The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. * A supermassive black hole at the very center of the central bulge * A near-spherical halo of stars, including many in globular clusters * A central stellar bulge of mainly older stars, which resembles an elliptical galaxy * A flat, rotating disc of stars and interstellar matter of which spiral arms are prominent components Spiral galaxies may consist of several distinct components: They are mostly found in low-density regions and are rare in the centers of galaxy clusters. ![]() Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. The most convincing evidence for the stars forming a bar in the Galactic Center comes from several recent surveys, including the Spitzer Space Telescope. The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc. The proportion of barred spirals relative to barless spirals 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. 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 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. Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed, according to a 2010 Hubble Space Telescope survey. The Milky Way – the galaxy that includes Earth and our solar system – is an example of a spiral galaxy. ![]() Most of the galaxies that scientists have discovered so far are spiral galaxies, as opposed to the other two main categories of galaxy shapes - elliptical and irregular. Spiral galaxies are twisted collections of stars and gas that often have beautiful shapes and are made up of hot young stars.
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