Transcript rood_ozma50
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