1. 3. Stellar Formation and Evolution in the lecture "Life and Exoplanets" Big bangNuclear fusion in starsSupernova nucleosynthesisPlanetary formation

2. Milky way and the Sun

3. Nuclear Fusion ( 핵융합 ) in Stars Compare nuclear fission

4. Stars are giant nuclear reactors ( 핵 반응기 ). In the center of stars, atoms ( 원자 ) are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright.

5. Nuclear fusion ( 핵융합 ) is an atomic reaction that fuels stars. In fusion, many nuclei (the centers of atoms) combine together to make a larger one (which is a different element). A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition. The net mass of the fused atomic nuclei is smaller than the sum of the constituents. This lost mass is released as electromagnetic energy, according to the mass-energy equivalence relationship: E=  mc 2.

6. ProtonNeutron Electron Proton Electron 1 H (Hydrogen) 2 H (Deuterium) ProtonNeutron 3 H (Tritium) The proton is a subatomic particle with an electric charge of +1 elementary charge. The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton. Isotope

7. The hydrogen fusion process is temperature-sensitive. In the Sun, with a 10 million K core, hydrogen fuses to form helium in the proton-proton chain reaction. 4 1 H → 2 2 H + 2e + + 2ν 2 1 H + 2 2 H → 2 3 He + 2γ 2 3 He → 4 He + 2 1 H These reactions result in the overall reaction: 4 1 H → 4 He + 2e + + 2γ + 2ν mass of 4  1 H nuclei > mass of 1  4 He  Energy according to E=  mc 2

9. The hydrogen fusion process is temperature-sensitive, so a moderate increase in the core temperature will result in a significant increase in the fusion rate. In evolved stars, helium can be transformed into carbon in the triple-alpha process. 3 4 He → 12 C + γ + 7.2 MeV

11. Hertzsprung–Russell diagram (HR-diagram, 헤르츠스프룽 - 러셀 도표 ) and Stellar Classification ( 분류 )

12. Luminosity vs. Temperature or Absolute Magnitude vs. Spectral Class Stars fall into 4 main regions on the graph –Main sequence (90% of stars) –Giants (Large & Cool) –Supergiants (Large) –White Dwarfs (Small & Hot) Each region refers to a different stage in a star's evolution

13. Luminosity Spectral Class (or Color) Red giants Supergiants White dwarfs Main sequence

14. Oh, Be A Fine Girl, Kiss Me

15. Formation of Stars

16. 1.Stars are formed within extended regions of higher density in the interstellar medium. 2.These regions are called molecular clouds mainly composed of hydrogen plus helium. 3.As massive stars are formed from molecular clouds, they powerfully illuminate those clouds, ionizing hydrogen and creating an H II region.

18. 4.The star forming clouds are initially in hydrostatic equilibrium ( 정수 압 평형 ). 5.The formation of a star begins with a gravitational instability inside a molecular cloud 6.It is triggered by shock waves from supernovae or the collision of two galaxies. Or stellar wind and radiation pressure from massive young stellar objects may compress interstellar medium. 7.Once a region reaches a sufficient density of matter when the internal gas pressure is not strong enough to prevent gravitational collapse (gravitational instaility), it begins to collapse under its own gravitational force. Protostar formation

19. hydrostatic equilibrium

20. As the cloud collapses, dense dust and gas form globules. As a globule collapses and the density increases, the gravitational energy is converted into heat. When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium, a protostar forms at the core. These pre-main sequence stars are often surrounded by a protoplanetary disk (explain later).

21. NGC 3603, an open cluster of stars surrounded by massive cloud M 83, a barred spiral galaxy

22. The Antennae Galaxies, very high starburst galaxy occurring from the collision of two galaxies

23. HST image of globules in H II region, dark clouds of dense dust and gas 8K, one of the coldest place in the universe Mass of 2-50 M  ( 1 M  = Solar mass)

25. Upper limit: 150 M  Radiation pressure too great. Lower limit: 0.08 M  Too cool for H-fusion to begin Brown dwarf (Jupiter mass = 0.001M  )

26. Main Sequence

27. Stars spend about 90% of their lifetime at this stage, fusing hydrogen to produce helium near the core. Such stars are said to be on the main sequence. Once a star is born, the proportion of helium in a star's core will steadily increase. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. For example, the Sun is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.

28. Fusion process differs by mass. For low mass stars, proton- proton chain is dominant process for nuclear fusion at the core, whereas for high mass stars, carbon-nitrogen- oxygen cycle is dominant.

29. These reactions result in the overall reaction: 4 1 H → 4 He mass of 4  1 H nuclei > mass of 1  4 He  Energy according to E=  mc 2

31. Post-Main Sequence

32. Massive stars process up to iron  explode in Supernova events Average or Low Mass stars stop before iron  gently blow themselves to death forming planetary nebulas

33. Red Giant When stars > 0.4 M  run out their hydrogen fuel in their core, their outer layers expand and cool to form a red giant. In a red giant of up to 2 M , hydrogen fusion proceeds in a shell-layer surrounding the core. Eventually the core is compressed enough to start helium fusion. Stars shrinks in radius and increases its surface temperature. After the star has consumed the helium at the core, fusion continues in a shell around a hot core of carbon and oxygen. Eventually the outer layers of the star will be shed, creating a planetary nebula, with only a white dwarf left behind.

35. Planetary nebula

37. Red Supergiant (High Mass Star) After a helium-burning runs out of helium fuel in its core, the star's core starts to collapse and heat up. This causes the outer layers of the star to expand and cool, similar to the process that occurred after the star ran out of hydrogen fuel and left the main sequence. As the star becomes larger and larger, it eventually becomes a red supergiant. Extremely massive supergiants can generate high enough pressure and temperature to fuse elements even heavier than carbon and oxygen. Near the end of the red supergiant phase, a high mass star will develop several "onion layers" of heavier and heavier elements. Eventually stars this massive die …

39. Death of High Mass Stars: Supernova (Type II) The "Type II" supernovae are the result of a massive star consuming all of its nuclear fuel and then exploding. These stars have large H-rich envelopes, hence the presence of H in the spectra Elements heavier than Mg produced during explosion. Lighter elements produced during preceding stellar evolution

40. Crab Nebula (M1), a supernova remnant in the constellation of Taurus

41. The Sun The Sun is a Population I, or heavy element-rich, star. –Population I: metal rich –Population II : metal poor –Population III: metal free, which is believed to form in the early universe The formation of the Sun may have been triggered by shockwaves from nearby supernovae. This is suggested by a high abundance of heavy elements in the Solar System, such as uranium, relative to the abundances of these elements in Population II stars.

42. Photosphere (6000K) Core (13,600,000 K) with 0.25 solar radi: Thermonuclear reaction