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Modern Astronomy Stars & Galaxies Lecture 7 The Milky Way Galaxy: Putting it all together Geraint F. Lewis University of Sydney 2005 Outline Galactic Structure Where are the stars? Rotation and spiral structure Into the middle Formation of the Galaxy Milky Way and friends The Discovery of the Milky Way Views are obscured by dust Discovering the true structure of the galaxy took a lot of detective work Eventually the mystery was solved when Hubble discovered Island Universes The Andromeda Galaxy, the Milky Way’s sister Galactic Makeup 100-400 billion stars Disk, bulge & halo 25kpc (80,000 lyrs) across 8kpc (30,000 lyrs) from the centre 300pc (1,000 lyrs) thick at the Sun 5kpc (16,000 lyrs) thick at the centre Monty Python Our galaxy itself contains 100 billion stars. It’s a 100 thousand light years side to side. It bulges in the middle, 16 thousand light years thick, But out by us, it’s just 3000 light years wide. We’re 30,000 light years from galactic central point. We go ‘round every 200 million years, And our galaxy is only one of millions of billions In this amazing and expanding Universe From the Meaning of Life In context… Imagine each star in the Milky Way is the size of pea The 200 billion stars in the Galaxy would be a cube of peas 34m aside Scaling to the Milky Way, the typical distance between peas is 220km The Milky Way would stretch between the Earth and the Moon The Galactic Disk: Stars 82% of stars Star on circular orbits Many young stars (active star formation) Metal rich stars (Pop I) Pronounced spiral arms Thin & thick disks The Galactic Disk: Gas & Dust • ISM is ~10% mass of stars • 1-2% of the ISM is dust • Dust is visible in the infrared • Dust made in evolved stars • Made in supernovae How do we see hydrogen? The Galactic Disk: Hydrogen • Atoms start in a low energy state, with proton and electron unaligned. •When atoms bump, the atoms can become excited, with the proton and electron aligned. •The electron can return to the low energy state by emitting radio waves at 21cm. •If we had radio eyes sensitive to this wavelength, the disk of the galaxy would glow brightly. Seeing hydrogen Hydrogen: Stellar raw material Dominant gas in the Galaxy Neutral hydrogen clouds (HI) Molecular complexes (H2) Spiral arms in hydrogen Stellar Life Cycle Star Birth Star Death The Bulge 18% of star Very little gas & dust Little star formation Metal poor stars (Pop II) Something energetic in centre Stars on random orbits Bulge Orbits Unlike the Earth orbiting the Sun, the stars in the Bulge are not orbiting a single, massive object. It is the gravitational attraction of all the stars together that keeps the bulge as a single object. In effect, they are orbiting nothing! Out into the halo 2% of the star Virtually no gas or dust No star formation Larger, emptier version of the bulge ~200 globular clusters Very metal poor stars Appears to be a very boring place! Metal poor stars Stars must have formed a long time ago out of almost pristine material. Stars must have small masses to have lived this long! Velocity of the Sun The Sun orbits the Galaxy at 220km/s At this velocity, it takes 240 million years to complete a single orbit It also bobs up and down When the Sun was last at this location, it was the Early Triassic, a world with the first dinosaurs, but no birds, grass or flowering plants! What about other stars? How does the Milky Way rotate? Is like a wheel? Like the Planets We can tell by looking at the Rotation Curve Using the Doppler Shift As objects move towards or away from us, their spectrum of light is shifted via the Doppler effect Towards red for things moving away Towards blue for things coming toward Using this we can map the radial velocities of stars in the galaxy and work out how the Milky Way is rotating. Rotation curve http://www-astronomy.mps.ohio-state.edu/~pogge A flat rotation curve? Given the distribution of stars and gas in the Milky Way, we would not expect a flat rotation curve. Either: We have problems with physics More matter is present than seen A lot (10-100x) more than seen!!! The dark matter halo The identification of a massive dark matter halo enveloping the Milky Way has some important implications Stars, gas & dust are made from protons & neutrons (baryons). Dark matter cannot be dark baryonic matter So you, me, all the stars, gas and dust, everything you have seen in a NASA press release, all of it, are a minor (tiny) player in a Milky Way dominated by dark matter! The dark matter halo Spiral structure Where does spiral structure come from? We have seen the disk doesn’t rotate like a solid wheel and so it would appear the spirals cannot be “fixed”! The spiral structure is seem in hot, young blue stars and gas, but not in old red stars. Do the spirals just rotate with the stars? Overwinding So spirals should rapidly become over wound! Density waves Density waves are enhancements in the surface density of the galaxy Density waves rotate slowly as coherent structure Stars pass through density waves, but slow down as they do so. Density waves Density waves Unlike stars, clouds of gas do not pass through density waves unscathed Gas clouds are compressed as they slow down Gas clouds rear-end one another Both cause cloud collapse & forming new stars, especially hot, luminous (OB) stars. These light up the spiral arms. Density waves OB stars evolve quickly, reaching the end of their lives in a few million years. These stars explode before they leave the spiral arms, promoting even more star formation. The cooler, longer lived stars leave the spiral arms and mix with the galactic population. This is why spiral arms are obvious in OB stars, but not older stars. Seeding density waves What causes density waves? Smaller perturbations can be induced in galactic disks via interactions with other systems These grow to become pronounced waves. Seen in all kings of disks, including protostellar disks! Journey to the centre So, what is occurring in the Galactic centre? The region is clearly very energetic! Right in the middle What is this nothing which the gas and stars are rapidly orbiting? Only real possibility is a Black Hole! Must have a mass of 2.7£106M¯. Not a black hole from a single stellar collapse, but must be built up over time. We shall meet these again when we come to look at quasars, but we have to ask “Just how active was the young Milky Way”? The formation of the Milky Way The first stars formed in clouds of mainly hydrogen and helium (metal poor) Merge into a rotating protogalaxy of dark matter, stars, gas & dust Gas cools and collapses into a disk, taking the dust with it Stars in the halo & bulge just age, while in the disk they continually forming Nice picture, but it does not explain everything!!! The Milky Way & friends The Magellanic Clouds The Local Group Conclusions The Milky Way does not live alone! We live in the Local Group, dominated by little Dwarf Galaxies The closest ones are being disrupted, but where is this material going? The End