Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar- like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to their barless cousins has likely changed over the history of the Universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars.Our own Milky Way is a barred spiral, although the bar itself is difficult to observe from the Earth 's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope.Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.
Spiral galaxies may consist of several distinct components:
A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components
A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
A bar-shaped distribution of stars
A near-spherical halo of stars, including many in globular clusters
A supermassive black hole at the very center of the central bulge
A near-spherical dark matter haloThe relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy.
