AST 115 1st Edition Lecture 31 Outline of Last Lecture I. Types of Galaxiesa. ClassificationII. Cause of Spiral ArmsIII. Hubble’s Classification SchemeIV. Properties of GalaxiesV. The Distribution of Galaxies in the UniverseOutline of Current Lecture I. The Distribution of Galaxies in the Universe continuedII. Interacting GalaxiesIII. Dark Matter in Other GalaxiesCurrent Lecture The Distribution of Galaxies in the Universe continued…o The Virgo Cluster is the nearest to us (d = 50 MLY). Rich cluster of galaxies with more than 1,000 members. We see numerous galaxies within this cluster.o Superclusters Coma (super) cluster- d ~ 300 MLY- Can have as many as 10,000 galaxieso The Hubble Deep Fields Images from telescopes are exposed for a very long time (days). On several occasions, the Hubble Space Telescope made very long exposures at great distances. Galaxies were found to limit the Hubble Space Telescope. The universe is filled with galaxies throughout, each with millions and millions of stars.o Other studies in selected directions reveal a “sponge-like” structure of the universe at a very large scale. The superclusters of galaxies are like the material of a sponge that surrounds empty pockets of bubbles. The mostly empty pockets in the universe are called voids. Voids are typically 150 MLY across.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.o On a very large scale (~ 600 MLY or more), the distribution of galaxies in the universe is said to be “isotropic and homogeneous”. Isotropic: the same in all directions Homogeneous: uniform in space. Interacting Galaxieso Galaxies collide, stars don’t. Reason for this: separation between galaxies is 10s of times their diameters; separation between stars is 10,000,000s of times their diameters.o Collisions of galaxies can strip them of gas. This heated gas remains in the intergalactic space in a cluster. Ex: The “Mice” galaxieso Compression of gas within the colliding galaxies can lead to impressive rates of new star formation in “starburst galaxies”.o Giant elliptical may form in centers of clusters of galaxies by “mergers” and “cannibalism”. In mergers, two roughly equal galaxies combine. In cannibalism, a large galaxy devours a smaller one.- Ex: M87 in the Virgo Cluster Dark Matter in Other Galaxieso (In our Galaxy, a flat galactic rotation curve implies the presence of dark matter).o Around other spiral galaxies: Studies in rotation rates of spirals lead to mass estimates. Flat rotation curves imply dark matter for most spirals, as in our Galaxy.o In clusters of galaxies: Clusters of galaxies are maintained in spite of insufficient visible numbers of galaxies.o Galaxies should fly away from each other, but they don’t. There must be mass wedon’t see. The mass required to gravitationally hold a cluster together is thought to be in the form of dark matter.o There is evidence that dark matter makes up ~ 23% of the universe. Normal matter is only ~ 4%. We still don’t know what dark matter is.o The remaining ~ 73% of the universe is thought to be mysterious “dark
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