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The ISM and Stellar Birth Extinction and Reddening Rayleigh Scattering Extinction and Reddening Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs. That is, a star 1000pc from Earth will look about 2 magnitudes fainter than if space were empty completely • Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs. That is, a star 1000pc from Earth will look about 2 magnitudes fainter than if space were empty completely • Dust thought to come from stellar ‘winds’, blowing out molecules of hydrogen, carbon, oxygen and other elements which cool and coalesce into dust grains Carbon IR Visible UV 2. Nebulae Emission Nebulae: • Light is from emission spectrum • Reminder: result of a low density gas excited to emit light. The light is emitted at specific wavelengths • The gas is excited by light from hot stars > 25,000K (B1). It does not shine under it’s own light. • Sometimes called HII regions, as they mostly contain hydrogen that has been ionised by the light from stars • Density: 100-1000 atoms per cubic cm • Pink due to red, blue and violet Balmer emission lines Orion Nebula (M42) Eagle Nebula Reflection Nebulae: • Light reflected (scattered) by dust/gas much like the moon reflects the Sun’s light – so doesn’t generate its own light • Scatter light from cooler stars • Mostly scatters blue light (like our atmosphere) – so they appear blue. • Dust grains must have sizes ranging from 0.01mm down to 100 nm • See absorption spectrum of nebula in the star’s spectrum • Doppler broadening due to motion of gas molecules • Lines split into more than one component indicates light travelled through different gas/dust clouds with different radial velocities Witch Head Nebula Merope Nebula Dark Nebulae: • More dense clouds of dust and gas obscure light from background stars • Very cool (10 - 100 K) Horse’s head nebula Snake nebula The components of the Interstellar Medium (ISM): 1. HI clouds • seen through interstellar absorption lines and 21 cm radio radiation • Neutral Hydrogen • 50 – 150pc diameter • Few hundred K • 10 – 100 molecules / cubic cm • Twisted into long filaments • Near Stars, it is ionized to form HII regions The components of the Interstellar Medium (ISM): 2. Hot intercloud medium • Between HI clouds • Few thousand K • 0.1 molecule / cubic cm • Mostly hydrogen (HII) ionized by ultraviolet light from stars The components of the Interstellar Medium (ISM): 3. Giant molecular clouds (GMCs) • Contain larger molecules, sometimes organic, although still mostly hydrogen • 10K • 1000 – 100,000 molecules / cubic cm • 15 – 60pc across • Often seen as dark nebulae Our solar system . The components of the Interstellar Medium (ISM): 4. Coronal gas • 100,000 – 1,000,000 K • 0.001 – 0.0001 atoms / cubic cm • Ionized atoms • From supernovae or very hot stars • Emit X-Rays • Nothing to do with the Sun’s corona! We see evidence for the interstellar medium through... • Extinction and reddening • Emission nebulae • Dark nebulae • Reflection nebulae • 21cm radiation • X-rays from hot gas between stars ...from which we can figure out the components of the ISM: • HI clouds • Hot Intercloud medium • Giant molecular clouds • Coronal gas • Stars are born when a small part of a giant molecular cloud collapses • Resistance to collapse: 1. Heat energy - 10K: average speed of hydrogen Molecule is 800mph 2. Magnetic fields – act as springs 3. Rotation 4. Turbulence Need a triggering mechanism: shock waves Need a triggering mechanism: shock waves Shock wave from: • Supernova explosions • Ignition of hot nearby stars • Collision of molecular clouds • Spiral pattern of galaxy NGC 1999 – Reflection nebula containing a small clump of a giant molecular cloud collapsing to form stars Protostars Clumps of compressed gas resulting from the shock wave passing through the gmc start to collapse under their own gravity • As gas molecules fall in, their speed increases • They collide with other molecules and randomize their speeds • Temperature is just a measure of how fast, on average, the random motion of molecules is • So as the cloud collapses, its temperature increases Protostars Protostar Dust Free Zone IR photon Outer Envelope cloaks protostar of Gas and Dust Protostars •As cloud collapses, it flattens out into a disk due to rotation • Protostar continues contracting and heating up until the center becomes hot enough to start fusing hydrogen into helium > the star is born! • Drive away their cocoon of dust and gas Birth line Hayashi track