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CH11 - Astronomy 1F03 2010/11 Fall Term 2010/11...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 11 Stars & The Interstellar Medium H-R Diagram: Features Main Sequence: Red Dwarfs up to Blue Giants Blue Most Stars lie on MS MS A Fair Sample: Nearby Stars Fair Stars within 5 pc of the Sun Nearby stars are mostly small and red mostly Stars – snapshots in time The Main Sequence stands out because stars spend most time there stars Stars on the Main Sequence are compact, stable and in equilibrium compact, Young Stars Young Stars must evolve through a pre-main Stars main sequence phase sequence New stars form from collapsed gas clouds (c.f. solar nebula idea) clouds To find young stars we have to look into the Interstellar Medium… into The Interstellar Medium The Floating between the stars (white points) are clouds of gas and dust of The Interstellar Medium: ISM The The Interstellar Medium is very empty by Earth standards: is 1 particle per cc Compare: Water: 1023 per cc Water: Air: 1019 per cc Air: Best Earth Vacuum: 1000 per cc Best Gas: Composition Gas: Mostly Hydrogen (3/4 by weight) Mostly 90% of gas particles Helium: 9% of particles Helium: Other: 1% Other: Similar to Sun or Jovian Planets Dust Dust Dust is similar to soot or smoke Dust It is made of the 1% “other” Carbon, Oxygen, Silicon, Magnesium, Iron noticeably depleted from the gas – assumed to have formed dust Effects of dust Effects Dust absorbs/scatters light effectively, better than gas better Dust absorbs bluer light most Dust Colours appear redder though dust: Colours Stars Stars Sunsets Sunsets Effects of dust Effects Spectral lines identifiable despite dust reddening – can still what can kind of star it is kind Colour becomes Colour becomes redder – can can use to detect dust in front dust ISM: Distribution ISM: Non-uniform distribution Non Forms clouds with wide range of densities, favour disk of Milky Way favour Densest clouds form stars… Densest Milky Way Stars Dust Clouds Star Forming Regions Star The Gas is normally hard to see because it is cold (under 100 K) because Near sources of heating (e.g. young stars) the gas is heated and glows stars) Illuminated gas clouds are called Emission Nebulae Emission Nebulae Nebulae The Triffid Triffid nebula nebula glowing in the light of young hot stars stars Nebulae Nebulae Hot Young Stars (e.g. O,B) ionize and blow away nearby gas blow The gas emits characteristic pinkish light of Hydrogen gas light Lagoon Nebula Lagoon and the “Hourglass” Massive Hot Stars & Nebulae Massive Why are these hot stars and gas so often Why near each other? near O and B stars are short-lived and They live and die near the gas clouds they formed in they Cloud dispersion Cloud The clouds are heated/illuminated by young stars young Bright, fuzzy gas is visible blowing off the cloud the Pillars of M6 HST Image Dark Dust Clouds Dark 99% of Interstellar space is dark but not empty not No visible emission because too cold (100K or less) (100K Detected via: Silhouettes Silhouettes 21 cm radiation… 21 Horsehead Nebula 21 cm Radiation 21 Electrons and Protons have spin Electrons Flipping to opposite spins emits a 21 cm wavelength photon wavelength This allows us to see Atomic Hydrogen This Long wavelength unaffected by dust – can see very far Molecular Gas Molecular Denser clouds for molecules Denser Naturally mostly hydrogen: H2 Naturally Hydrogen Molecules hard to detect Hydrogen Observe tracer molecules at radio wavelengths: wavelengths: Carbon Monoxide, Hydrogen Cyanide, Ammonia, Water, Formaldehyde Ammonia, Molecular Gas Molecular Dense Molecular Gas forms complexes 50 parsecs 50 Up to millions of solar masses Up Dense: 1 million particles per mL mL Extremely Cold: 20 K Extremely Likely sites for star formation Overview: Overview: Densities in the ISM Density Temperature 100-1000 K 100 K 10-20 K 20-2000 K 1000-100 million K (particles per cc) Average medium Atomic Clouds Molecular Clouds Solar Nebula Star 1 1000-1 million 1000 1 million to 1 billion 10 billion-1015 1020-1029 Compare: Water: 1023 per cc (1 gram per cc) Water: Air: 1019 per cc (0.001 grams per cc) Air: Densities in the ISM Densities Density Temperature 100-1000 K 100 K 10-20 K 20-2000 K 1000-100 million K (particles per mL) (particles mL Average medium Atomic Clouds Molecular Clouds Solar Nebula Star 1 1000-1 million 1000 1 million to 1 billion 10 billion-1015 1020-1029 Interstellar Material Lower density ISM not strongly affected by self-gravity Densities in the ISM Densities Density Temperature 100-1000 K 100 K 10-20 K 20-2000 K 1000-100 million K (particles per mL) (particles mL Average medium Atomic Clouds Molecular Clouds Solar Nebula Star 1 1000-1 million 1000 1 million to 1 billion 10 billion-1015 1020-1029 Star Formation Process Molecular clouds feel their own gravity Molecular Gas Clouds Molecular Dense Molecular Gas forms complexes 50 parsecs 50 Up to millions of solar masses Up Dense: 1 million particles per mL mL Extremely Cold: 20 K Extremely Dense enough to feel own gravity and undergo Dense runaway collapse runaway Infrared image of Perseus Perseus Molecular Molecular cloud complex complex (IRAS – (IRAS infrared) Lifetime of Molecular Clouds Lifetime Molecular Clouds are cold and dense Molecular Their pressure can’t hold them up Their hold against gravity against Lifetime of Molecular Clouds Lifetime Molecular clouds are turbulent and contain magnetic fields magnetic This support dissipates in This 1-10 million years Simulation by Simulation Tilley & Pudritz Pudritz Molecular Cloud collapse Molecular Given enough resolution it is possible to simulate collapse of clouds down to spinning disks and starlike starlike blobs Bate et al simulation Bate et Fixed temperatures Simulations: Cautionary Tale Simulations: Top simulations are expensive: up to 1,000,000 hours of supercomputer time 1,000,000 Even so, it is normally not possible to include all important physics and still finish in a reasonable time reasonable Simulators take shortcuts (e.g. Bate): Simulators Constant temperatures Constant No dust No Fixed sized stars Fixed No radiation No Star Formation in a nutshell Star The defining characteristic of a star is nuclear burning nuclear Nuclear burning requires confined high temperatures: 10,000,000 K temperatures: Star formation therefore involves collapsing ISM gas clouds down to dense hot balls: stars dense Slowing down collapse Slowing Collapsing gas heats up as gravitational energy is released gravitational Heat leads to pressure that can oppose gravity to stop collapse oppose The time to collapse can depend on the time it takes to radiate away the heat away Stages of Star Formation Stages 1. Molecular Cloud Initial Size: 1014 km (3-30 parsecs) Initial Cold cloud collapses Heat continuously radiated away (Temperature 10K) (Temperature The cloud fragments… The Gravitational Fragmentation Gravitational For a specific temperature and density there is a minimum mass that can collapse called the Jeans Mass Jeans For a constant temperature cloud the Jeans Mass shrinks as the cloud collapses Mass Parts of the cloud collapse faster than the cloud overall cloud The cloud breaks up into fragments The Jeans Mass and collapse Jeans Simulations by Simulations David Tilley David Small Jeans Mass Small (high density) (high Large Jeans Mass Large (low density) (low Stages of Star Formation Stages 2. Cloud cores Size: 1012 km (0.1 pc, 10,000 AU) Size: Temperature starts to rise ~ 100 K because radiation no longer escapes easily easily The cloud no longer fragments The This core will form a star system (perhaps a binary) (perhaps Cloud Cores Cloud Simulated Cloud cores cores Richard Klein, Berkeley Stages of Star Formation Stages 3. Protostellar Nebula 3. Protostellar Size 1010 km (100 AU = Solar Nebula) Size Rotation is starting to dominate Rotation Central regions quite opaque to radiation radiation Central temperature rises to 10,000 K Central Protostar powered by gravity not fusion Protostar Orion Nebula Orion Protostars Protostars Star dimly visible Star Very red within dusty disk dusty Stages of Star Formation Stages 4. Protostar 4. Protostar Size: 108 km (0.4 AU = Mercury’s orbit) Size: Gravity powered still Gravity Very luminous, red Very Temperature: 1,000,000 K centre, few 1000 K surface 1000 Slowly contracting… Slowly Proto-star on Proto star the H-R diagram the The protostar can now protostar can be placed on the Hertzsprung-Russell Hertzsprung Russell diagram diagram Redder and more luminous than final main sequence position main Stages of Star Formation Stages 5. T-Tauri Star Size: 107 km (10x Sun) Size: Star collapses further Hotter core and surface Hotter Strong winds and outflows as last material falls onto star falls “T-Tauri” phase named phase after first star of this type known type Herbig-Haro objects Outflows Outflows Outflows allow the star to shed excess rotation and excess energy excess Outflows are associated with material falling onto the star the Star collapse overview Star The process is initially as fast as it takes to fall it It slows because it takes longer for the denser protostar to shed protostar to the heat of contraction contraction Stages of Star Formation Stages 6. Star ignition Size: 2 million km (1.5x Sun) Size: Surface temperature: 4500 K (red) Surface Central temperature 10,000,000 K Central Nuclear burning starts Nuclear Star contracts towards main sequence position position Stages of Star Formation Stages 7. Zero Age Main Sequence Size: 1.4 million km (Sun) Size: Surface Temperature 5500 K (yellow) Surface Central temperature 15,000,000 K Central 10 billion years of hydrogen burning ahead… ahead Pre-Main Sequence Evolution Pre For a Solar Mass Star Heating and contraction until nuclear fusion starts nuclear Do we see stars doing this on the H-R this diagram? diagram? Obscured Stars Obscured In general there is a lot of gas and dust around young stars around Often they are easier to see in the infrared to Large stars can remove dust and gas and are seen earlier and Orion New Star Cluster New Pre-Main Sequence Evolution Pre at different masses Larger stars contract faster contract Heat up and begin nuclear reactions sooner sooner Failed Stars: Brown Dwarfs Failed Some stars never get hot enough to start nuclear burning start Cut-off 0.08 Solar Masses (80 Jupiters) Cut off Jupiters > 0.08 star, < 0.08 brown dwarf 0.08 These stars slowly cool and get fainter over time over Star Clusters Star Stars form from large clouds with 1000Stars 1 million solar masses We expect stars to form in groups called star clusters star Pleiades cluster Star Clusters Star Open Cluster: 100-10,000 stars Open Association: Loose collection of few Association Loose 100 stars, drifting apart 100 These clusters tend to be young and seen in the disk of the galaxy seen Young Clusters Young Young star clusters still have large blue stars have The main sequences extends far to the blue in the H-R diagram in Globular Clusters Globular Globular clusters are tight clusters of 10,000’s of stars 10,000 They are most often found far out in the galaxy galaxy Globular clusters are almost always very old almost Old Star Clusters Old Large blue stars burn out relatively quickly relatively Stars of 1 solar mass or larger are gone in 10 billion years billion Old star clusters have only a red main sequence only Star Clusters and Star Star Formation Efficiency Large stars interact strongly with gas clouds Most emission nebulae are associated emission are with sites of star clusters (1000’s of with of stars) stars) Nebulae indicate regions where molecular clouds are being destroyed by star clusters destroyed Star Clusters and Star Star Formation Efficiency Around nebulae most of the gas is pushed away and/or ionized before it can form stars can This prevents complete conversion of gas into stars – star formation is star inefficient inefficient Gas clouds: maybe 20% into stars, 80% dispersed dispersed ...
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