spiral galaxy

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spiral galaxy

(spiral) See galaxies; Hubble classification.

Spiral Galaxy


a giant star system that, when observed through a telescope, appears as a bright nucleus (a large aggregate of closely spaced stars) from which there extend spiral arms that coil around the nucleus.

Spiral galaxies most often have two arms that curl in the same direction. Sometimes several independent arms are observed, which, in some cases, branch like the branches of a tree. In exceptional cases, just one arm is observed. All the arms lie virtually in the same plane, which coincides with the galaxy’s plane of rotation. In some galaxies, the arms open wide; in other cases, the arms are wound so tightly that they are almost rings. The medial line of a spiral arm satisfies well the equation of a logarithmic spiral.

A special type of spiral galaxy is the barred spiral. Here, the nucleus is crossed by a short or long bar, and the spiral arms begin at the ends of the bar.

The arms of spiral galaxies consist of a great number of stars and tenuous neutral gas, primarily hydrogen. The gas is usually detected by radio-astronomical means. In regions containing very hot stars, however, the hydrogen ionizes and is luminous. Spiral arms are characteristically composed of hot stars, Cepheids, bright gaseous nebulas, and dark and bright dust clouds.

The Galaxy is a spiral galaxy. Astronomical observation has permitted the positions of the Galaxy’s arms to be determined. It has been established that the solar system lies in the space between two arms.

The origin of the spiral arms has not been completely explained. Their existence seems to be maintained by density waves in the plane of the galaxy. The patchiness of the arms is a sign of intensive star formation: the gas is condensing into groups of stars. (SeeGALAXIES; GALAXY, THE.)


spiral galaxy

[′spī·rəl ′gal·ik·sē]
A type of galaxy classified on the basis of appearance of its photographic image; this type includes two main groups: normal spirals with circular symmetry of the nucleus and of the spiral arms, and barred spirals in which the dominant form is a luminous bar crossing the nucleus with spiral arms starting at the ends of the bar or tangent to a luminous rim on which the bar terminates.