A spiral galaxy is a type of galaxy characterized by a flat, rotating disk that is filled with stars, gas, and dust, along with a central concentration of stars known as the bulge. The disk of a spiral galaxy is often marked by distinct spiral arms that wind outward from the center, where higher concentrations of stars and interstellar matter exist. These arms are formed by regions of star formation and can appear very bright due to the presence of young, hot stars.
Spiral galaxies are categorized into several types, including:
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Normal Spirals: These galaxies have well-defined spiral arms and a prominent bulge. They are classified further as "a", "b", and "c" based on the tightness and prominence of the spiral arms.
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Barred Spirals: These galaxies contain a central bar-shaped structure, with spiral arms that extend from the ends of the bar. They are similarly classified into types (e.g., SBa, SBb, SBc) based on the characteristics of the bar and arms.
Spiral galaxies are typically found in groups or clusters of galaxies and are among the most common types of galaxies in the universe. They often host significant amounts of ongoing star formation, particularly in the spiral arms, where molecular clouds provide the material for new stars. Notable examples of spiral galaxies include the Milky Way, the Andromeda Galaxy (M31), and the Whirlpool Galaxy (M51).