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PHYS 183 Lecture Notes - Molecular Cloud

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PHYS 183
Tracy Webb

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PHYS 183 The Milky Way Inside and Out Tracy Webb Winter 2013
Lecture 21: February 27th, 2013
we are going to start with star birth
a star birth is more like a formation event
the single most important feature of a star is it’s mass it determines everything
but what determines the mass of a star?
nebula: large cloud of gas in the universe
interstellar medium: the gas and dust between stars in the universe where stars form
newborn stars produce white patches in the cloud where starlight illuminates surrounding gas
clouds look dark where dust particles block the light from more distant stars dust in the
universe is tiny little grains of soot-like ash
interstellar medium is a place where you find a lot of extra energy or energy that is being blocked
most of the matter in giant molecular clouds is in the form of molecules (H2, CO) as opposed to
single atoms
molecular clouds usually have very distinct boundaries/edges
they have typical temperatures of 10-30 K (very cold) & densities of 300 molecules per cubic cm
(best vacuum’s in physics labs are ~100 molecules per cubic cm) very cold & very dense
interstellar dust is made of tiny solid particles made of carbon, silicon, iron & oxygen larger
than molecules & made of heavier elements
it also blocks the light from stars behind them
interstellar reddening
o dust is better at blocking blue light than red
o recall: light interacts with orbits larger than it
o there is a general ring of apparently red stars at the edge of the molecular cloud
o observing the infrared light from a cloud can reveal new stars within it
wavelengths (like infrared) provide a lot of information about hidden processes
dust grains in the gas glows in the infrared since they absorb energy from new stars (anything
with a temperature glows)
stars form due to the idea of gravity
formation of star:
o simulation begins with turbulent gas cloud
o random motions in cloud cause it to become lumpy
o if gravity can overcome pressure in dense regions, they can collapse to form even denser
lumps of matter as it collapses, it loses potential energy which is turned into thermal
energy, so the star is heating up which creates pressure
o large cloud fragments into many smaller lumps of matter
o each lump can go on to form one or more new stars
why do stars form? due to collapses of high density areas in clouds of gas
stars can only form if gravity can overcome the thermal pressure of gas thermal pressure
depends on temperature & density
gravity pushes inwards, pressure pushes outwards
there is a constant war between gravity and pressure
cooling emission lines from molecules can relieve some of the thermal pressure
thermal energy is converted to light which is then radiated away (usually in infrared & radio)
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