Astrophysics (Index)About

galactic disk

(galactic disc, disk)
(the "disk" portion of a disk galaxy)

A galactic disk (or galactic disc) is the disk portion of a disk galaxy, i.e., not including the galactic halo, the latter of which includes dark matter and can include globular clusters, field stars, clouds, and even (within the dark matter halo) dwarf galaxies. The galactic disk includes most of the stars, thus comprising the visible portion of the galaxy, and most of the interstellar medium (ISM) (gas and dust). The edge between galactic disk and surroundings is not clear-cut, and cited dimensions of the disk are generally based upon a defined criteria regarding the exponential falloff of emitted electromagnetic radiation over directions across the galaxy's plane away from the center, and over the two (opposite) directions perpendicular to the galaxy's plane.

Further reading:

Referenced by pages:
corotation resonance (CR)
disk galaxy
galactic bulge
galactic north
giant elliptical galaxy
Gould's Belt
Kapteyn universe
Milky Way
Monoceros Ring
NGC 6946
peak star-formation epoch
Smith Cloud
spiral arm