Transcript Powerpoint
Another galaxy: NGC 4414. The Milky Way roughly resembles it. Today’s Lab: Galaxies & Cepheids - Part I. Finding distances to galaxies - Part 2. Shapes of Galaxies - Part 3. Color of Galaxies - Part 4. Size of Galaxies Finding Distances In the Solar System => Radar Signal goes out and back at speed of light Near Stars => Geometric Parallax Farther Stars => Spectroscopic Parallax (Classify the main sequence star by spectral type - surface temperature and use that to find Luminosity and M, then use m-M dimming factor) Star Clusters Use dimming factor/modulus = m – M => distance (we did this last week) - - How do we find distances to nearest galaxies Similar to what we did for clusters (using the difference between actual brightness M & apparent brightness m) but need way to find Luminosity / Absolute Magnitude Shapley (1917) found that Sun was not at center of Milky Way Shapley used distances to variable “RR Lyrae” stars (a kind of Horizontal Branch star) in Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc. Measuring the Milky Way The usefulness of these stars comes from their period–luminosity relationship. Both RR Lyrae & Cepheids are post main-sequence Using Cepheid to find m and M mavg = (mmax + mmin) / 2 = (24.7 + 25.7) / 2 = 25.2 Period = 25.3 day Once we have the period, go back and get luminosity from graph like that on previous slide with the known period Measuring the Milky Way Period of Cepheid C46 is 25.3 days What is its absolute magnitude M? Measuring the Milky Way We have now expanded our cosmic distance ladder one more step. Hubble’s Galaxy Classification Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning. Hubble’s Galaxy Classification Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest. Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively less tight, although the correlation is not perfect. The components of spiral galaxies are the same as in our own Galaxy: disk, core, halo, bulge, spiral arms. Hubble’s Galaxy Classification Spiral galaxies are classified according to the size of their central bulge. Hubble’s Galaxy Classification Similar to the spiral galaxies are the barred spirals. Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of fewer than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy. Ellipticals are classified according to their shape, from E0 (almost spherical) to E7 (the most elongated). S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas. The irregular galaxies have a wide variety of shapes. These galaxies appear to be undergoing interactions with other galaxies. Irregular galaxies are the most common type of galaxy Hubble’s Galaxy Classification A summary of galaxy properties by type Local Group of Galaxies Here is the distribution of galaxies within about 1 Mpc of the Milky Way. Determining the amount of Red Light and Blue Light coming from a Galaxy using SIPS Watch TA demonstration Finding angular size and actual size Step 1: Find Angular Size in arcminutes Angular Size = (angular size of window) x (Pixel size of object) / (Pixel size of window) This gives angular size in arcminutes / Pixel size-length using pixel ruler Step 2: Find Angular Size in radians Convert angular size in arcminutes to arcseconds and then radians Angular size (arcseconds) = Angular size (arcminutes) x 60 arcseconds / 1 arcminute Angular size (radians) = Angular size (arcseconds) x 1 radian/ 206,000 arcseconds Step 3: Find Actual Size Actual Size (pc) = Angular Size (radians) x Distance (pc)