Knowra Spiral galaxy Spiral galaxy A galaxy with a flattened disk of stars, gas, and dust organized into spiral arms around a central bulge. Its arms are patterns of enhanced density, not fixed chains of stars.
Density wave theory : A theory that explains spiral arms as regions of enhanced density moving through a rotating galactic disk. It explains how spiral patterns can persist while stars move through the arms.
Galactic bulge : A dense, roughly spheroidal concentration of stars surrounding the center of many galaxies. It is the central component around which a spiral galaxy’s disk is organized.
Milky Way : The barred spiral galaxy that contains the Solar System. It is the spiral galaxy whose structure astronomers infer from within its dusty disk.
Elliptical galaxy : A galaxy with a smooth, rounded or elongated shape and little cold gas or organized disk structure. It lacks the prominent star-forming disk and spiral arms characteristic of spiral galaxies.
Hubble sequence : A classification scheme arranging galaxies by their visible shapes, including ellipticals, spirals, and irregulars. It places spiral galaxies within a broader system for comparing galaxy morphologies.
Differential rotation : Rotation in which different parts of a system orbit at different angular speeds. A galaxy’s faster inner orbits would wind material into tight spirals without other mechanisms.
Galactic disk : A flattened, rotating region of stars, gas, and dust in a galaxy. The disk contains the spiral arms and most of the galaxy’s cold gas.
Andromeda Galaxy : A large spiral galaxy about 2.5 million light-years from Earth in the Local Group. Its nearly face-on view makes its broad spiral structure comparatively easy to study.
Lenticular galaxy : A disk galaxy with a central bulge but no prominent spiral arms and little ongoing star formation. It retains a disk yet lacks the obvious arms that distinguish spiral galaxies.
Galaxy evolution : The changes in a galaxy’s structure, contents, and activity over cosmic time. Gas loss, star formation, and interactions can alter a spiral galaxy’s appearance and future.
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