Spiral galaxy

A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized

Related Topics:
Galaxy - Hubble sequence

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by the following physical properties:

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• A considerable total angular momentum
• Composed of a central bulge surrounded by a disk
• The bulge resembles an elliptical galaxy, containing many old, so-called "Population II" stars, and usually a supermassive black hole at its center.
• The disk is a flat, rotating assembly consisting of interstellar matter, young "Population I" stars and open star clusters.

Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.

Related Topics:
Star formation - Logarithmically - Lenticular

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The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.

Related Topics:
Spheroid - Globular cluster

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Our galaxy, the Milky Way, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research, however, suggests that it may in fact be a barred spiral.

Related Topics:
Milky Way - Barred spiral

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Spiral galaxy: Origin of the spiral structure ►