2. The Birth of Stars of the stars

3. 18-1 How astronomers have pieced together the story of stellar evolution 18-2 What interstellar nebulae are and what they are made of 18-3 What happens as a star begins to form 18-4 The stages of growth from young protostars to main- sequence stars 18-5 How stars gain and lose mass during their growth 18-6 What insights star clusters add to our understanding of stellar evolution By reading this chapter, you will learn Chapter 18 18-7 Where new stars form within galaxies 18-8 How the death of old stars can trigger the birth of new stars

4. Interstellar matter: cold near absolute zero mostly H&He, Low density Different types: Reflection nebula, emission nebula and absorption (dark) nebula

8. Interstellar matter 1.Interstellar matter is the ingredients to make new stars. These are generally found more commonly in: a-disk, b-center, c- outskirt : of a spiral galaxy. 2.(a-Emission, b-absorption, c-reflection, d-planetary) nebula has signature color of blue. 3.Interstellar matter is mostly made of: a- carbon, b- hydrogen & helium, c-Oxygen & Nitrogen, d-iron, e- different composition depending on the location within a galaxy. 4.When the light from a far away star travels long distance, often the color is shifted towards: a-blue, b- green, c-white, d-red Click on “enter” to review your answers: 1. a, 2. c, 3. b, 4. d

11. Protostars form in cold, dark nebulae

12. Size of the stars: If the Giant molecular cloud is too large & cloud of gas reaches Jeans’ instability, the cloud of gas is now too warm to collapse. If the protostar is too small: The interior temperature never reaches hot enough for hydrogen fusion. These failed stars are called Brown dwarfs

13. Mass gain / loss Low-mass stars that vigorously eject gas are called T Tauri stars (a protostar may eject large amounts of gas into space before starting H fusion)

14. Mass loss

15. Young star clusters: star formation and evolution

16. The spiral arms of our Galaxy are laced with giant molecular clouds, immense nebulae so cold that their constituent atoms can form into molecules Star birth can begin in giant molecular clouds

17. Star-forming regions appear when a giant molecular cloud is compressed This can be caused by the cloud’s passage through one of the spiral arms of our Galaxy, by a supernova explosion, or by other mechanisms OB associations give energy for emission nebula and trigger star formation

18. Review EM radiation 1.Which range of EM radiation has highest energy? 2.Which range of EM radiation has low frequency? Hint: f=v: (wave length x frequency = speed) 3.Which range of EM radiation has lowest energy? 4.Which range of EM radiation is commonly referred to as “heat”? 5.Which range of EM radiation has fastest speed? 6.What wavelength of light can human eyes detect? Click on “enter” to review your answers: 1. Gamma rays, 2. radio, 3. radio, 4. infra red, 5.all travels @ speed of light through vacuumed 6. 400nm – 700nm

21. Supernovae compress the interstellar medium and can trigger star birth

22. Birth of the stars 1.Where are we likely to see the stellar formation? A- anywhere on the disk of spiral galaxy, b-at the core of any spiral galaxy, c- in spiral arms. D- only at 4kpc from the center. 2.All the stars first form as part of: a- binary system, b-globular star cluster, c-OB associations or open star cluster, d- galactic clusster 3.Observing in (a-visible range, b- UV range, c-X ray range, d- IR range) tells us a lot about stellar formation. 4.T-tauri stars are: a- high mass star that have just completed its formation, b- low-mass star that have just completed its formation, c-. high mass star that is about to start helium fusion, d-low mass stars that is losing mass right before staring its hydrogen fusion 5.Super novae could start star formations. Supernova is: a-birth of high mass star, b- change in state during main sequence of a high mass star, c- after glow of energy after the high mass star dies. d- explosive dyeing process of high mass star. Click on “enter” to review your answers: 1. c, 2. c, 3. d, 4. d, 5.d