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Solar Nebula Theory Basic properties of the Solar System that need to be explained: 1. All planets orbit the Sun in the same direction as the Sun’s rotation 2. All planetary orbits are confined to the same general plane 3. Terrestrial planets form near the Sun, Jovian planets further out Other aspects include: - similar direction of rotation - ring systems - asteroid/Kuiper belt - formation of natural satellites - angular momentum problem Star Formation Interstellar Medium (ISM): Gas & dust between stars (100 – 200 atoms/cm3) Composition Gas: 70% H (neutral H, H+, H2) 22 – 25% He 3 – 5% “metals” Dust: Silicate grains (rock/sand) Graphite (Carbon) Basic organic material Star Formation Nebula (Large cloud of ISM) - low density (200 atoms/cm3) - T ~ 20 K - D ~ 150 L.Y. - M ~ 104 – 106 MSuns Gravity causes: - globule contraction - material accumulation - central region to heat up Rotation causes: - Densest region to become spherical (proto-star) - Outer gases cast off into proto-planetary disk Proto-planetary Disks in Orion Condensation Sequence Nebula material is uniformly spread through the proto-planetary disk Different materials condense at different temperatures which leads to rock/metal worlds close to Sun & gas giants further away Solar Nebula Theory Inner disk: metals/silicates (Terrestrial Planets) Outer disk: ices and gases (Jovian Planets) Habitable Zone: region around a star where H2O can exist as liquid Proto-Sun & Proto-planetary Disk Accretion Dust grains stick together through electricity to form planetesimals Collisions between planetesimals melt surface which acts as a “glue” causing them to grow larger Largest planetesimals pull in material through gravity and become proto-planets Terrestrial Planet Formation Terrestrial - Interior heating causes differentiation; leads to layered interior - Primitive H, He atmospheres heated away - Out-gassing creates CO2 dominated atmosphere - Water vapor washes out CO2; life forms oxygenate atmosphere Jovian Planet Formation Jovian - Gases (H, He) and ices (NH3, CH4) accrete more quickly which causes planets to grow more massive - Once Sun ignites, solar wind sweeps gases and dust from Solar System Formation of the Moon Large Impact Theory 1) Mars-sized object strikes Earth early on in its history vaporizing crust 2) Debris settles into a disk that lies within the ecliptic plane 3) Material accretes to form the Moon Explains Moon’s age, density, orbit, & lack of water Star Formation A protostar’s mass will determine the star’s: !Position on - temperature (Spectral Class) Main Sequence - luminosity (Absolute Magnitude) - length of protostar stage (in fact, all stages) A star’s evolution depends on how massive it is: Low Mass (0.08 MSuns < M < 2 MSuns) Intermediate Mass (2 MSuns < M < 8 MSuns) High Mass (M > 8 MSuns) We’ll trace Sun’s evolution then compare more massive stars’ to it Star Formation HR Diagram is used to map out each star’s evolutionary cycle. 1. Cool protostar collapses under gravity 2. Pressure inside builds up which increases temperature 3. Protostar shrinks & heats up more 4. Once Tcore ~ 15 million K, H " He fusion reactions start in core Stage 1: Protostar Protostar Protostar’s mass determines R, T, L, & formation time. Higher mass # more gravity • larger in size • hotter • more luminous • quicker formation Vice versa for lower mass stars Upper: 120 MSun # Prad too great Lower: 0.08 MSun # too cool for H-fusion to begin (Brown Dwarf) Stage 2: Main Sequence A STAR IS BORN!!! (Once fusion reactions in the core begin & the star stabilizes) Hydrostatic Equilibrium: Equilibrium Inward Gravity = Outward Pressure Main Sequence Longest stage (90%) of star’s life Core Hydrogen gradually replaced with Helium (Presently the is Sun’s core is 35% H, 64% He) Solar Structure Solar Structure We cannot see beneath the surface, but we create a model based on computer simulations & physical equations Solar Core Diameter = 280,000 km (0.2 DSun) Density = 160 g/cm3 Tcore = 15 million Kelvin Temperature is high enough to: • Create a fully ionized plasma • Generate energy through nuclear fusion Nuclear Fusion Nuclear Fusion 4 H " 1 He (+ 2 e+ + 2! + 2") Mass of 4 H nuclei > mass of 1 He nucleus Difference in mass is converted into energy. $ m " energy according to E = mc2 Astronomers estimate that the Sun will run out of core Hydrogen in a little under 10 Billion years. Radiation Zone (350,000 km region that surrounds core) Energy is transported by absorption/emission of photons by atoms: • photons are emitted in random directions Radiation Zone Core Atoms How long does it take the average photon to make it out of the Sun? ~ 1 Million Years!!! Convection Zone (210,000 km above radiation zone) • energy is transported mostly through convection currents Hot gas (bright) rises and heats surroundings; cool gas (dark) sinks Photospheric Granulation Photosphere • marks the Sun’s outer boundary (500 km) • photons emitted into space (“light sphere”) • Solar spectrum made from gases here Chromosphere • Gaseous region (2300 km) above photosphere • Appears red, red due to strong H% emission Can only view using proper filter (H%) or during a Solar Eclipse Note: Helium was first observed in here Corona Large (106 km), hot (106 K) region of diffuse gas surrounding Sun • Heated to such high temperatures by the Sun’s magnetic field • Visible during a total solar eclipse or with use of coronagraph Solar Wind Continual stream of high energy ions and electrons that travel through interplanetary space at speeds of ~ 300 km/s. Earth’s magnetic field deflects the solar wind Sunspots Sunspots: large, dark areas that appear on the photosphere Central dark region (umbra) umbra surrounded by flowing gases (penumbra) penumbra D ~ 10,000 – 50,000 km; T ~ 4000 – 5000 K Spectrum of sunspot indicates magnetic influence Solar Dynamo Differential rotation causes magnetic field to wrap around Sun, forming kinks that penetrate the photosphere Solar Cycle Continual wrapping # more “kinks” # more activity Once max. occurs, magnetic field re-stabilizes (with magnetic poles reversed) and the cycle begins again. Solar Activity Solar Flares – violent eruptions of solar material (106 H-bombs) Coronal Mass Ejection High energy explosions (1012 H-bombs) resulting from coronal gas getting trapped within tangled magnetic kinks Coronal Mass Ejections If directed toward Earth, they can: - Cause power outages - Disrupt satellite function - Harm astronauts - Destroy atmosphere/ozone? Red Giant Once the core Helium supply runs out: - Inert Carbon core is left behind and starts to contract - Helium just above core fuses into Carbon (He-burning shell) (H-burning shell remains above) - Excess heat causes the star to expand (2nd Red Giant stage) 2nd Red Giant Stage Low mass stars won’t get hot enough to start fusing Carbon Post-Main Sequence (Intermediate/High Mass Stars) Same process occurs for higher mass stars when core Hydrogen runs out but the stars become Supergiants (Stage 3) Hot enough to fuse Carbon and heavier elements New Supergiant stage begins with every core fusion reaction Supergiants Multiple layers of fusion shells form around the core Iron (Fe) is the last element to be created inside stellar cores Supergiants can become highly unstable due to imbalance between Inward Gravity & Outward Pressure Pulsating Variable Stars Stars found in the Instability Strip of the HR Diagram experience a cyclic change in size & temperature # changes in luminosity.