Stellar Evolution and the Life Cycles of the Stars: Stellar Deaths

1. Stellar Evolution and the Life Cycles of the Stars:Stellar Deaths

2. Stellar Evolution and the Life Cycles of the Stars:In which we learn where the atoms in your body came from…

3. This Week: • Chapters 13, 14, 15 • Assigned question due Thursday April 28: Question 5 from Chapter 15.

4. Stellar Evolution • There are several distinct phases in the life cycle of a star. The evolutionary path depends on the initial mass of the star. • Although there is a continuous range of masses, we often talk about “lightweight” stars (masses similar to the Sun) and “heavyweight” stars (masses about about 10 solar masses).

5. Stellar Evolution

6. Stellar Evolution • The basic steps are: • Gas cloud • Main sequence • Red giant • Rapid mass loss (planetary nebula or supernova explosion) • Remnant • The length of time spent in each stage, and the details of what happens at the end depend on the initial mass.

7. Points to Remember: • How to counter gravity: • Heat pressure from nuclear fusion in the core (no mass limit) • Electron “degeneracy” pressure (mass limit 1.4 solar masses) • Stars experience rapid mass loss near the end of their “lives”, so the final mass can be much less than the initial mass.

8. Points to Remember: • Sources of energy: • Nuclear fusion: • needs very high temperatures • about 0.7% efficiency for hydrogen into helium. • Gravitational “accretion” energy: • Drop matter from a high “potential” • About 10% efficient when falling onto massive bodies with very small radii.

9. After the Main Sequence: Low Mass • After hydrogen fusion is completed, the core collapses, and the outer parts of the star expand. • The core may fuse helium into carbon for a short time, after which the core collapses further. • The outer parts of the star expand by large amounts, and eventually escape into space, forming a planetary nebula. Matter is recycled back into space.

10. Planetary Nebulae • These objects resembled planets in crude telescopes, hence the name “planetary nebula.” • They are basically bubbles of glowing gas.

11. Planetary Nebulae • They are basically bubbles of glowing gas. • The ring shape is a result of the viewing geometry. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

12. Planetary Nebulae • The red light is the Balmer alpha line of hydrogen, and the green line is due to oxygen. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

13. Planetary Nebulae • This HST image shows freshly ejected material interacting with previously ejected material. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

14. Planetary Nebulae • The outer layers of the star are ejected, thereby returning material to the interstellar medium. What about the core?

15. The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses.

16. The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses. • To what?

17. The Remnant: Low Mass • After all of the helium in the core is used up, a low mass star cannot get hot enough to go to the next step of carbon fusion. There is no more energy source to support the core, so it collapses. • To what? • But first: a historical mystery involving the brightest star in the sky: Sirius (the “dog” star).

18. Sirius • This bright star is relatively close to the Sun. The spectral type is A1V, and its mass is about twice the Sun’s mass. • In the 1830s it was discovered that Sirius moves in the plane of the sky (roughly 1 arcsecond per year).

19. Sirius • This bright star is relatively close to the Sun. The spectral type is A1V, and its mass is about twice the Sun’s mass. • In the 1830s it was discovered that Sirius moves in the plane of the sky (roughly 1 arcsecond per year). However, the motion was not in a straight line: Sirius has a binary companion.

20. Sirius • From the size of the wobble, it was estimated that the companion star had a mass roughly equal to the Sun’s mass.

21. Sirius • From the size of the wobble, it was estimated that the companion star had a mass roughly equal to the Sun’s mass. • However, this object was extremely faint, and observers tried for decades to spot it without success.

22. Sirius • From the size of the wobble, it was estimated that the companion star had a mass roughly equal to the Sun’s mass. • However, this object was extremely faint, and observers tried for decades to spot it without success. • The famous telescope maker Clark spotted the faint companion in the 1870s when testing out his latest refracting telescope.

23. Sirius • Clark discovered the faint companion was roughly 10,000 times fainter than Sirius.

24. Sirius • Clark discovered the faint companion was roughly 10,000 times fainter than Sirius but bluer. • Here is a modern image, early on it was relatively hard to study the faint star owing to the high contrast.

25. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s.

26. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius!

27. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius! • The density is roughly 1.4 million grams per cubic centimeter!

28. The Puzzle • Sirius B has a mass roughly equal to the Sun’s mass, but it is about 10,000 times fainter than the Sun while being having a surface temperature about 10 times higher than the Sun’s. • To be so faint while being hot, the radius of Sirius B must be 1% of the Sun’s radius! • The density is roughly 1.4 million grams per cubic centimeter! ????

29. Degenerate Matter • The nature of Sirius B was solved in the 1920s and 1930s. It has to do with what happens to the star when pressure can no longer support it…

30. Degenerate Matter • Once the internal pressure stops, the gravitational collapse begins. • Eventually, the gas becomes supercompressed so that the particles are touching. The the gas is said to be degenerate, and acts more like a solid. • For a star with an initial mass of less than about 8 solar masses, the final object has a radius of only about 1% of the solar radius, and is extremely hot (and therefore blue).

31. Degenerate Matter • Once the internal pressure stops, the gravitational collapse begins. • Eventually, the gas becomes supercompressed so that the particles are touching. The the gas is said to be degenerate, and acts more like a solid. • For a star with an initial mass of less than about 8 solar masses, the final object has a radius of only about 1% of the solar radius, and is extremely hot (and therefore blue). These are the white dwarf stars.

32. After the Main Sequence: Low Mass • The red giants are stars that just finished up fusing hydrogen in their cores. • The white dwarfs are the left over cores of red giants that have shed their mass in planetary nebulae. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

33. White Dwarfs • White dwarfs are supported by “electron degeneracy pressure”, which is explained by quantum mechanics: basically no two electrons can occupy the same space. • The density can be 106 times that of water. • A higher mass white dwarf has a smaller radius than a lower mass white dwarf. • The maximum allowed mass is 1.4 solar masses.

34. After the Main Sequence: Low Mass • The core collapses until the gas is “degenerate”, at which point it acts like a solid. It becomes a white dwarf: • The density is more than 1 million times that of water. • The source of support is the “electron degeneracy” pressure. The maximum mass that can be supported is 1.4 solar masses. • There is no internal source of energy, and the white dwarf cools down slowly over time. Initially, the white dwarf is relatively hot (several times the solar temperature).

35. After the Main Sequence: Low Mass • The red giants are stars that just finished up fusing hydrogen in their cores. • The white dwarfs are the left over cores of red giants that have shed their mass in planetary nebulae. Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com)

36. After the Main Sequence: High Mass • A massive star (more than about 10 to 15 solar masses) will use up its core hydrogen relatively quickly. The core will collapse. • The core heats up, and helium is fused into carbon. After this, carbon and helium can fuse into oxygen since the high mass gives rise to very high temperatures. • Eventually elements up to iron are formed in successive stages.

37. After the Main Sequence: High Mass • Eventually elements up to iron are formed in successive stages. • Iron fusion does not produce energy, so there is no energy source to halt the gravitational collapse. • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure.

38. After the Main Sequence: High Mass • Eventually elements up to iron are formed in successive stages. • Iron fusion does not produce energy, so there is no energy source to halt the gravitational collapse. • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure. The collapse continues.

39. After the Main Sequence: High Mass • If the initial mass of the star is more than about 8 solar masses, the core will be too massive to form a white dwarf, since at that stage the gravity is stronger than the electron degeneracy pressure. The collapse continues. • Protons and electrons are fused to form neutrons and neutrinos. The core collapses to a very tiny size, liberating a huge amount of energy. The outer layers are blown off in a supernova explosion.

40. Supernovae • A supernova can be a billion times brighter than the Sun at its peak.

41. Supernovae • Supernovae are rare events. One occurred in a relatively nearby galaxy in 1987.

42. Supernovae • Supernovae are rare events. One occurred in a relatively nearby galaxy in 1987. • It has been closely studied since with the Space Telescope and other telescopes.

43. Supernovae • Several solar masses of material is ejected into space by the explosion. • Many “supernova” remnants are known.

44. Supernovae • Material is returned to the interstellar medium, to be recycled in the next generation of stars. • Owing to the high temperatures, lots of exotic nuclear reactions occur, resulting in the production of various elements. All of the elements past helium were produced in supernovae.

45. Supernovae • Material is returned to the interstellar medium, to be recycled in the next generation of stars. • Owing to the high temperatures, lots of exotic nuclear reactions occur, resulting in the production of various elements. All of the elements past helium were produced in supernovae. • Most of the atoms in your body came from a massive star!

46. The Remnant: High Mass • What happened to the core?

47. The Remnant: High Mass • What happened to the core? • Gravity overcame the electron degeneracy pressure, so the collapse continued. • Protons and electrons form neutrons, and the gas is compressed so that the neutrons become degenerate (i.e. they are basically touching). • The resulting remnant has a radius of about 10 km, and a typical mass of 1.4 solar masses. This is a neutron star. • The density is 6.4 x 1014 grams/cc. • The surface gravity is 1011 times that of Earth.

48. Points to Remember: • How to counter gravity: • Heat pressure from nuclear fusion in the core (no mass limit) • Electron “degeneracy” pressure (mass limit 1.4 solar masses) • Neutron “degeneracy” pressure (mass limit about 3 solar masses)

49. Neutron Stars • According to model computations, a neutron star should be very small (radius of about 10 km), and very hot (temperatures more than 1 million degrees). • Is there any hope of observing them?

50. Neutron Stars • According to model computations, a neutron star should be very small (radius of about 10 km), and very hot (temperatures more than 1 million degrees). • Is there any hope of observing them? • Yes: there are some exotic phenomena that are best explained by neutron stars.