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SCI 238
Mike Fich

Galaxies: classified by their morphology (appearance/shape) - galaxies come in a wide range of types  spirals (like our own Milky Way): with spiral structure, disks, gas and dust, active star formation.  ellipticals: large rounded balls of stars, with little gas and dust, no disks - lenticulars: intermediate in type between spirals and ellipticals. with disks but no spiral arms, some small amount of gas and dust - barred spiral: – spiral galaxies with a bar in the middle galaxy classification is morphological: based on appearance… spiral galaxies vary in appearance: nucleus, arms, bar... spiral galaxy classification: considers bulge vs disc, spiral arm appearance, bar or not... edge-on spirals show dust, gas, bulge galaxies are large, but appear small because they are so far away •the apparent angular size of the Andromeda galaxy is only a few times greater than the Moons •but Andromeda is 700 kpc (0.7 Mpc) away from us •its true (linear) diameter is >10 times greater than the Moon!  lenticular galaxies have disc and bulge but no spiral arms or gas  Elliptical galaxies classified by “ellipticity” o Elliptical galaxies vary greatly in size and mass o this galaxy contains many globular star clusters  irregular galaxies have little symmetry, often include gas  Large Magellanic Cloud  Small Magellanic Cloud  Properties of Galaxies Spirals Barred Spirals Lenticulars Ellipticals Irregulars Appearance flattened disk bulge disk bulge halo no disk no obvious spherical halo spiral structure arms ellipsoidal distribution Gas/Dust in disk 3-10% little or none 0-1% little or none lots 10-20% (mass %) <0.1% stars old in halo young,old inold old young and old disk star formation in spiral arms none none vigorous stellar circular orbits in disk;circular orbits in random random motions random in halo disk; random in halo The Extragalactic Distance Scale  Cepheid Variable Star (Period-Luminosity Law) o good to distances of 25 Mpc  Hubble Law – expansion of the Universe
  Tulley-Fisher (more massive galaxies are more luminous)  Supernova The Hubble Law and the Expanding Universe The expanding Universe Hubble first detected the recession of galaxies in the 1920s - with a small sample of nearby galaxies galaxy radial velocity correlates with distance: v=H d0 Hubble observed that virtually all galaxies are moving away from our own Milky Way galaxy the distances between galaxies are increasing The Hubble Law: v=H d
 0  measure the radial velocities of galaxies using the Doppler shift o virtually ALL galaxies are moving away from us
  plot the distance to the galaxies versus their velocities (d o distance known from methods already described o the more distant a galaxy is from us, the faster it is receding
 o discovered by Edwin Hubble in the 1920s
 o a linear relationship between velocity and distance o the slope of the line is called Hubbles Constant: H o o the velocity in km/s = H o the distance in Mpc o best (current) estimate of H os 70.4±1.5 km/s·Mpc using the Hubble Law to determine distance  measure the radial velocity  invert the equation: d = v/H o  the galaxy radial velocity is often referred to as redshift z (objects moving away from us have their spectral 
 lines Doppler shifted to longer wavelengths; longest visible wavelength is red...)  z is the fractional change in the measured wavelength: z = o 
 if no change in wavelength, z=0; this is the redshift of the local Universe
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