Download Basic Observations Star Formation favors smaller stars (more

Chapter 18: The Formation of Stars and Planets
Basic Observations
Star Formation
favors smaller stars (more smaller stars observed than
larger stars)
favors binary or multiple star systems
young stars tend to be found in
clusters (=> formation occurs in localized regions
bound together by gravity)
associations (formed near each other, but too far
apart to be bound by mutual gravitation)
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Basic Observations (cont’d)
Solar system clues
all orbital planes of planets and equatorial plane of sun
(approximately) the same
planets orbit in the same direction, most rotate same direction
planetary orbits are nearly circular
regular satellites of outer planets display same patterns as
planets
Sun has 99% of solar system mass, but less than 1% of the
angular momentum (“Angular Momentum Problem”)
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Star Birth
Molecular clouds: concentrations of gas and dust
[Fig18.1,2-4(orion nebula),5 (IRAS view of molecular cloud), eagle_nebula_GPN-2000-000987.jpg]
relatively cool => contain molecules (vs. atoms and ions)
=> radio telescopes used to measure spectrum
Giant Molecular Clouds
~10 pc across
~ 1 million M in gas and dust
Clumps ~1000 -10,000 M with cloud cores
Cloud Cores (formation of cores still a puzzle)
cores collapse gravitationally, slowed by magnetic fields [fig 18.6]
cores contain intense infrared sources => protostars
warmer cores => larger stars, multiple stars,
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cooler cores =>smaller stars
Evolution of a protostar
central region: gravity strongest, collapse fastest
collapse generates heat (and infrared radiation)
density increases until material becomes opaque
gas temperature increases, pressure balances gravity
continued accumulation of material by protostar
rotation of core causes material to accumulate in disk around
protostar (angular momentum) [Figure 18.7-9,13, 12(disk edge on vs flat) pancake.avi, 0708.mov]
friction transfers angular momentum to exterior regions
eventually dispersed by winds, or forms planetary
system/binary companion
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Young Stars
H-R diagram [Figure 18-10,11]
T-Tauri stars: light (less than 3M ) pre main sequence
Ae, Be stars: more massive pre-main sequence stars
gravitational contraction → PE converted to KE (heat) →
density/pressure in center of protostar start hydrogen fusion
more massive stars collapse, start fusion more quickly
reach main sequence before dust and gas dispersed
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Young Stars (cont’d)
strong stellar winds
possibly caused by strong magnetic activity caused by initiation
of fusion (large energy release in core → convection)
strong wind halts infalling material, eventually blowing away
surrounding cloud [Figure 18-15]
bipolar outflows [ Fig 18.15, 16,beta_pictoris_01l.jpg]
strong wind blows away surrounding gas/dust
→ stars become visible
most massive stars reach main sequence before becoming
visible
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Planetary Systems [2507.mov]
planets formed from disk of material: the solar nebula
abundance of radioactive isotopes provide timeline
most meteorites formed within 20 million years of each other
earth formed within the next 100 million years
=> solar system formed ~ 4.6 billion years ago, took ~10 to
100 million years to form
solar nebula has same chemical composition as outer region of
the sun.
planets contain more heavier elements (small percentage of
solar mix)
=> solar nebula was many times the mass of the planets ~
1/10 to 1/100 x M
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Cooling and Condensation
nebula began to cool as accumulation of infalling material by
sun ceased
inner regions hottest
as nebula cools, materials condense (at condensation
temperatures)
the condensation sequence [Figure 18.17,18] determines what
materials condense
=> chemical makeup of the planet
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Planet formation [ejs_gravitation2.jar]
microscopic grains of condensed materials accumulated into larger
bodies: planetesimals (up to 1000’s kms in size)
largest bodies continue to accumulate material (accrete)
bombardment era
two models for giant planets [Figure 18.19]
protoplanets accumulate gas from nebula
gas forms unstable rings which form protoplanets
possible final collisions (formation of earth’s moon, stripping of
mercury’s silicate exterior, etc.)
satellite systems
generally miniature versions of solar system formation
some satellites are captured planetesimals
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evidence for Extrasolar planetary systems
pulsars (timing of pulses affected by star/large planet orbiting
each other, similar to doppler shift)
periodically doppler shifted stellar spectra
see, http://exoplanets.org/
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