The ISM and Stellar Birth

Extinction and Reddening Rayleigh Scattering

Extinction and Reddening

Near the Sun, Extinction amounts to 2 magnitudes per 1000 parsecs 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 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): 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): 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): 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): 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: Heat energy 10K: average speed of hydrogen Molecule is 800mph 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