AST 115 1st Edition Lecture 30 Outline of Last Lecture I. Spiral Structure of the Galaxy’s DiskII. Rotation of the GalaxyIII. Mass of the GalaxyIV. Dark MatterOutline of Current Lecture I. Types of Galaxiesa. ClassificationII. Cause of Spiral ArmsIII. Hubble’s Classification SchemeIV. Properties of GalaxiesV. The Distribution of Galaxies in the UniverseCurrent Lecture Types of Galaxieso Classification → based on its apparent shape in the sky Elliptical (E)- Range from circular (E0) to elongated (E7).- Contain little or no dust.- No current star formation. Normal Spirals (NS)- Range from large nucleus with tightly-wound arms (Sa) to small nucleus with loosely-wound arms (Sc). Barred Spiral (SB)- Range from (SBa through SBc), as in normal spirals, but arms emerge from a “bar” of material through the nucleus. Irregular (Irr)- These galaxies show no symmetry.- Like the spirals, they contain both gas and dust and have both young and old stars. Flocculent Spiral Galaxies- This means “fleecy”.- Have chaotic or poorly defined arms.- Star formation triggers more star formation.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. Grand – Design Spiral Galaxies- Have symmetric and well defined arms.- Rotation triggers star formation in smooth spiral arms.- Example: M81 in Ursa Major. Cause of Spiral Armso Regions of increased density or density waves form naturally in a rotating gas disk. Leads to formation of stars in spiral patterns, thus spiral galaxies.o Not understood = how spiral arms are maintained over several rotations of the galaxy. Shouldn’t the arms wrap up tight? We don’t know. Hubble’s Classification Scheme Properties of Galaxieso Irregular galaxies = tend to be smallest, dimmest, and ones with fewest stars.o Spirals = large, but the largest galaxies are the ellipticals. The elliptical also have the greatest range in size, brightness, and number of stars.o Ellipticals have little or no dust or gas and thus no new star formation, unlike the spirals and the irregulars.o Most of the apparently bright galaxies are of the spiral type, but if we include faint dwarf elliptical and faint irregulars, spirals are actually in the minority. The Distribution of Galaxies in the Universeo Large scale studies of distribution show that they congregate in clusters and superclusters of galaxies.o Notes: Regular clusters of galaxies mainly contain ellipticals Irregular clusters of galaxies also have spirals in them “Rich” = above average number of galaxies “Poor” = below average number of galaxies The “Local Group” of galaxies is in the poor cluster that our Milky Way Galaxy is
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