An elongated bar-shaped structure composed of stars present in
some spiral galaxies. About two-third of
such galaxies contain bars that cross their centers.
Bars, like → spiral arms, result
from a → density wave
in which stars take very elliptical orbits.
They form when the → galactic disk
dominates the → galactic bulge,
→ Ostriker-Peebles criterion.
Bars play an extremely important role in a galaxy’s
evolution. The gravity from a bar is the mechanism that drives
→ interstellar gas from the outer parts of a
→ spiral galaxy inward
toward the central regions,
and into the galactic nucleus itself. This causes tremendous
bursts of star formation. Therefore, a majority of massive stars are born in such
starbursts in the nuclei of galaxies. Bars may also channel the
material that falls into black holes within active galactic nuclei,
releasing enormous power in radiation and particles from tiny regions
at the centers of some galaxies. Bars disappear as galactic centers grow more
massive (after some 2 to 8 Gyr).
See also: → galactic; → bar.
