Astronomy: Perspective

Astronomy: Perspective

Bob Rood

U. Virginia

The Milky Way might look like this.

It contains billyuns & billyuns of stars

25,000 lySun

Green Bank Scale Model

On GB scale model 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

SN1054

Supernovae & other stars make heavy elements.

Molecules

Volatile

Hydrogen H2

Water H2O

Carbon monoxide CO

Carbon dioxide CO2

Methane CH4

Ammonia NH3

Refractory

Silicon dioxide SiO2

Number of atoms per 10,000,000 of

hydrogen

hydrogen 10,000,000 sulfur 95

helium 1,400,000 iron 80

oxygen 6,800 argon 42

carbon 3,000 aluminum 19

neon 2,800 sodium 17

nitrogen 910 calcium 17

magnesium 290 all other elements 50

silicon 250

“Heavy or metals”

• 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

Age (Gyr)

Meta

llici

ty

Metallicity built up rapidly and has remained almost constant

Emission nebula

Reflection Nebula

Dust lanes

Embedded newly formed stars

Star formation continues in Giant Molecular Clouds

The 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

Gas being entrained in the Solar windDust being blown away

by Solar radiation pressure

Closeup of an evaporating icy planetesimal:The nucleus of Comet Halley in 1986

Nucleus covered with a layer of black crud

Gas boils out of cracks

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 changesby 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

ImpactsOn the 108 year timescale there is an impact large enough to lead to a major extinction event.KT event:Bad for dinosaursGood 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 mini-extinction event.

Galactic -ray burst

A -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 -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 -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 -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

Fragments of Comet Shoemaker-Levy 1993

Last big accretion event in the Solar System.

An ETI Gotcha

Jupiter eats comets

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