Sun, the core is not hot enough to fuse He into a heavier element. Helium is practically “ash”. The shrinkage of the helium core releases gravitational energy and ups the central temperature.

9. Hydrogen Shell Burning As the temperature rises, even more hydrogen is burned up at a quicker pace. Hydrogen outside the core is quickly burned up as well. A stars response to running out of fuel? It gets much more bright! Star after this stage are called subgiants. Their temperature falls, but radius goes way up and luminosity increases slightly.

10. 20.2 Evolution of a Sun-like Star Stages of a star leaving the main sequence:

12. 20.2 Evolution of a Sun-like Star Stage 9: The Red-Giant Branch As the He core continues to shrink, the outer layers of the star expand and cool until a red giant is formed. If our Sun were a red giant it would extend out as far as the orbit of Mercury. Despite its cooler temperature, its luminosity increases enormously due to its large size.

13. 20.2 Evolution of a Sun-like Star The red giant stage on the H–R diagram: Notice the near vertical leap from Stage 8. What properties go up?

14. 20.2 Evolution of a Sun-like Star Stage 10: Helium fusion into carbon Once the core temperature has risen to 100,000,000 K, the helium in the core starts to fuse, through a triple-alpha process: The 8 Be nucleus is highly unstable, and will decay in about 10 –12 s unless an alpha particle fuses with it first.

15. 20.2 Evolution of a Sun-like Star The helium flash: Helium begins to fuse extremely rapidly; within hours the enormous energy output is over, and the star once again reaches equilibrium: Stage 10 horizontal branch – star is steadily burning He What changed?

16. 20.2 Evolution of a Sun-like Star Stage 11: Back to the giant branch As the helium in the core fuses to carbon, the core becomes hotter and hotter, and the helium burns faster and faster. The star is now similar to its condition just as it left the main sequence, except now there are two shells:

18. 20.2 Evolution of a Sun-like Star The star has become a red giant for the second time: using the Stage 11 asymptotic- giant branch Why does a star get brighter as it runs out of fuel in its core? - Non-burning inner core begins to shrink releasing gravitational pull and making the star larger, allowing it shine brighter.

19. Some stars are large enough to fuse carbon into a heavier element, but the Sun is not one of them. Some carbon might be converted to oxygen, but there is simply not enough temperature or pressure to continue on. Our Sun now enter Stage 12: A Planetary Nebula There is no more outward fusion pressure being generated in the core, which continues to contract. Meanwhile, the outer layers of the star expand to 300 times the Sun’s size (engulfing Mars)

20. 20.3 The Death of a Low-Mass Star Virtually all of the stars envelope is ejected into space at about 10km per second. The star forms a planetary nebula (above and left):

21. 20.3 The Death of a Low-Mass Star The star now has two parts: A small, extremely dense carbon core An envelope about the size of our solar system. The envelope is called a planetary nebula, even though it has nothing to do with planets – early astronomers viewing the fuzzy envelope thought it resembled a planetary system.

22. 20.3 The Death of a Low-Mass Star Planetary nebulae can have many shapes: As the dead core of the star cools, the nebula continues to expand, and dissipates into the surroundings.

23. 20.3 The Death of a Low-Mass Star Stages 13 and 14: White and black dwarfs Once the nebula has gone, the remaining core is extremely dense and extremely hot, but quite small. It is luminous only due to its high temperature.

24. 20.3 The Death of a Low-Mass Star The Hubble Space Telescope has detected white dwarf stars (circled) in globular clusters:

25. 20.3 The Death of a Low-Mass Star As the white dwarf cools, its size does not change significantly; it simply gets dimmer and dimmer as it burns helium (or hydrogen or carbon, depending on original size), and finally ceases to glow. Black Dwarf – a cold dense burned out ember in space.

26. 20.3 The Death of a Low-Mass Star This graphic shows the entire evolution of a Sun-like star. Such stars never become hot enough for fusion past carbon to take place. - The star’s brief stay in the horizontal and asymptotic stages are not shown

27. Why does fusion cease in the core of a low mass star? -Because the temperature is not great enough to fuse heavier metals beyond carbon. -https://www.youtube.com/watch?v=vya79mD8 OvU

28. 20.4 Evolution of Stars More Massive than the Sun High mass stars evolve much more rapidly than their low mass counterparts. The Sun’s lifetime is 10 billion years, but a high mass (B) star only lasts for 100 million years while an even higher mass (O) star will last for a short 20 million years.

29. 20.4 Evolution of Stars More Massive than the Sun High-mass stars, like all stars, leave the main sequence when there is no more hydrogen fuel in their cores. The first few events are similar to those in lower-mass stars – first a hydrogen shell, then a core burning helium to carbon, surrounded by helium- and hydrogen-burning shells.

30. 20.4 Evolution of Stars More Massive than the Sun Stars with masses more than 2.5 solar masses do not experience a helium flash – helium burning starts gradually. A 4-solar-mass star makes no sharp moves on the H–R diagram – it moves smoothly back and forth.

31. 20.4 Evolution of Stars More Massive than the Sun A star of more than 8 solar masses can fuse elements far beyond carbon in its core, leading to a very different fate. Its path across the H–R diagram is essentially a straight line – it stays at just about the same luminosity as it cools off. Eventually the star dies in a violent explosion called a supernova.

32. What is the essential evolutionary difference between high mass and low mass stars?

33. 20.4 Evolution of Stars More Massive than the Sun In summary:

34. 20.5 Observing Stellar Evolution in Star Clusters The following series of H–R diagrams shows how stars of the same age, but different masses, appear as the cluster as a whole ages. After 10 million years, the most massive stars have already left the main sequence, while many of the least massive have not even reached it yet.

35. 20.5 Observing Stellar Evolution in Star Clusters After 100 million years, a distinct main-sequence turnoff begins to develop. This shows the highest- mass stars that are still on the main sequence. After 1 billion years, the main-sequence turnoff is much clearer.

36. 20.5 Observing Stellar Evolution in Star Clusters After 10 billion years, a number of features are evident: The red-giant, subgiant, asymptotic giant, and horizontal branches are all clearly populated. White dwarfs, indicating that solar-mass stars are in their last phases, also appear.

37. 20.5 Observing Stellar Evolution in Star Clusters This double cluster, h and chi Persei, must be quite young – its H–R diagram is that of a newborn cluster. Its age cannot be more than about 10 million years.

38. 20.5 Observing Stellar Evolution in Star Clusters The Hyades cluster, shown here, is also rather young; its main-sequence turnoff indicates an age of about 600 million years.

39. 20.5 Observing Stellar Evolution in Star Clusters This globular cluster, 47 Tucanae, is about 10–12 billion years old, much older than the previous examples.

40. Why are the observations of star clusters so important to the theory of stellar evolution?

41. 20.6 The Evolution of Binary-Star Systems Does a star in a binary system with another star have any affect on its evolution? Yes & No! If the stars in a binary-star system are relatively widely separated, their evolution proceeds much as it would have if they were not companions. If they are closer, it is possible for material to transfer from one star to another, leading to unusual evolutionary paths.

42. 20.6 The Evolution of Binary-Star Systems Each star is surrounded by its own Roche lobe; particles inside the lobe “belong” to the central star. It is the “zone of influence” of gravity. The Lagrangian point is where the gravitational forces between Roche lobes are equal.

43. 20.6 The Evolution of Binary-Star Systems There are different types of binary-star systems, depending on how close the stars are. In a detached binary, each star has its own Roche lobe:

44. 20.6 The Evolution of Binary-Star Systems In a semidetached binary, one star can transfer mass to the other:

45. 20.6 The Evolution of Binary-Star Systems In a contact binary, much of the mass is shared between the two stars:

46. 20.6 The Evolution of Binary-Star Systems As the stars evolve, the type of binary system can evolve as well. This is the Algol system. It is thought to have begun as a detached binary:

47. 20.6 The Evolution of Binary-Star Systems As the blue-giant star entered its red-giant phase, it expanded to the point where mass transfer occurred (b). Eventually enough mass accreted onto the smaller star that it became a blue giant, leaving the other star as a red subgiant (c). https://www.youtube.co m/watch?v=FBbQHmuw 9Lo