spiral nebula
Definition
Noun: 1. A spiral galaxy: A type of galaxy characterized by a flat, rotating disk containing stars, gas, and dust, and a central concentration of older stars known as a bulge. Distinctive spiral "arms" that contain younger stars and stellar nurseries extend outward from this central region.
Usage
- The term "spiral nebula" is a historical term. In the early 20th century, distant galaxies were often called "nebulae" because they appeared as fuzzy patches of light through telescopes. The "spiral" descriptor referred to their observed shape.
- Modern astronomy distinguishes between a nebula (a cloud of gas and dust within a galaxy) and a galaxy (a massive, gravitationally bound system of stars). Therefore, the contemporary term is spiral galaxy.
- Example:
Advanced Usage
- Historical Context: The use of "spiral nebula" was central to the "Great Debate" of 1920 regarding the scale of the universe. Astronomers debated whether these spirals were small, nearby objects within our Milky Way or "island universes" (galaxies) in their own right.
- Structure: The spiral arms are not solid structures but density waves, where the passage of the wave triggers star formation, making the arms appear brighter due to the presence of young, hot stars.
Variants and Related Words
- Spiral galaxy (n): The modern, precise term for a spiral nebula.
- Barred spiral galaxy (n): A spiral galaxy with a central bar-shaped structure composed of stars, from which the arms spiral.
- Nebula (n): A cloud of gas and dust in interstellar space. Can be an emission nebula, reflection nebula, or planetary nebula, but is distinct from a galaxy.
- Galaxy (n): A vast system of stars, stellar remnants, interstellar gas, dust, and dark matter, bound together by gravity.
Synonyms
- Spiral galaxy
- Island universe (historical/poetic)
Antonyms
- Elliptical galaxy: A galaxy with an ellipsoidal shape and little interstellar gas or ongoing star formation.
- Irregular galaxy: A galaxy that lacks a distinct regular shape, unlike a spiral or elliptical galaxy.
Noun
-
a galaxy having a spiral structure; arms containing younger stars spiral out from old stars at the center
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