Week 4 The Large Scale Universe Galaxies, Cosmology, The Theory of Everything
Our Galaxy – from the Inside In visible light
Our Galaxy in History Discovery • 200 years ago astronomers assumed stars were evenly distributed through space! • 100 years ago – we were the center of a disk of stars
Our Galaxy • Variable stars were the trick • RR Lyra and Cephid = Henrietta Leavitt • Instability Strip on H-R Diagram upper right • Period Luminosity Relationship right • Hubble in 1923 found Cephids in other galaxies = same distance calculations but on an intergalactic scale!
Our Galaxy pg 542-543 • Galactic Structure right top • A Center (nuclear bulge) right bottom • Disk • Halo belowOpen clusters vs. Globular Clusters
Galactic Structure • Mass of a Galaxy • Not only is the overall mass important, but the distribution of mass • Disk orbits and Halo orbits lower left
Dark Matter • Keplerian motion is expected • Falls apart at the outer edge! • DARK MATTER • A form of matter that does not react to E-M forces, or the Strong or Weak nuclear forces… only gravity • A Halo around the galaxy • Extends as far as 7X the visible radius of the galaxy • Contains 10-100X the mass of the visible galaxy in it! • But we can see through it! • Particles? Another force? A problem here and elsewhere.
Galactic Structure • Formation • Similar to the Solar System Theory in principle right • Age • From oldest open clusters = 7 billion years • From globular clusters (Hipparcos observations) = 11 billion years • Stars in the disk have metals (things heavier than Hydrogen and Helium), stars in the halo are almost entirely Hydrogen and Helium)
Formation of Galaxies Dark Matter a factor as well?
Galactic Arms • We are in the Orion-Cygnus Arm • The next arm out is centered in Perseus (near Cassiopeia), the center of the galaxy is in Sagittarius • Radio observations give us much of this structure (since light can’t get through the dust) • The arms are NOT solid objects held together by some force
More Galactic Structure • Spiral Arms make up the outer reaches of our Galaxy
What makes arms? • Best theory = the Density Wave Theory- they are like sound waves/shock waves traveling through the disk • O and B stars live their short lives before the wave passes (so we see the brightest stars • BUT what starts the waves? How do you get branches? = Unknown.
More Galactic Structure • The Nucleus: • Core region about as big as the full moon in our sky • Massive Black Hole = 2.7 million star mass!! • Stars 860 AU away circle the core in 16 years!!! • Event Horizon (Schwarzschild Radius) = 13X larger than our sun in size.
Galaxies in General • Arms and Stars= TYPES of Galaxies • Elliptical Galaxies left • Spiral Galaxies + Barred Spirals center • Irregular Galaxies lower right • NEW! Dark Matter finding!!
An old idea • Edwin Hubble = Evolution Diagram • E0 (round) to E7 (flat) elliptical • S0 (part elliptical and part spiral) , Sa to Sc – looser spiral • SBa to SBc lower barred spiral • Irr Irregular
Galactic Winds – Young Galaxies? More on thislater…
Galaxies in General Distances to the Galaxies • Light years and parsecs are too small now – We’ll use megaparsec (Mpc) • One Mpc = 3.26 million light years or 2x1019 miles • Cepheids can be seen for 200 Mpc (.6 billion light years = the universe is about 14 billion light years in radius)
Far Out Galaxies • Beyond this we have to use the overall brightness of galaxies themselves (similar shape and detail) • Supernova can help too (but are rare) • LOOK BACK TIME (Moon = 1.3 seconds, Sun= 8 minutes, Alpha Centauri = 4 years, Andromeda Galaxy 2 million years, The ‘edge’ of the universe = 13.7 billion years)
Looking Out at Galaxies • The most distant galaxies visible = 3000 Mpc (10 billion light years) • The Hubble Law • Vr=Hd (Vr= velocity of galaxy away from us, d= distance in Mpc, H = Hubble Constant (km/sec/Mpc) • Newest results put H near 70-80 km/sec/Mpc • 71 ± .4 km/sec/Mpc EVEN MORE RECENT from WMAP
Looking Out at Galaxies • OTHER CRITTERS • Colliding Galaxies
Active Galactic Nuclei • Galaxies with Active Nuclei – wind again • Pump out radio and X-ray radiation and jets (from the central Black Hole) • Comes from Collisions = newest theory
Quasars • Quasars • Special active nuclei cores near the beginning of the universe (the furthest things visible in the universe) • Moving away from us at a good fraction of the speed of light!!! • Massive Black Holes • Gravitational lens (Einstein)
A good quasar! Jets are common
The discovery of quasars… Incredible Red Shift!!