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Lecture 15:
Star formation and the ISM
Astronomy 111
Wednesday October 19, 2016
Reminders
• Homework #7 due Monday
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Star Birth
• Where do stars come
from?
• How do stars form?
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Stars are very far apart!
• Although they look close together in the
night sky, stars are very far apart from
each other
– Remember that the next closest star to the
Sun is 4.3 light-years
– That’s 25 trillion miles!
• What is in all that space?
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Interstellar medium
• ISM is the stuff
between stars
• Composition (by mass)
– 75% Hydrogen
– 23% Helium
– 2% Heavy metals
– Also has some dust (about 1%)
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Interstellar medium
• How much is
there?
• The density of the ISM is only 1
atom/cm3
• Earth’s atmosphere has a density of
2.5x1019 molecules/cm3!
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Dust
• Tiny particles in the ISM
made of “heavy elements”
– Iron, silicon, carbon
• The half of ISM that is not in the form of
hydrogen and helium is dust
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Sky: Optical
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Sky: Infrared
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In-class assignment
• Dust absorbs light from stars and reradiates it like a blackbody with a
temperature of T=10-100K
• Use Wien’s law
max
2,900,000 nm

T
to calculate the range of wavelengths of
peak emission of dust in the Universe
• What instrument would best be used to
study this emission?
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Blackbody spectrum of dust
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Glowing dust
Wide-field Infrared Survey Explorer (WISE)
3.3, 4.7, 12, and 23 micron composite image
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Glowing dust
Dust glows
when it
absorbs light
and re-radiates
as heat
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Glowing dust
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Interstellar clouds
• Regions of cool, dense gas are called
“clouds”
– Usually have T~100K
• Even cooler, denser regions are called
“molecular clouds”
– T~10K
– Lots of dust and molecular hydrogen
• These clouds hold together in space
because they are self-gravitating
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Barnard 68
Molecular clouds
Horsehead nebula
Rosette nebula
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Where are stars formed?
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Where are stars formed?
• Stars are formed deep within molecular
clouds
• If a molecular cloud is massive enough, it
will collapse through its self-gravity
• Gravitational collapse is balanced by
–
–
–
–
Outward pressure from heat within the cloud
Conservation of angular momentum
Turbulence within the cloud
Magnetic fields
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How are stars formed?
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How are stars formed?
• The collapse of a molecular cloud is relatively
slow and complicated
• The molecular cloud will fragment as it
collapses, resulting in multiple knots of dense
gas
Computer simulation of star formation during
collapse of a molecular cloud:
http://www.youtube.com/watch?v=YbdwTwB8jtc
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How are stars formed?
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How are stars formed?
• Collapse of these knots is much like the
formation of a planetary system
• The collapsed material forms an
accretion disk around a protostar
• The protostar may eventually form a
planetary system
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A star is born
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A star is born
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A star is born
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Disks and jets
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From protostar to star
• The protostar
begins to
gravitationally
collapse
• Material from the
molecular cloud
falls on it, adding
energy to its
collapse
• This raises the
temperature
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From protostar to star
• Eventually the
interior gets hot
enough to begin
fusing hydrogen
into helium
• The protostar
becomes a main
sequence star
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From protostar to star
Evolving onto the
Main Sequence
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Stellar mass range
• Massive stars
are rare
• Many more
low-mass stars
(M<Msun)
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Summary
• Interstellar medium contains gas and dust
• Cool clouds within the ISM can
gravitationally collapse to form stars
• A protostar will heat up as it collapses
• This heat will cause the protostar to begin
fusing hydrogen into helium, which is when
it becomes a main-sequence star
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