Chapter 11 The Formation of Stars
A World of Dust 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
Bare-Eye Nebula: Orion One example of an interstellar gas cloud (nebula) is visible to the bare eye: the Orion nebula
Three Kinds of Nebulae (1) 1) Emission Nebulae Hot star illuminates a gas cloud; excites and/or ionizes the gas (electrons kicked into higher energy states); electrons falling back to ground state produce energy. The Trifid The Fox Fur Nebula NGC 2246 Nebula
Three Kinds of Nebulae (2) Star illuminates gas and dust cloud; 2) Reflection Nebulae star light is reflected by the dust; reflection nebula appear 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).
Three Kinds of Nebulae (3) Dense clouds of gas and dust absorb the light from the stars behind; 3) Dark (absorption) Nebulae appear dark in front of the brighter background; Bernard 86 Horsehead Nebula
The Life Cycle of Stars Dense, dark clouds, possibly forming stars in the future Aging supergiant Young stars, still in their birth nebulae
Shocks Triggering Star Formation Globules = sites where stars are being born right now! Trifid Nebula
Sources of Shock Waves Triggering Star Formation (1) Previous star formation can trigger further star formation through: a) Shocks from supernovae (explosions of massive stars):
Sources of Shock Waves Triggering Star Formation (2) Previous star formation can trigger further star formation through: b) Energy released from hot, massive O or B stars which produce a lot of UV radiation:
Sources of Shock Waves Triggering Star Formation (3) Giant molecular clouds are very large and may occasionally collide with each other c) Collisions of giant molecular clouds.
Sources of Shock Waves Triggering Star Formation (4) d) Spiral arms in galaxies like our Milky Way: Spirals arms create rotating shock waves
Protostars Protostars = pre-birth state of stars: Hydrogen to Helium fusion not yet ignited Still enshrouded in opaque cocoons of dust => barely visible in the optical, but bright in the infrared.
Protostellar Disks As protostars contract, they heat Up. leads to the formation of protostellar disks birth place of planets and moons
From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of H He fusion processes
Evidence of Star Formation Star Forming Region RCW 38
Globules Bok Globules: ~ 10 to 1000 solar masses; Contracting to form protostars
Globules (2) Evaporating Gaseous Globules ( EGGs ): Newly forming stars exposed by the energy released from nearby massive stars
The Source of Stellar Energy Recall from our discussion of the sun: Stars produce energy by nuclear fusion of hydrogen into helium.
The Source of Stellar Energy (2) Only 30% of the sun s mass is hot enough to fuse! Every fusion reaction only produces a small amount of energy. Stars give off so much energy because there are so many fusion reactions happening all the time!!! 2 x 10^45 reactions every second!!!!
Energy Transport Energy generated in the star s center must be transported to the surface. Inner layers: Radiative energy transport γ-rays Gas particles of solar interior Cool gas sinking down Outer layers (including photosphere): Convection Bubbles of hot gas rising up
Stellar Structure Flow of energy Energy transport via convection Energy transport via radiation Energy generation via nuclear fusion Sun Basically the same structure for all stars with approx. 1 solar mass or less. Temperature, density and pressure decreasing
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
Energy Transport Structure Inner convective, outer radiative zone Inner radiative, outer convective zone