Astronomy:%20Perspective

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Astronomy Perspective

• Bob Rood
• U. Virginia

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25,000 ly
Sun
The Milky Way might look like this. It contains
billyuns billyuns of stars
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Green Bank Scale Model
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On GB scale model t Ceti is at geosynchronous
orbit Center of Milky Way close to Mercurys
orbit
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The age of the oldest stars in the Milky Way is
about 13Gyr 1Gyr 1 billion years
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The age of the oldest meteorites, and by
inference the Solar System, is 4.57 Gyr, i.e., ltlt
Age of MW
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Supernovae other stars make heavy elements.
SN1054
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Molecules Volatile Hydrogen
H2 Water H2O Carbon
monoxide CO Carbon dioxide CO2 Methane
CH4 Ammonia
NH3 Refractory Silicon dioxide SiO2

Heavy or metals
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• We are made out of common stuff
• The ratios of the various elements are pretty
  much the same throughout the MW
• H2O should be ubiquitous

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Metallicity
Age (Gyr)
Metallicity built up rapidly and has remained
almost constant
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Emission nebula
Reflection Nebula
Embedded newly formed stars
Dust lanes
Star formation continues in Giant Molecular Clouds
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The r Ophiuchi molecular cloud one of the
closest of the dark clouds.
And smaller cold dark clouds
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The rate of star formation was much higher early
in the Galaxy If the best targets are solar type
stars close (lt 3000 ly) to the Sun and 5 Gyr old
then R? 1 star/1000 yr
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disk
Protostars are typically surrounded by a dusty
disk
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The dust collects into km size planetestimals. The
se collide and build up planets or planet cores.
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A surviving rocky planetesimal the asteroid
Gaspara
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An evaporating icy planetesimal Comet West
Gas being entrained in the Solar wind
Dust being blown away by Solar radiation pressure
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Closeup of an evaporating icy planetesimal The
nucleus of Comet Halley in 1986
Gas boils out of cracks
Nucleus covered with a layer of black crud
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Classical Planet Formation Terrestrial planets
form in inner Solar System from rocky
planetesimals In outer SS icy planetesimals
accrete to form a core of perhaps 10M? which has
sufficient gravity to suck on H and He to make
Jovian planets.
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Stellar Mass-Luminosity Relation
Luminosity increases rapidly as mass increases
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Stellar lifetime decreases rapidly as stellar
mass increases
Stars with M gt 1.2M? dont live long enough for
complex life to develop.
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Of the 30 brightest stars, all except 2 are more
luminous than the Sun. Almost half are more
luminous than 1000 L ? .
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In an unbiased sample of all stars closer than 10
pc, the vast majority are less luminous than the
Sun. The typical star is a dinky little thing
with L lt L ? /100.
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The consequence is that the familiar bright stars
are not good SETI targets. SETI scientists are
aware of this. The general public and most
science fiction writers are not.
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In the 4.6 Gyr since the Sun formed its
luminosity has increased by 25. This has
important consequences for the Earth.
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Ice ages -7C ? 8 change in L CO2 Greenhouse
3C ? 4 change in L
Major climate change with if L changes by a few
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Faint young Sun problem ? Early Greenhouse must
have been substantially enhanced Greenhouse must
evolved as L increases keeping T? just right.
(The Goldilocks Problem) Potential crisis when
the atmosphere becomes oxidizing.
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Evolution of the early terrestrial Greenhouse

• mid 1970s ammonia
• late 1970s methane ammonia
• late 1980s lots and lots of CO2
• 2000s methane protected by photochemical haze
• 2010s ?

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What is an Earthlike planet?
Liquid H2O on the surface for Gyrs
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Theres certainly more to it than Mltfew M? and
roughly the right distance from the star. E.g.,

• Too massive ? initial outgassing of CO2 leads to
  runaway greenhouse
• Too small ? vulcanism stops and atmosphere
  almost vanishes like Mars

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Cosmic Catastrophes
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Impacts
On the 108 year timescale there is an impact
large enough to lead to a major extinction
event. KT event Bad for dinosaurs Good for
mammals
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Nearby Supernova
E.g., Fields Ellis, (1999, New Astronomy, 4,
419) suggest that deep-ocean 60Fe is a fossil of
a near-earth (30 pc) supernova and might be
associated with a mini-extinction event.
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Galactic g-ray burst
A g-ray burst at a distance of 10kpc and pointed
at the Earth would produce a radiation dose of
6500 rads (65 grays) inside the ISS. 65 x
fatal. Very bad for a civilization that had moved
to space colonies.
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Galactic g-ray burst (cont)

• Worse than biggest solar flares because
• No warning
• No shielding by magnetic fields
• Requires more mass shielding than protons from
  flares

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Galactic g-ray burst (cont)

• Worse than biggest solar flares because
• No warning
• No shielding by magnetic fields
• Requires more mass shielding than protons from
  flares

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Galactic g-ray burst (cont)
Frequency perhaps one per 107 yr even correcting
for the fact that bursts are more common in lower
metallicity galaxies
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Gotchas were playing Calvinball
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There is no fJ in the Drake Equation
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An ETI Gotcha
Fragments of Comet Shoemaker-Levy 1993
Jupiter eats comets
Last big accretion event in the Solar System.
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Without Jupiter there would be a major extinction
event every 100,000 years. (Wetherill, 1994, Ap
Sp Sci, 212, 23)
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Classical picture Whether you get a Jupiter or
not is a contest between building the core of icy
plantesimals and the stars blowing away the H
He. If the star wins no Jupiter
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On the other hand if a Jupiter is formed too
quickly while there is still a lot material in
the disk, it spirals inward to become a hot
Jupiter and eats any Earth-like planets on the
way.
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Time to wakeup for Coffee