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Astronomy 305 Frontiers in Astronomy Professor Lynn Cominsky Department of Physics and Astronomy Offices: Darwin 329A and NASA EPO (707) 664-2655 Best way to reach me: [email protected] 9/23/03 Prof. Lynn Cominsky 1 How do stars evolve and planets form? Properties of stars Life cycles of stars Solar systems Planet formation 9/23/03 Prof. Lynn Cominsky 2 The Nearest Stars Distance to Alpha or Proxima Centauri is ~4 x 1011 km or ~4.2 light years Distance between Alpha and Proxima Centauri is ~23 AU 9/23/03 Prof. Lynn Cominsky 3 The Solar Neighborhood Some stars within about 2 x 1014 km (~ 20 light years) 9/23/03 Prof. Lynn Cominsky 4 Classifying Stars Hertzsprung-Russell diagram 9/23/03 Prof. Lynn Cominsky 5 Classes of Stars Bigger stars are brighter than smaller stars because they have more surface area Hotter stars make more light per square meter. So, for a given size, hotter stars are brighter than cooler stars. • White dwarfs - small and can be very hot (Class VII) • Main sequence stars - range from hotter and larger to smaller and cooler (Class V) • Giants - rather large and cool (Class III) • Supergiants - cool and very large (Class I) 9/23/03 Prof. Lynn Cominsky 6 Properties of Stars Temperature (degrees K) - color of star light. All stars with the same blackbody temperature are the same color. Specific spectral lines appear for each temperature range classification. Astronomers name temperature ranges in decreasing order as: O B A F G K M Surface gravity - measured from the shapes of the stellar absorption lines. Distinguishes classes of stars: supergiants, giants, main sequence stars and white dwarfs. 9/23/03 Prof. Lynn Cominsky 7 Populations of Stars Population I – young, recently formed stars. Contain more metals than older stars, as they were created from debris from previous stellar explosions. Population II – older stars that have evolved and are almost as old as the Universe itself. Population III – the original stars that were formed about 200 million years after the Big Bang. They should be nearly all H and He 9/23/03 Prof. Lynn Cominsky 8 Life Cycles of Stars 9/23/03 Prof. Lynn Cominsky 9 Life Cycles of Stars 9/23/03 Prof. Lynn Cominsky 10 The very first stars Simulations by Tom Abel, Mike Norman and Greg Bryan 13 million years after the Big Bang, a piece of the Universe has collapsed due to a slightly higher density of dark matter. It forms a 100 million solar mass protogalaxy, and at the center of this protogalaxy, a star is born! Density movie Temperature movie 9/23/03 Prof. Lynn Cominsky 11 Life and death of the very first star From The Unfolding Universe, directed by Tom Lucas, simulation by Tom Abel 9/23/03 Prof. Lynn Cominsky 12 Molecular clouds and protostars Giant molecular clouds are very cold, thin and wispy– they stretch out over tens of light years at temperatures from 10-100K, with a warmer core They are 1000s of time more dense than the local interstellar medium, and collapse further under their own gravity to form protostars at their cores Simulation with narration by Jack Welch (UCB) Orion in mm radio (BIMA) 9/23/03 Prof. Lynn Cominsky 13 Protostars Orion nebula/Trapezium stars (in the sword) About 1500 light years away HST/ 2.5 light years 9/23/03 Chandra/10 light years Prof. Lynn Cominsky 14 Stellar nurseries Pillars of HST/Eagle Nebula in M16 dense gas Newly born stars may emerge at the ends of the pillars About 7000 light years away 9/23/03 Prof. Lynn Cominsky 15 Main Sequence Stars Stars spend most of their lives on the “main sequence” where they burn hydrogen in nuclear reactions in their cores Burning rate is higher for more massive stars - hence their lifetimes on the main sequence are much shorter and they are rather rare Red dwarf stars are the most common as they burn hydrogen slowly and live the longest Often called dwarfs (but not the same as White Dwarfs) because they are smaller than giants or supergiants Our sun is considered a G2V star. It has been on the main sequence for about 4.5 billion years, with another ~5 billion to go 9/23/03 Prof. Lynn Cominsky 16 Stars that are not likely to have solar systems that can support life White dwarfs neutron stars black holes Wildly variable stars Planetary nebulae Very young stars Stars in clusters Stars in binaries or triple systems 9/23/03 Prof. Lynn Cominsky 17 How stars die Stars that are below about 8 Mo form red giants at the end of their lives on the main sequence Red giants evolve into white dwarfs, often accompanied by planetary nebulae More massive stars form red supergiants Red supergiants undergo supernova explosions, often leaving behind a stellar core which is a neutron star, or perhaps a black hole (see Lecture 2) 9/23/03 Prof. Lynn Cominsky 18 Red Giants and Supergiants Hydrogen burns in outer shell around the core Heavier elements burn in inner shells 9/23/03 Prof. Lynn Cominsky 19 White dwarf stars Red giants (but not supergiants) turn into white dwarf stars as they run out of fuel White dwarf mass must be less than 1.4 Mo White dwarfs do not collapse because of quantum mechanical pressure from degenerate electrons White dwarf radius is about the same as the Earth A teaspoon of a white dwarf would weigh 10 tons Some white dwarfs have magnetic fields as high as 109 Gauss White dwarfs eventually radiate away all their heat and end up as black dwarfs in billions of years 9/23/03 Prof. Lynn Cominsky 20 Neutron Stars Neutron stars are 10 km in radius (about the size of San Francisco) and about 1.4 Mo One teaspoon of NS material weighs 100 million tons! Neutron stars can have magnetic fields as strong as 1013 Gauss Neutron stars can rotate as fast as 1000 times per second Neutron stars do not collapse because of quantum mechanical pressure from degenerate neutrons If Neutron stars accrete too much material, and go over 3 Mo they can collapse to a black hole 9/23/03 Prof. Lynn Cominsky 21 Planetary nebulae Planetary nebulae are not the origin of planets Outer ejected shells of red giant illuminated by a white dwarf formed from the giant’s burnt-out core Not always formed 9/23/03 HST/WFPC2 Eskimo nebula 5000 light years Prof. Lynn Cominsky 22 Variable stars Most stars vary in brightness Periodic variability can be due to: Eclipses by the companion star Repeated flaring Pulsations as the star changes size or temperature Novae are stars which repeatedly blow off their outer layers in huge flares Flare stars have regions which explode Pulsating stars have an unstable equilibrium between the competing forces of gas pressure and gravity 9/23/03 Prof. Lynn Cominsky 23 Pleiades Star Cluster A star cluster has a group of stars which are all located at approximately the same distance The stars in the Pleiades were all formed at about the same time, from a single cloud of dust and gas This is a very young system (25 million years) 9/23/03 Prof. Lynn Cominsky D = 116 pc 24 Open Star Clusters Open Cluster NGC 3293 d = 8000 c-yr 20 -1000 stars diameter ~ 10 pc young stars (Pop I) mostly located in spiral arms of our Galaxy and other galaxies solar metal abundance 9/23/03 Prof. Lynn Cominsky 25 Globular Star Clusters Globular Cluster 47 Tuc d=20,000 c-yr 104 - 106 stars diameter ~ 30 pc centrally condensed old stars (Pop II) galaxy halo low in metals 9/23/03 Prof. Lynn Cominsky 26 How planets form Our solar system Architecture Formation of inner solar system Formation of Moon Formation of outer solar system Disks around stars 9/23/03 Prof. Lynn Cominsky 27 Formation of the Solar System Activity Examine the figures and tables that are provided in the handout Answer the questions on the worksheet Feel free to discuss them with your neighbor! 9/23/03 Prof. Lynn Cominsky 28 Solar system architecture The planets are isolated from each other without bunching, and they are placed at orderly intervals The planets' orbits are nearly circular, except for those of Mercury and Pluto. Their orbits are nearly in the same plane; Mercury and Pluto are again exceptions. All the planets and asteroids revolve around the Sun in the same direction that the Sun rotates (from west to east). 9/23/03 Prof. Lynn Cominsky 29 Solar system architecture Except for Venus, Uranus, and Pluto, the planets also rotate around their axes from west to east. Studies of chemical composition suggest that the small, dense Terrestrial planets are rocky bodies that are poor in hydrogen; the large, low-density Jovian planets are fluidlike bodies that are rich in hydrogen; and most of the outer planets' satellites, comets, and Pluto are icy bodies. 9/23/03 Prof. Lynn Cominsky 30 Solar system architecture The Terrestrial planets have high mean densities and relatively thin or no atmospheres, rotate slowly, and possess few or no satellites--points that are undoubtedly related to their smallness and closeness to the Sun. The giant planets have low mean densities, relatively thick atmospheres, and many satellites, and they rotate rapidly--all related to their great mass and distance from the Sun. 9/23/03 Prof. Lynn Cominsky 31 Formation of the solar system Animation shows a simplified model 9/23/03 Prof. Lynn Cominsky 32 Solar system formation Protoplanetary Nebula hypothesis: Fragment of interstellar cloud separates Central region of this fragment collapses to form solar nebula, with thin disk of solids and thicker disk of gas surrounding it Disk of gas rotates and fragments around dust nuclei– each fragment spins faster as it collapses (to conserve angular momentum) Accretion and collisions build up the mass of the fragments into planetesimals Planetesimals coalesce to form larger bodies 9/23/03 Prof. Lynn Cominsky 33 Solar System Formation Formation of the Sun Solar nebula central bulge collapsed to form protosun Contraction raised core temperature When temperature reaches 106 K, nuclear burning can start Solar winds could have blown away remaining nearby gas and dust, clearing out the inner solar system 9/23/03 Prof. Lynn Cominsky 34 Formation of Inner Planets While the terrestrial planets formed (and shortly thereafter), they were bombarded by many planetesimals Bombardment made craters and produced heat which melted the surfaces, releasing gases to form atmospheres, and forming layered structures (core, mantle, crust) Additional heat provided by gravitational contraction and radioactivity 9/23/03 Prof. Lynn Cominsky 35 Elements in the Planets Chemical composition at formation depended on temperature (mostly determined by distance from Sun) Asteroid belt had lower temperature, so carbon and water-rich minerals could coalesce in the planetesimals From Jupiter outwards, temperatures were much lower, so frozen water coalesced with frozen rocky material, or at even lower temperatures, frozen methane or ammonia 9/23/03 Prof. Lynn Cominsky 36 Formation of Moon Lunar samples from Apollo revealed the similarity (but some differences) between the materials in the Earth’s crust and mantle and the Moon Collisional ejection would explain these similarities – a Mars sized body impacts the cooling Earth – part is absorbed, part splashes out material which cools to form the Moon Problems remain with the lunar orbital plane vs. the equatorial plane of the Earth 9/23/03 Prof. Lynn Cominsky 37 Formation of Earth’s Moon Simulation shows formation of Moon due to impact on Earth 9/23/03 Prof. Lynn Cominsky 38 Formation of Outer Planets In the outer, cooler regions, icy planetesimals collided and adhered. Hydrogen and helium were then accreted onto these Earth-sized bodies. More H and He adhere to larger bodies, explaining their relative lack in Uranus and Neptune Uranus and Neptune are richer in heavier elements such as C, N, O, Si & Fe 9/23/03 Prof. Lynn Cominsky 39 Formation of Outer Planets Formation of moons of Jupiter and Saturn are mini-versions of the solar system evolution Heat from Jupiter when it formed resulted in inner moons that are rocky, and outer moons that are icy Comets and Kuiper belt objects are remnants of original icy planetesimals, located far from Sun 9/23/03 Prof. Lynn Cominsky 40 Rings Saturn has 7 named Jupiter has faint rings (A-F) dark rings A-ring B-ring Encke division 9/23/03 Cassini division Prof. Lynn Cominsky 41 Rings Uranus has 11 Neptune has 3 dark rings known rings HST image of Uranus and its rings 9/23/03 HST image of Neptune Prof. Lynn Cominsky 42 Formation of Rings Rings appear too young to be primordial – maybe only 108 y - i.e., they must have formed after the planets Rings are ubiquitous in the outer planets – whereas we once thought they were rare (only Saturn had rings) Perhaps collisions between moons and interlopers provides material for the rings – seems to work for Uranus and Neptune, but not for Jupiter and Saturn 9/23/03 Prof. Lynn Cominsky 43 Formation of Rings Saturn’s rings have a resonant relationship with its satellites – i.e., the satellites sweep out gaps between the rings and create fine structure in the patterns seen in the rings A-ring Resonance – the satellite Janus orbits Saturn 6 times while the ring material orbits 7 times, creating a six-lobed structure at the ring’s outer edge Cassini gap – Mimas has a 2:1 resonance with the outer edge of the B-ring at the gap 9/23/03 Prof. Lynn Cominsky 44 Disks around stars There is much evidence of disks with gaps (presumably caused by planets) around bright, nearby stars, such as Beta Pic 9/23/03 Prof. Lynn Cominsky 45 Web Resources Astronomy picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html Imagine the Universe http://imagine.gsfc.nasa.gov Ned Wright’s ABCs of Distance http://www.astro.ucla.edu/~wright/distance.htm National Geographic Star Journey http://www.nationalgeographic.com/features/97/stars/i ndex.html Zoom Star Types Site http://www.enchantedlearning.com/subjects/astronomy/stars/startypes. shtml 9/23/03 Prof. Lynn Cominsky 46 Web Resources John Blondin’s supercomputer models http://www.physics.ncsu.edu/people/faculty.html Cepheid variables http://zebu.uoregon.edu/~soper/MilkyWay/cepheid.html U Washington Star Age Lab http://www.astro.washington.edu/labs/clearinghouse/labs/Clusterhr/ color_mag.html First star simulations http://cosmos.ucsd.edu/~tabel/GB/gb.html Molecular cloud - protostar simulations http://archive.ncsa.uiuc.edu/Cyberia/Bima/StarForm.html 9/23/03 Prof. Lynn Cominsky 47