Transcript Document
Galaxies – Types and Structures • Other galaxies like our own Milky Way? Appear as small globules like globular clusters in our galaxy. • Harlow Shapeley – NO, only other globular cluster of stars • Heber Curtis – YES, other galaxies like ‘island universes’ • Hubble settled the debate by measuring the large distances to other galaxies using the bright Cepheid stars Twin galaxies: Spiral and Dwarf Whirlpool Galaxy disintegrates its small neighbor Mult-wavelength Far-Infrared map of M81 Bode’s Galaxy 12 million Lys Ursa Major constellation Andromeda Variety of Spiral Galaxies Elliptical Galaxies Types of Galaxies • Spirals – nucleus, bulge, halo, spiral arms • Barred Spirals – barred nucleus, …..”….. • Ellipticals – various kinds of ellipticity, from nearcircular E0 to highly oval and flat E7 (need to distinguish from edge-on view) – no disks, spiral arms, or dust lanes • Irregulars – Not like spirals or ellipticals • Hubble Classification – Tuning Fork Diagram Ordinary Spirals Ellipticals Barred Spirals Hubble Classification • Ordinary Spirals – classified according to relative bulge strength and tightness of spiral arms - Sa: prominent bulge and tight but indistinct arms - Sb: less prominent bulge and looser arm structure - Sc: small bulge and loose and clearly seen arms i.e. from Sa to Sc, from tight to unwinding arms • Barred Spirals – bar-shaped nucleus (jet??); as many as ordinary spirals; bar rotates like solid; spiral arms emerge from either end (SBa, SBb, SBc) • Irregulars – chaotic structure, no systematic rotation, many dwarf irregular galaxies (classified as “dI”) Barred Spirals: Powered by Rotating Jets New Galaxy (General) Catalog (NGC) Clusters of galaxies Local Group of Galaxies Around Milky Way Collision of Galaxies • Galaxy-galaxy collision can induce gravitational tidal effects and lead to “starbursts” – rapid stellar formation Colliding Galaxies (Simulations) Constant rotation Curves of Galaxies: Dark Matter Properties of Galaxies Stellar Birthrate: Ellipticals have older stars than spirals No significant star formation after 1 billion years Ongoing star formation Distance Scale: Hubble’s law • Hubble also discovered that the farther a galaxy is, the faster it is receding from us the Universe is expanding Big Bang ! • Hubble’s Law: Velocity is proportional to distance v=Hd (H – Hubble’s constant) H = 71 km/s/Mpc • Observe the “redshift” (like Doppler shift) from the spectrum and determine the distance Redshift of the Ca II line in the spectra of galaxies Cosmological Distance Ladder • Several methods: - Trigonometric parallax (d = 1/p), Earth as baseline up to 100 pc (gd based) - 1 kpc (Hipparcos Satellite) - Spectroscopic parallax (spectral type of star gives absolute L on H-R diagram, up to 50-60 kpc - Cepheids and RR Lyrae, up to ~30-40 Mpc (using Hubble Space Telescope), out to about Virgo cluster - Tully-Fisher Relation: L is proportional to the Doppler width of the 21 cm H-line (proportional to mass and L) - Supernovae Ia up to a few hundred Mpc (using HST) • Each step calibrates the next one – “bootstrap method” Observed Flux and Luminosity Distance Modulus: m – M = 5 Log (d/10) m – measured (apparent) magnitude M – absolute magnitude at 10 pc Period-Luminosity Relation: Pulsating Cepheid, RR Lyrae Stars Apparent Magnitude (m) vs. T(d) The Hydrogen 21-cm radio map of the Sky and the Galaxy Tully-Fisher Relation: Width of 21-cm line, due to Doppler blue and redshifts, is proportional to mass of the galaxy, and therefore to intrinsic Luminosity L Distance Modulus (m-M) gives d H I 21 cm Hyperfine Transition Light Curves of Supernovae Ho depends fit to data Methods to determine the cosmological distance scale Multiple images by gravitational lensing Gravitational Lensing and Multiple Images Gravitational lensing of a quasar – two images a,b