CH14 - Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson...

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Unformatted text preview: Astronomy 1F03 2010/11 Fall Term 2010/11 Chaisson & McMillan, Astronomy Chapter 14 The Milky Way The Milky Way The Milky Way Magellanic Clouds The Milky Way Galaxy Galaxy The Sun and all the stars we see in the night sky inhabit a large collection called “the Galaxy” or the Milky Way or Galaxy Galaxy We can see the vast number of stars in Milky Way stretched across the sky in a band band The Milky Way Galaxy The The Milky way is a flat disk with spiral patterns There is a bulge of redder stars in the bulge of centre centre A thin halo of old halo of stars is around the around the outside around Andromeda: Nearest External Spiral Nearest Galaxy Galaxy The Milky way Galaxy The The Milky way contains billions of stars, gas and dust gas There are vast gaps of empty space between it and other nearby galaxies between Other galaxies mostly appear as tiny smudges on the sky The idea of a Galaxy The The Milky Way Galaxy is a large collection of stars including the Sun collection It is a modern idea, less than 100 years old old What distinguishes galaxies within a larger universe? larger History: History: Beyond the Solar System… The Big Questions of 300 years ago: How are stars distributed around the Sun? Sun? Is there a preferred direction or shape? Is Does it go on forever – just more and just more stars? more Where the Sun lives… Where Historically it was felt that the Universe was probably an endless sea of stars was There was no way to measure distances to stars to It was realized that there are more stars in some directions than others…. stars Herschel’s Universe: Herschel Star Counts William Herschel (18th C) counted the William C) number of stars in a every direction to estimate the shape of the “Universe” The Sun Herschel’s Universe Herschel’s Universe Herschel Assumed uniform star density Assumed Comparing to reality: Comparing Edge is Edge apparent: Dust hides the stars hides Edge is real: Edge Ran out of stars Ran Our local universe (the Our Galaxy) is a dusty disk of stars of Measuring the Galaxy: Measuring The distance Ladder Ways of measuring Ways distance distance “spectroscopic spectroscopic parallax”: star clusters parallax star within 10,000 parsecs within Ideally, we’d llike a way ike Ideally, to tell how bright a star is even at Galactic distances distances We can use variable stars… stars Variable Stars Variable For some stars, nuclear burning is not stable stable The star varies its brightness in a regular way over time way Detecting Variables: Two observations at Two different times different Variable Stars Variable For RR-Lyrae and RR and Cepheid variable stars, the period of the variation indicates the luminosity (absolute magnitude) (absolute This can be used to measure distance measure Discovered by Henrietta Leavitt (1908) (1908) Cepheids are are brightest brightest The Size of the Milky Way The Within the halo of stars around the Milky Way disk there are old globular Milky globular clusters clusters From 1915-1919 Shapley used RRFrom 1919 RR Lyrae variables to measure their variables distances distances They form a 30,000 parsec distribution around the Milky way around Shapley’s Galaxy: Shapley Globular Clusters Shapley assumed that globular clusters trace the overall shape of the galaxy the This puts the Sun 8000 parsecs from the centre of the Milky Way from Note: 8 kpc = 8000 parsecs kpc 1 pc = 3.26 Light years, pc 8 kpc = 26000 light years kpc Island Universes: Island The Great Debate of 1920 Shapley felt the Galaxy was the whole universe whole The debate was whether the nebulae (non-point like fuzzy places nebulae point on the sky) were all near objects or other “island universes” At first galaxies were called “iisland sland universes” universes Shapley vs. Curtis The Great Debate: Conclusion The In 1921 Edwin Hubble found a cepheid cepheid variable in the Andromeda Galaxy variable (M31) (M31) It put Andromeda 700,000 parsecs away (2 million light years) away Most nebulae are island universes: Most galaxies comparable to the Milky Way! The Modern Galaxy: The Key Elements Milky Way in the Infrared (2Mass) Milky Note: less obscuration by dust Mapping the Milky Way Mapping Dust makes it hard to see stars 21 cm radiation from atomic Hydrogen lets us map the whole disk in gas lets It is a rotating disk around 30,000 pc in diameter, 300 pc thick diameter, Milky Way at 21 cm: Milky Atomic Hydrogen Atomic Gas in the Galactic Disk Gas Dopper shifts indicate rotation Structure of the Milky Way Structure Sun Magellanic clouds: Nearby dwarf Nearby galaxies galaxies Star motions in the Galaxy the Halo stars move in plunging radial orbits plunging Stars in the disk move in nearly circular orbits circular These are clues for forming the Galaxy forming Star Populations Star Astronomers call the few percent of elements Astronomers that aren’t hydrogen and helium metals that hydrogen metals Population I: Young (bluer) Stars with more Population Young metals are found in the disk disk Population II: Old (redder) Stars with few Population Old metals are found in the halo halo Forming a Milky Way Forming Population I (young, disk) stars are recent generation of stars formed from the gas disk the They inherit the circular motion of the gas disk gas Prior generations of stars created the extra metals seen in these stars extra Forming a Milky Way Forming Population II (old, red) stars were not formed in the Milky Way disk formed They formed outside the disk and fell in They Modern theory indicates galaxies form by combining smaller clumps: by Gas goes to the disk Gas Stars pass right through on plunging orbits Stars Galaxy Formation Formation Cartoon Version Galaxy Formation Movie Galaxy See movies/MW1 See Simulation & Movie by Greg Stinson using Simulation GASOLINE code Young stars: blue Young Older stars: red Older Gas: green Gas: Spiral Arms in Galaxies Spiral The Milky way (and other disk galaxies) have beautiful spiral arm patterns have Why? Why? M51 The M51 “Whirlpool” Galaxy Milky Way Milky artists conception artists Spiral Arms Spiral A visual pattern of gas and dust only? visual The inner galaxy rotates faster so a pattern would coil up over time pattern A Spiral Wave Spiral Lin and Shu (1960’s) Shu s) proposed that a wave of stars and gas was passing through the disk through This wave is long-lived This These spiral waves form in other disks – llike the ones ike that form planets that Aside: Aside: A wave example Waves in traffic: A disturbance creates denser, slower traffic denser, Spiral Density Waves Spiral The wave moves through the gas of the disk of The gas is compressed compressed New stars are formed behind NGC1566 the wave the Other ways to get patterns… Other Propagating Star Propagating Formation Formation Supernovae from one generation of stars compress gas for two new generations on either side either How fast does the Milky Way Galaxy Rotate? Milky We can measure the rotation at different distances from the centre different The Sun takes 225 million years to orbit once once The Mass of the Galaxy The total mass (solar masses) = This implies the Milky Way is has a mass of 100 billion solar masses inside the Sun’s orbit the Most of the visible material in the Galaxy is within the Sun’s orbit Galaxy Dark Halo? Dark At 15 kpc there is practically no visible kpc there material left material The Curve implies 200 billion solar masses within 15 kpc kpc The Rotation Curve of the Milky Way (and other disk galaxies) doesn’t fall off as other fall expected expected This implies there is 600 billion solar masses out to 50 kpc kpc What is the Dark Matter? What Failed or Dead Stars: Brown Dwarfs, White Dwarfs, Neutron Stars… (Hard to White (Hard see) see) Black Holes? Black Exotic Particles? (Neutrinos, WIMPs) WIMPs WIMP: Weakly Interactive Massive WIMP: Particle Particle Stellar Dark Matter Stellar MACHOs: MAssive Compact Halo Objects MAssive Dark stars cannot emit light Dark Their gravity can bend light like a lens can This gravitational lensing effect can lensing effect amplify light from objects behind the dark object dark Currently MACHOs can’t do it alone… MACHOs Gravitational Lensing Lensing The MACHO curves space near it near Temporarily, more light from a background star reaches the observer observer The Galactic Centre The In the Galactic centre the star density is 1 million times greater than average than Galactic Centre Galactic Radio Xrays (Chandra) Infrared Radio The Galactic Centre The At the Centre of the Galaxy there is a strong radio source called Sgr A* Sgr (Sagitarius A*) There is a rotating ring of molecular gas around it Sgr A* Sgr Supermassive Blackhole Motions of Stars and gas indicate a compact 2 million solar mass object compact Probably a huge black hole Probably Sgr A* Sgr Supermassive Blackhole Sgr A* pours a combined total of more than a Sgr A* million times the luminosity of the Sun million It is mostly radio, sub-mm and infrared but It mm also X-rays and Gamma rays also There are contributions from massive stars, dust and gas, supernovae and the black hole – there’s a lot going on there! Bubbles of Energy (Fermi Satellite, Gamma Rays Nov 9, 2010) Gamma Past Black Hole Activity in the Milky Way? NASA ...
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