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Download In the beginning… Astronomical Observations of Star Formation
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In the beginning… Today’s Lecture: Modern version of the creation story: how stars are born, and how our solar system formed EAS 302 1 Astronomical Observations of Star Formation Just as with humans, birth and death of stars is an “everyday” everyday” occurrence. Giant Nebulae (e.g., Tarantula Nebula in the Large Magellanic Cloud) Cloud) are the stellar nurseries. Stars form by collapse of parts of these great clouds of gas and dust. (These are also regions of nucleosynthesis since many of the stars born in them are massive and end their short lives as supernovae). EAS 302 2 1 Great Nebula in Orion EAS 302 3 Star Birth in the Orion Nebula EAS 302 4 2 “Proplyds” in Orion Nebula EAS 302 5 Edge on View of a Proplyd EAS 302 6 3 Formation of Our Solar System Widely accepted “nebular” or “protoplanetary” theory of solar system evolution has evolved from ideas of Immanuel Kant (1724-1804) and Pierre Laplace (17491827). Local instability in nebula causes gravitational collapse Shockwave from supernova might provide a trigger. Calculations predict collapse into central sphere Because of conservation of angular momentum, sphere (proto-star) surrounded by disk. To conserve angular momentum, gas must spin faster as it collapses. Observations, combined with physics-based calculations, yield following scenario of 4 stages EAS 302 7 EAS 302 8 4 Formation of the central star and disk EAS 302 9 Stage 1: Collapse of cloud ~ 106 yrs Temperatures reach 1500K in inner solar system heat due to gravitational collapse (conversion of kinetic to thermal energy), but is short-lived. EAS 302 10 5 Nebular Temperatures & Condensation Strong radial temperature gradient in inner solar system produces variation in the amount and composition of the dust in the disk Because of temperature most elements are in gas in inner most solar system In outer solar system, most elements are in dust (but most mass in gas). A key factor in planet formation is condensation of water (ice) , which makes grains sticky. EAS 302 11 Stage 2: Disk begins to dissipate Bipolar winds eject mass from system magnetic fields, asymmetry pull matter in, angular momentum out. Disk begins to cool, dust condenses. Some material heated during cycling close to star Material is accreted from disk to star Disk begins to clear from center outward Young star becomes visible in center of disk EAS 302 12 6 Stage 3: T-Tauri: Terminal Accumulation of Sun surface of star is now 4000 K Shines due to gravitational heating final stages of accumulation of star Dust is accumulating into planetesimals disk continues to dissipate Giant planets form planetesimal formation begins EAS 302 13 Stage 4: Residual Static Nebula Final dissipation of nebula (photoevaporation may help). Terrestrial planets accumulate from planetesimals Fusion ignites in stellar cores and star becomes ‘main sequence’ Process has taken ~10 million years for Sun-sized star (much less for giant stars). EAS 302 14 7 Summary Solar system formed 4.56 b.y. ago by collapse of a cloud of gas and dust Kinetic heating of inner solar system. Planets formed by oligarchic growth, starting with micron and mm-sized grains that coalesced to form larger and larger objects. Isotopic anomalies support the possibility that a Supernova may have triggered collapse of an interstellar cloud to form the solar system EAS 302 15 Let’s Compare the theory with observations EAS 302 16 8 Observations from Meteorites Primitive meteorites (chondrites) Presence of chondrules and refractory inclusions tell of at least locally high temperatures at the distance of the asteroid belt Differentiated meteorites Pieces of planetesimals that melted and differentiated to form iron cores and silicate mantles Ages of differentiated meteorites are within a few 10’s of millions of years of primitive meteorites Therefore: planetesimal-sized bodies formed and differentiated within a few 10’s of millions of years at most EAS 302 17 Data on the Planets Mass (kg) Sun 1.99!1030 Terrestrial Planets Mercury 3.35 !1023 Venus 4.87 !1024 Earth 5.98 !1024 Moon 7.35 !1022 Mars 6.42 !1023 asteroids 4 !1021 Giant Planets Jupiter 1.90 !1027 Saturn 5.69 !1026 Icy Planets Uranus 8.73 !1025 Neptune 1.03 !1026 Pluto 2.88!1022 Anhydrous Chondrit e s Radial distance (AU) Radius (km) 6.96!105 Density 1 atm (g/cc) density (g/cc) 1.4 Atmospheric components 1.6 2.44!103 6.05!103 6.38!103 1.74!103 3.39!103 2.8 !103 5.2 9.6 6.99!104 5.95!104 1.31 0.69 H, He H, He 19.1 30.8 39.4 2.54!104 2.13!104 1.15!103 1.30 1.76 ~2.0 3.4-3.9 H, He, CH4 H, He, CH4 — 0.39 0.72 1.0 5.42 5.24 5.52 3.3 3.93 5.3 3.95 4.03 3.4 3.7 — CO2, N2, Ar N2, O2, Ar — CO2, N2, Ar EAS 302 18 9 Terrestrial Planets: Mercury, Venus, Earth, and Mars Rocky (silicate) outer parts (crust and mantle) and inner cores of Fe-Ni metal. Silicates are compounds of silica (SiO2) and the oxides of other metals. Common silicates: Olivine: (Mg,Fe)SiO4 Pyroxene: (Mg,Ca,Fe)2 Si2O6 feldspar (e.g., (Na,K)AlSi3O8) mica (e.g., biotite K(Mg,Fe)3 AlSi3 O 10(OH)). EAS 302 19 The Giant Planets: Jupiter and Saturn Like the Sun, they consist primarily of H and He. Thus they are approximately “solar” in composition (though not exactly). Small rocky core overlain by metallic H layer, overlain by molecular H, He layer (gradual transitions from liquid to gas). EAS 302 20 10 The Icy Planets: Uranus and Neptune These too consist of largely of H and He, but they are enriched in C and N compared to Jupiter and Saturn. Their solid parts consist of ices of methane and nitrogen. EAS 302 21 So What? Pattern consistent with nebular model of solar system formation: the inner planets are extremely volatile-depleted compared to the solar composition. The giant planets are only slightly depleted in the most volatile components (H and He) compared to the Sun The outer planets, while distinctly depleted in H and He relative to the Sun, are nevertheless very rich in volatiles (C and N) compared to the Earth. EAS 302 22 11 Conclusions from this: The giant planets must have formed early, before the gas of the nebula dissipated. Position of Jupiter at the “snow line critical”: Condensation of water ice makes particles sticky, which greatly speeds accretion into larger bodies. Icy planets may form more slowly because of lower densities in outer part of nebula Process of oligarchic growth in inner solar system begins while inner solar system is still hot. Partial nebular dissipation results in H and He depletion The terrestrial planets are depleted not only in gaseous elements such as H, He, C, and N, but in “moderately volatile” elements as well. These include the alkalis (Na, K, Rb, Cs) and elements such as sulfur, lead, and indium. Planetesimal accretion took place before these elements could condense. Final assembly of the terrestrial planets took longer and was not complete until gas had cleared from inner solar system. EAS 302 23 Formation of the Terrestrial Planets Idea is that planets formed by “Oligarchic Growth”, i.e., progressive accretion. Collisions between bodies forms progressively larger bodies. Process is initially fast, but eventually slows when there are only a few bodies left in each orbital zone. Final stage of planetary accretion would involve collisions between large bodies. EAS 302 24 12 Lunar constraints on planet formation Moon is the only other planetary body we have directly sampled and explored Moon and Earth and closely related, both physically and chemically Therefore, they probably have a shared history ‘Hadean’ record preserved on the Moon Oldest rocks from the Moon are 4.45 Ga Record of the ‘early Earth’ (Hadean period) is missing, destroyed by subsequent events Oldest terrestrial mineral grain is 4.4 Ga EAS 302 25 The Moon: some key observations No other planet has such a large moon relative to its size (except Pluto). Moon has only a very small iron core Moon has a bulk density about the same as the Earth’s mantle (suggests compositional similarity). Highly depleted in highly volatile elements (gaseous elements); depleted in moderately volatile elements. Has identical oxygen isotope composition to the Earth Bottom line: Earth and Moon are both similar and different. EAS 302 26 13