Chapter 9. The Formation and Structure of Stars

Transcription

1 Chapter 9 The Formation and Structure of Stars

2 The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the interstellar medium because: a) Dense interstellar clouds are the birth place of stars. b) Dark clouds alter and absorb the light from stars behind them.

3 Three kinds of nebulae 1) Emission Nebulae (HII Regions) A hot star illuminates a gas cloud; excites and/or ionizes the gas (electrons kicked into higher energy states); electrons recombining, falling back to ground state produce emission lines The Fox Fur Nebula NGC The Trifid 2246 Nebula

4 2) Reflection Nebulae Star illuminates gas and dust cloud star light is reflected by the dust reflection nebula appears blue because blue light is scattered by larger angles than red light same phenomenon makes the day sky appear blue (if it s not cloudy)

5 Emission and Reflection Nebulae

6 3) Dark Nebulae Dense clouds of gas and dust absorb the light from the stars behind; appear dark in front of the brighter background Barnard 86 Horsehead Nebula

7 Interstellar Reddening Blue light is strongly scattered and absorbed by interstellar clouds. Red light can more easily penetrate the cloud, but it is still absorbed to some extent. Barnard 68 Visible Infrared Infrared radiation is hardly absorbed at all. Interstellar clouds make background stars appear redder.

8 Interstellar Absorption Lines The interstellar medium also produces absorption lines in the spectra of stars.

9 HI clouds: Structure of the ISM The ISM occurs in two main types of clouds: Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 a few hundred atoms/cm 3 ); size: ~ 100 pc Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm 3 ); gas can remain ionized because of very low density

10 Fragmentation of a Cloud This simulation begins with a turbulent cloud containing 50 solar masses of gas.

11 Fragmentation of a Cloud The random motions of different sections of the cloud cause it to become lumpy.

12 Fragmentation of a Cloud Each lump of the cloud in which gravity can overcome pressure can go on to become a star. A large cloud can make a whole cluster of stars.

13 Shocks Triggering Star Formation The gas in the ISM needs to be compressed in order to collapse and form stars: Shocks traveling through interstellar space can do this.

14 Shocks Triggering Star Formation

15 Compression of the ISM by Winds from Hot Stars

16 The Contraction of a Protostar

17 From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of H He fusion processes

18 Collapse of the Solar Nebula Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms.

19 Flattening Collisions between particles in the cloud cause it to flatten into a disk.

20 Formation of Jets Rotation also causes jets of matter to shoot out along the rotation axis.

21 Evidence of Star Formation Nebula around S Monocerotis: Contains many massive, very young stars, including T Tauri Stars: strongly variable; bright in the infrared.

22 T Tauri Stars Very young stars, still in the forming stage Ttypically 100, million years old

23

24 Jets are observed coming from the centers of disks around protostars.

25 Protostellar Disks and Jets Herbig Haro Objects Disks of matter accreted onto the protostar ( accretion disks ) often lead to the formation of jets (directed outflows; bipolar outflows): Herbig Haro Objects

26 Globules Bok Globules: ~ solar masses; Contracting to form protostars

27 Globules Evaporating Gaseous Globules ( EGGs ): Newly forming stars exposed by the ionizing radiation from nearby massive stars.

28 Winds from Hot Stars Very young, hot stars produce massive stellar winds, blowing parts of it away into interstellar space. Eta Carinae

29 The Orion Nebula An Active Star-Forming Region

30 Atomic Terminology Atomic Number = # of protons in nucleus Atomic Mass Number = # of protons + # of neutrons

31 How does nuclear fusion occur in the Sun?

32 Fission Big nucleus splits into smaller pieces. (Nuclear power plants) Fusion Small nuclei stick together to make a bigger one. (Sun, stars)

33 High temperatures enable nuclear fusion to happen in the core.

34 The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

35 The Source of Stellar Energy Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium In the sun, this happens primarily through the proton-proton (PP) chain.

36 The CNO Cycle In stars slightly more massive than the sun, a more powerful energy generation mechanism than the PP chain takes over. The CNO Cycle

37 Fusion into Heavier Elements Fusion into heavier elements than C, O: requires very high temperatures; occurs only in very massive stars (more than 8 solar masses)

38 Hydrostatic Equilibrium Imagine a star s interior composed of individual shells. Within each shell, two forces have to be in equilibrium with each other: Outward pressure from the interior Gravity, i.e. the weight from all layers above

39 Hydrostatic Equilibrium Outward pressure force must exactly balance the weight of all layers above everywhere in the star. This condition uniquely determines the interior structure of the star. This is why we find stable stars on such a narrow strip (Main Sequence) in the Hertzsprung-Russell diagram.

40 Gravitational equilibrium: Gravity pulling in balances pressure pushing out. Energy balance: Thermal energy released by fusion in core balances radiative energy lost from surface.

41 Energy Transport Energy generated in the star s center must be transported to the surface. Inner layers of the sun: Radiative energy transport Outer layers of the sun (including photosphere): Convection

42 Flow of energy Stellar Structure Energy transport via convection Sun Energy transport via radiation Energy generation via nuclear fusion Basically the same structure for all stars with approx. 1 solar mass or less. Temperature, density and pressure decreasing

43 Core: Energy generated by nuclear fusion ~ 15 million K

44 Radiation zone: Energy transported upward by photons

45 Convection zone: Energy transported upward by rising hot gas

46 Hydrostatic equilibrium Energy transport Conservation of mass Conservation of energy Stellar Models The structure and evolution of a star is determined by the laws of: A star s mass (and chemical composition) completely determines its properties. That s why stars initially all line up along the main sequence.

47 Interactions of Stars and their Environment Supernova explosions of the most massive stars inflate and blow away remaining gas of star forming regions. Young, massive stars excite the remaining gas of their star forming regions, forming HII regions.

48 The Life of Main Sequence Stars Stars gradually exhaust their hydrogen fuel. In this process of aging, they are gradually becoming brighter, evolving off the zero-age main sequence.

49 The Lifetimes of Stars on the Main Sequence