Astrophysics: Stellar Evolution 1 Stellar Birth Stellar Formation A. Interstellar Clouds (Nebulae) B. Protostellar Clouds 2 C. Protostars Dr. Bill Pezzaglia Updated: 10/02/2006 A. Interstellar Clouds 1. The Stuff of the Universe a) The Galaxy 1. x 2. x. x Only 10% of mass is stars 90% is raw material Stars form in the spiral arms 1b. What is the stuff 74% Hydrogen (element #1) 25% Helium (element #2) 1% Dust grains Made of heavier elements 2. Evidence of Stuff (a) Extinction of Light (less stars seen!) 1
2b. Emission Nebulae 2c. Interstellar Reddening HII region (ionized hydrogen gas) Note also less stars seen between the two nebula due to a dark cloud! Note NGC60 (left) is more red than NGC576 (right) because it is twice as far away. Short wavelength Blue light is absorbed more than Red Star Birth Collapse of the Cloud Stars begin their lives in a MOLECULAR CLOUD. Cold (10-0 K) Made mostly of H and He Dense compared to interstellar space, but not dense compared to vacuum on Earth Cold temp & relatively high density allow gravity to overcome thermal pressure, leading to gravitational collapse of cloud. During collapse, cloud remains <100 K, glows in infrared light. Molecular cloud Barnard 68, 500 light years away, 1/2 light year across. Orion Nebula, 1500 light years away, Trapezium cluster of O stars (HST) 1. Protostar Accretion disk Formation of Protostar False Color image of protostar Red: emission from ionized gas, jets along disk s rotation axis. Green: starlight scattered from dust particle in accretion disk. Disk is edge-on hence the dark band Cloud continues to collapse, increasing the density. Radiation becomes trapped, temperature rises. Protostar forms: thermal energy cannot escape, internal temp & pressure increase --> this rising pressure begins to fight the crush of gravity. Orion Nebula, 1500 light years away, infrared image. Color corresponds to temperature of emitting gas. (HST) 2
Formation of Protostellar disk To conserve angular momentum, a protostellar disk must form encircling the protostar. Cloud fragments spin faster as it collapses. Rotating cloud flattens to form protostellar disk. Protostellar disks become planetary systems? Protostellar disk thermal dust emission, NGC 758 S, rotating disk of gas = 100 solar masses, compact dense core cloud = 1000 solar masses. Distance = 10,000 light years. Green = cloud core, yellow = protostellar disk, red = protostar. Contours show disk rotation (red = receding disk, blue = disk coming towards us). Protostellar disk slows rotation of protostar Protostar rotation generates magnetic field. Magnetic field: 1. Transfer angular momentum outward to slow rotation, 2. Generates a protostellar wind BINARY STARS: if can t get rid of enough angular momentum --> unstable and split into 2 stars JETS: two high-speed streams of gas along rotation axis. How form?? Protostar Mizar and Alcor Herbig-Haro Objects T Tauri Stars High Velocity (100 to 1000 km/sec) bi-polar flow from young (T-Tauri) star hits interstellar medium. Causes heating, and creates an emission nebula. Pre-main sequence star Variable About to ignite in fusion. Herbig-Haro Object HH47. The scale bar represents 1000 AU. Star is born when core temperature > 10 million K (for hydrogen fusion) Fusion starts, gravity contraction stops. Internal structure stabilizes, thermal energy balances gravity --> MAIN SEQUENCE STAR A Star is Born! How Long Does It Take to be Born? Depends on the mass. Rule of thumb: Massive stars do everything faster!!
How Long Does It Take to be Born? Depends on the mass. Rule of thumb: Massive stars do everything faster!! High mass protostar (> 8 solar masses): million years or less Sun-like protostar: 50 million years Small star (< 2 solar masses): 100 million years Stellar Birth Weights Stars are born in CLUSTERS because molecular clouds can contain thousands of solar masses of gas. What processes govern the clumping and fragmenting of the gas into protostars with different masses? Unknown. In star clusters, more LOW MASS stars than HIGH MASS stars form. With time, the balance shifts even more towards lowmass stars as high-mass stars die away. So the most massive stars in a cluster may live and die before the smallest stars even finish being born. Upper limit: 100 M sun. Stellar Mass Limits Why? Such furious power that gravity cannot contain the internal pressure - blow themselves apart. Observations: No stars observed > 100 M sun. Lower limit: 0.08 M sun Why? Never reaches 10 million K for hydrogen fusion. Observations: Brown dwarfs. Degeneracy pressure halts gravitational contraction, no fusion --> Failed Star. Radiates internal thermal energy, cools with time, very dim (observed in infrared), gravity never can overtake degeneracy pressure (which doesn t decrease with time). Note 0.08 M sun = 80x M jupiter. 4
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Formation of the Solar System SOLAR NEBULA is the collapsed part of the giant interstellar cloud that formed the Sun. SOLAR NEBULA temp increases with collapse (gravitational potential energy converted to kinetic energy). SOLAR NEBULA is hottest at center, forming PROTOSUN. SOLAR NEBULA shrinks in radius, spins faster. Rotation ensures not all material falls into protosun (the more angular momentum of rotating cloud, the more spread out is the disk) SOLAR NEBULA flattens into PROTOPLANETARY DISK. Artist conception of protoplanetary disk 6