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Chapter 16

PHYS 183 Chapter Notes - Chapter 16: Accretion Disk, Radiation, Main Sequence

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PHYS 183
Tracy Webb

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PHYS 183 The Milky Way Inside and Out Tracy Webb Winter 2013
16: Star Birth
16.1 Stellar Nurseries
youngest star clusters always associated with dark clouds of gas & dust birthplace of stars
dark patches in the Milky Way are interstellar gas clouds
interstellar medium: the gas & dust that fill the spaces between stars within a galaxy; consists of
70% H, 25% He & 2% heavier elements
o virtually all has the same chemical composition but can look very different place to place
based on differences in temperature & density
molecular clouds: interstellar clouds that are particularly cold & dense that stars are born in
o H2 is the most abundant molecule but hard to detect since usually too cold for H2 to
produce emission lines, so usually use CO which produces radio emission lines
interstellar dust: half of the atoms of elements heavier than He in a molecular cloud are tiny,
solid grains; constitute ~1% of molecular cloud’s mass
detect reddening from Doppler shift since it doesn’t change wavelength of stars spectral lines so
determine amount of reddening by comparing observed colour to colour expected of that spectral
amount of reddening tells how much dusty gas lies between earth & star
infrared allows us to see directly through molecular clouds & also show new stars in clouds
most radiation produced by young stars in molecular cloud can’t escape cloud since dust grains
absorb it & heat up themselves, emitting thermal radiation in infrared & microwaves this is why
clouds that appear dark in visible light glow in infrared light
stars form when gravity causes molecular cloud to contract until central object become hot
enough to sustain nuclear fusion in core
molecular clouds are the only places in space that the force of gravity overcomes the form of
pressure depends on density & temperature
thermal pressure: temperature dependant pressure
o can resist gravity in most interstellar gas clouds due to low gas densities keeping gravity
degeneracy pressure: pressure that is not temperature dependant
gravity is stronger in molecular clouds since more mass in each cubic cm of volume
gravitational contraction: regions of molecular cloud in which gravitational attraction is
stronger than thermal pressure so forced to contract
o this converts some of the gravitational potential energy into thermal energy
o if can’t get rid of thermal energy quickly, it builds up inside of cloud which raises
temperature & pressure halting formation of star doesn’t usually happen
stars form in clusters since gravity stronger in high mass gas clouds
individual gas clumps within molecular clouds move at substantially different speeds indicating
overall gas motion is turbulent
magnetic fields can help the cloud resist gravity
large clouds of gas form many individual stars instead of one extremely massive star due to battle
between gravity and pressure
o gravity follows inverse square law strength of gravity increases as cloud shrinks in size
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