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The ISM and Stellar Birth

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.

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The ISM and Stellar Birth

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  1. The ISM and Stellar Birth

  2. Extinction and Reddening Rayleigh Scattering

  3. Extinction and Reddening

  4. 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

  5. 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

  6. 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

  7. Orion Nebula (M42) Eagle Nebula

  8. 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

  9. Witch Head Nebula Merope Nebula

  10. Dark Nebulae: • More dense clouds of dust and • gas obscure light from background • stars • Very cool (10 - 100 K)

  11. Horse’s head nebula Snake nebula

  12. The components of the Interstellar Medium (ISM): • 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

  13. The components of the Interstellar Medium (ISM): • Hot intercloud medium • Between HI clouds • Few thousand K • 0.1 molecule / cubic cm • Mostly hydrogen (HII) ionized by ultraviolet light from • stars

  14. The components of the Interstellar Medium (ISM): • 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

  15. Our solar system .

  16. The components of the Interstellar Medium (ISM): • 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!

  17. 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

  18. Stars are born when a small part of a giant molecular cloud collapses • Resistance to collapse: • Heat energy • 10K: average speed of hydrogen • Molecule is 800mph • Magnetic fields – act as springs • 3. Rotation • 4. Turbulence

  19. Need a triggering mechanism: shock waves

  20. Need a triggering mechanism: shock waves • Shock wave from: • Supernova explosions • Ignition of hot nearby stars • Collision of molecular clouds • Spiral pattern of galaxy

  21. NGC 1999 – Reflection nebula containing a small clump of a giant molecular cloud collapsing to form stars

  22. 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

  23. Protostars Protostar Dust Free Zone IR photon Outer Envelope cloaks protostar of Gas and Dust

  24. Protostars • As cloud collapses, it flattens out into a disk due to rotation

  25. 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

  26. Birth line Hayashi track

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