Milky Way is Spiral galaxy

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Astronomy & Astrophysics
Michael Reid

AST201 March 26, 2013. Milky Way is Spiral galaxy - Bulge, Disk, Halo (all embedded in this; mostly empty) Types of Galaxies - Three main types: spiral, elliptical (absence of spiral; giant bulges with no disk), irregular (miscellaneous category; look “weird”) - Sub-types, ex: barred, dwarf irregular, dwarf elliptical, lenticular, etc. Spiral Galaxies - Grand design spiral: M74 o Very famous galaxy o “grand design” because spiral is very obvious; fully fledged - Ring galaxy: Hoag’s Object o Empty ring; nothing inside it - M64, Black eye galaxy o Gas spins in two directions (inside spins other way than outside) o Not unique; other galaxies do this too o May be result of collision - NGC 4622 o Loose spiral o Arms rotate the opposite way they appear to - Barred spiral Elliptical Galaxy: judge bulge part of spiral galaxy - No disk shape; are spheres (or; ellipsoids) - M87: very famous o Giant elliptical galaxy o No dust o Very massive o No new stars forming; very old - NGC 5128 o Giant elliptical galaxy o Has eaten a spiral galaxy and is absorbing its dust o Super-massive black hole at centre o Bipolar jets Irregular Galaxies - Look weird; not spiral or elliptical - Small; often orbit larger galaxies AST201 March 26, 2013. - M82: starburst galaxy o Starburst: galaxy forms many stars at same time  Any galaxy can undergo starburst period  Can happen when two galaxies collide Hubble’s tuning-fork classification scheme: - Useful for remembering different types of galaxies - Left, very elliptical; right, very spiralled - Barred spirals on bottom, non-barred spirals on top - Does not tell us much - Do not know if Milky Way is barred or not Clues to edge-on spiral galaxies - Have dust - Are organized - Bulge, in middle of disk - Elliptical would look spherical, even edge-on Galaxy Mergers (a way to form new galaxies) - Mayall’s Object is collision of two galaxies - Clouds of gas smash toget
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