1. Chapter 12: Chapter 12: The Lives of Stars from Birth Through Middle Age NGC 2264

2. Star-Forming Regions A star forming region of Cygnus, DR21, is a strong infrared radio emitter, revealing many protostars. Star formation in nearby galaxy M33 in a region NGC 604

3. The Core of the Rosette Nebula Radiation from young stars has blown gas away from the center of this nebula, causing the gas to clump in dark globules where new star formation is taking place.

4. A Supernova Remnant X-ray image of the Cygnus Loop HST image of the Cygnus Loop

5. Pre–Main-Sequence Evolutionary Tracks

6. Star Formation The central gases are heating as they fall into the newly forming protostar. The dark region has just developed a Jeans instability.

7. Star Formation As the protostar grows in mass, its surface gets brighter while its core heats up. When little gas is left in the center of the dark core, the object becomes a pre–main- sequence star.

8. Brown Dwarf Gliese 229A, left Gliese 229B, right, Brown Dwarf

9. Plotting the Ages of Stars Star Cluster NGC 2264

10. Evolution of Stars from the Main Sequence

11. Planetary Nebulae NGC 7293, Helix Nebula NGC 6826Menzel 3

12. From Supergiants to White Dwarfs

13. Structure of an Old Low-Mass Star Near the end of its life, a low-mass star like the Sun travels up the AGB and becomes a supergiant.

14. Structure of an Old High-Mass Star

15. Supernovae Proceed Irregularly Computer simulations showing how chaotic the supernova is deep inside the star as it begins to explode.

16. Supernova 1987A

17. Rotating, Magnetized Neutron Star Charged particles are accelerated near a neutron star’s magnetic poles and produce two oppositely directed beams of radiation.

18. Accretion Disk Around a Supermassive Black Hole The gases spiraling inward in an accretion disk heat up as they approach the black hole.

19. WHAT DO YOU THINK? How do stars form? How do stars form? Are stars still forming today? Are stars still forming today? Do more massive stars shine longer? Do more massive stars shine longer?

20. You will discover… how stars form how stars form what a stellar “nursery” looks like what a stellar “nursery” looks like how astronomers use the physical properties of stars to learn about stellar life cycles how astronomers use the physical properties of stars to learn about stellar life cycles the remarkable transformations of older stars into giants and supergiants the remarkable transformations of older stars into giants and supergiants that some dying stars eject material that creates new generations of stars, while others act as beacons that enable astronomers to pinpoint distant galaxies that some dying stars eject material that creates new generations of stars, while others act as beacons that enable astronomers to pinpoint distant galaxies that the H-R diagram is your guide to the stellar life cycle that the H-R diagram is your guide to the stellar life cycle

21. Everything Ages 19351994

22. A Connection to Interstellar Space Polycyclic aromatic hydrocarbons are also found in interstellar clouds.

23. The Pleiades The blue glow surrounding the stars of the Pleiades is a reflection nebula created as radiation scatters off dust grains. Most of the cool, low-mass stars have arrived at the MS, indicating that hydrogen fusion has begun in their cores.

24. Interstellar Reddening Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576.

25. This radio map shows the extent of giant molecular clouds in Orion and Monoceros. The locations of four prominent star-forming nebulae are indicated on the star chart overlay. Note that the Orion and Horsehead nebulae are sites of intense CO emission, indicating that stars are forming in these regions. Gas- and Dust-Rich Region of Orion

26. The glowing gases in emission nebulae are excited by ultraviolet radiation from young, massive stars. Dust grains obscure part of NGC 2024; the distinctively shaped dust cloud Horsehead Nebula blocks the light from the background nebula IC 434. Above and to the left of the Horsehead Nebula is the reflection nebula NGC 2023, whose dust grains scatter blue light from stars between us and it more effectively than any other color.

27. A Dark Nebula Barnard 86

28. Cluster of Protostars Protostars in the nebula RCW 49 (indicated by yellow circles), observed in the infrared by the Spitzer Space Telescope

29. Mass Loss from a Supermassive Star Pistol formed nearly 3 million years ago and originally had 100–200 M. The star ejected the gas we see in two episodes 6000 and 4000 years ago.

30. Mass Loss from a Supermassive Star The largest, most massive known star, LBV 1806-20, is 5 million times brighter and about 150 times more massive than the Sun.

31. An H II Region

32. The Orion Nebula

33. Evolution of an OB Association Cone Nebula

35. Fully Convective Star

36. A Mass-Loss Star HD 148937 is continuously shedding its outer layers. Other vigorous outbursts in the past gave rise to the two symmetric shells of material, NGC 6164 and NGC 6165, on either side of the star.

37. The Sun Today and as a Giant

38. Red Giant Stars in a Star Cluster

40. Post–Main-Sequence Evolution

41. A Globular Cluster M10

42. H-R Diagram of a Globular Cluster

43. Structure of the H-R Diagram

45. Metal-Poor and Metal-Rich Stars

46. The Instability Strip

47. Analogy for Cepheid Variability (a) As pressure builds up in this pot, the force on the lid (Cepheid’s outer layers) increases. (b) When the pressure inside the pot is sufficient, it lifts the lid off (expands the star’s outer layers) and thereby allows some of the energy inside to escape.

48. Period-Luminosity Relation

49. Detached, Semidetached, Contact, and Overcontact Binaries

50. Three Close Binaries

51. Mass Exchange between Close Binary Stars

52. WHAT DID YOU THINK? How do stars form? How do stars form? Stars form from the mutual gravitational attraction between gas and dust inside giant molecular clouds. Stars form from the mutual gravitational attraction between gas and dust inside giant molecular clouds. Are stars forming today? Are stars forming today? Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars. Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars. Do stars with greater mass shine longer? Do stars with greater mass shine longer? Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars. Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars.

53. Key Terms accretion disk birth line Bok globule brown dwarf Cepheid variable contact binary core helium fusion dense core detached binary electron degeneracy pressure emission nebula evolutionary track giant molecular cloud globular cluster H II regions helium flash horizontal branch star hydrogen shell fusion instability strip interstellar extinction interstellar medium interstellar reddening Jeans instability nebulae (singular nebula) OB association open cluster overcontact binary Pauli exclusion principle period-luminosity relation Population I star Population II star pre–main-sequence star protostar red dwarf Roche lobe RR Lyrae variable semidetached binary stellar evolution supernova remnant T Tauri stars Type I Cepheid Type II Cepheid variable stars zero-age main sequence (ZAMS),