The ISM and Stellar Birth

Extinction and ReddeningRayleigh 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, oxygenand other elements which cool and coalesce into dust grains

IR Visible UV

Carbon

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 Balmeremission lines

Orion Nebula (M42)

Eagle Nebula

Reflection Nebulae:• Light reflected (scattered) by dust/gas much like the moonreflects 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 backgroundstars• Very cool (10 - 100 K)

Snake nebula

Horse’s head nebula

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 formHII 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, althoughstill mostly hydrogen• 10K• 1000 – 100,000 molecules / cubic cm• 15 – 60pc across• Often seen as dark nebulae

The components of the Interstellar Medium (ISM):

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Our solar system

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!

The components of the Interstellar Medium (ISM):

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 cloudcollapsing to form stars

Clumps of compressed gas resulting fromthe shock wave passing through the gmcstart 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, onaverage, the random motion of molecules is• So as the cloud collapses, its temperature increases

Protostars

Protostar

Dust Free Zone

Outer Envelope cloaks protostar of Gas and Dust

IR photon

Protostars

Protostars

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

• Protostar continues contracting and heating up until the center becomes hot enoughto start fusing hydrogen into helium > the star is born!• Drive away their cocoon of dust and gas

Birth line

Hayashi track