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Astronomy: Perspective
Bob Rood
U. Virginia
25,000 ly
Sun
The Milky Way might look like this.
It contains billyuns & billyuns of stars
Green Bank Scale Model
On GB scale model t Ceti is at
geosynchronous orbit
Center of Milky Way close to
Mercury’s orbit
The age of
the oldest
stars in the
Milky Way is
about 13Gyr
1Gyr = 1
billion years
The age of the oldest meteorites,
and by inference the Solar System,
is 4.57 Gyr, i.e., << Age of MW
Supernovae &
other stars
make heavy
elements.
SN1054
Molecules
Number of atoms per 10,000,000 of
hydrogen
hydrogen
helium
10,000,000
1,400,000
“Heavy or metals”
oxygen
6,800
sulfur
Volatile
95
iron
80
argon
42
carbon
3,000
aluminum
19
neon
2,800
sodium
17
Hydrogen
H2
Water
H2 O
Carbon monoxide
CO
Carbon dioxide
CO2
Methane
CH4
Ammonia
NH3
Refractory
nitrogen
910
calcium
17
magnesium
290
all other elements
50
silicon
250
Silicon dioxide
SiO2
• 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
Metallicity
Age (Gyr)
Metallicity built up rapidly and has
remained almost constant
Emission nebula
Reflection Nebula
Embedded newly
formed stars
Dust lanes
Star formation continues in Giant
Molecular Clouds
The r Ophiuchi molecular cloud: one of the closest of the “dark
clouds.”
This is it
And smaller cold dark clouds
The rate of star formation was much
higher early in the Galaxy
If the best targets are solar type
stars close (< 3000 ly) to the Sun and
5 Gyr old then
R = 1 star/1000 yr
disk
Protostars are typically surrounded by a dusty
disk
The dust collects into km size
planetestimals.
These collide and build up planets or
planet cores.
A surviving rocky planetesimal: the asteroid
Gaspara
An evaporating icy planetesimal:
Comet West
Dust being blown away
by Solar radiation
pressure
Gas being entrained in
the Solar wind
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
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.
Stellar Mass-Luminosity Relation
Luminosity
increases rapidly
as mass increases
Stellar
lifetime
decreases
rapidly as
stellar mass
increases
Stars with M > 1.2M don’t live long
enough for complex life to develop.
Of the 30 brightest stars, all except 2 are more luminous
than the Sun. Almost half are more luminous than 1000 L  .
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 < L  /100.
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.
In the 4.6 Gyr since the Sun formed its luminosity
has increased by 25%. This has important
consequences for the Earth.
Ice ages: -7C  8% change in L
CO2 Greenhouse: 3C  4% change in L
Major climate change with if L changes
by a few %
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.
Evolution of the early terrestrial
Greenhouse
•
•
•
•
mid 1970’s: ammonia
late 1970’s: methane + ammonia
late 1980’s: lots and lots of CO2
2000’s: methane protected by
photochemical haze
• 2010’s: ?
What is an Earthlike planet?
Liquid H2O on the surface for Gyrs
There’s certainly more to it than
M<few 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
Cosmic Catastrophes
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
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 miniextinction event.
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.
Galactic g-ray burst (cont)
Worse than biggest solar flares
because:
1. No warning
2. No shielding by magnetic fields
3. Requires more mass shielding
than protons from flares
Galactic g-ray burst (cont)
Worse than biggest solar flares
because:
1. No warning
2. No shielding by magnetic fields
3. Requires more mass shielding
than protons from flares
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
“Gotchas:” we’re playing Calvinball
There is no fJ in the Drake Equation
An ETI Gotcha
Fragments of Comet
Shoemaker-Levy 1993
Jupiter eats
comets
Last big accretion event in the Solar System.
Without Jupiter
there would be a
major extinction
event every
100,000 years.
(Wetherill,
1994, Ap & Sp
Sci, 212, 23)
Classical picture: Whether you
get a Jupiter or not is a contest
between building the core of icy
plantesimals and the star’s
blowing away the H & He.
If the star wins: no Jupiter
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.
Time to wakeup
for Coffee