Galaxy

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(Redirected from Galaxies)

A galaxy is a large collection of stars, glowing nebulae (clouds), gas, and dust bound together by gravity. Many scientists now believe that a black hole, the remains of a massive star, lies at the center of many galaxies. Galaxies are as plentiful in the universe as grains of sand on a beach. Although astronomers are not yet sure how galaxies formed and evolved, the process must have occurred quickly very early in the history of the universe. The age of the oldest galaxies appears to be about the same age as the universe, which is estimated to be 10 to 13 billion years old.

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[edit] The shape of galaxies

Galaxies can be spiral, elliptical, or irregular in shape. Spiral Galaxies have a group of objects at the center (stars and possibly a black hole) surrounded by a halo of stars and an invisible cloud of dark matter. From this nucleus or center, arms spiral out like a pinwheel. The spiral shape is formed because the entire galaxy is rotating, with the stars at the outer edges forming the arms. Most spiral galaxies have just one arm wrapped around the nucleus, although some have two or even three arms.

Spiral galaxies are divided into two types: barred and unbarred. In barred spirals, a thick bar of stars crosses the center of the galaxy while Unbarred spirals have no such feature.

An elliptical galaxy contains mostly older stars, with very little dust or gas. It can be round or oval, flattened or spherical, and resembles the nucleus of a spiral galaxy without the arms. Astronomers do not yet know whether elliptical galaxies eventually form arms and become spirals, or if spiral galaxies lose their arms to become elliptical.

About one-quarter of all galaxies are irregular in shape and are much smaller than spiral galaxies. The irregular shape may be caused by the formation of new stars in these galaxies or by the pull of a neighboring galaxy's gravitational field.

Some galaxies are variations of these types. There are the Seyfert galaxies (violent, fast-moving spirals); bright elliptical galaxies of super-giants that often consume other galaxies; ring galaxies that seem to have no nucleus; twisted starry ribbons formed when two galaxies collide; and others.

[edit] Words to Know

  • Barred spiral galaxy: A spiral galaxy in which the spiral arms start at the end of a central bar structure rather than the nucleus.
  • Black hole: The remains of a massive star that has burned out its nuclear fuel and collapsed under tremendous gravitational force into a single point of infinite mass and gravity.
  • Dark matter: Unseen matter that has a gravitational effect on the motions of galaxies within clusters of galaxies.
  • Halo: A distribution of older stars and clusters of stars surrounding the nucleus of a spiral galaxy.
  • Irregular galaxy: A galaxy that does not fit into the shape categories of elliptical and spiral galaxies.
  • Light-year: The distance light travels in one year, roughly 5.88 trillion miles (9.46 trillion kilometers).
  • Milky Way: The galaxy in which we are located.
  • Nebulae: Bright or dark clouds, often composed of gases, hovering in the space between the stars.
  • Nucleus: The central core of a galaxy.
  • Radio waves: Electromagnetic radiation, or energy emitted in the form of waves or particles.
  • Spiral arms: The regions where stars are concentrated that spiral out from the center of a spiral galaxy.
  • Spiral galaxy: A galaxy in which spiral arms wind outward from the nucleus.
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