Transcript 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
• 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