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Facts about Galaxies for Kids

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, and interstellar medium of gas and dust, and, it is hypothesized, an important but poorly understood component called dark matter. The word galaxy is derived from the Greek galaxias, literally “milky”, a reference to the Milky Way.

  • Examples of galaxies range from dwarfs with as few as ten million (10) stars to giants with a hundred trillion (10) stars, each orbiting their galaxy’s own center of mass.
  • In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays.
  • Such interactions between nearby galaxies, which may ultimately result in a merger, sometimes induce significantly increased incidents of star formation leading to starburst galaxies.
  • The majority of galaxies are organized into a hierarchy of associations known as groups and clusters, which, in turn usually form larger superclusters.
  • According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way’s parallax, and he thus “determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere.”
  • In the 10th century, the Persian astronomer Al-Sufi made the earliest recorded observation of the Andromeda Galaxy, describing it as a “small cloud”.
  • Thus they were not gravitationally bound to the Milky Way, and were unlikely to be a part of the galaxy.
  • To support his claim that the Great Andromeda Nebula was an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.
  • This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.
  • Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble.
  • Galaxies come in three main types: ellipticals, spirals, and irregulars.
  • Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) and activity in the core (in active galaxies).
  • The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated.
  • In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge.
  • In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.
  • Despite the prominence of large elliptical and spiral galaxies, most galaxies in the Universeappear to be dwarf galaxies.
  • The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region.