The Galaxy Family

  • Steven J. Dick
Part of the Astronomers' Universe book series (ASTRONOM)


In stark contrast to spiral galaxies like our own Milky Way, elliptical galaxies are almost featureless aggregations of stars distinguished by an ellipsoidal or spherical morphology. The ellipsoidal morphology was identified early on, even before these objects were realized to be external galaxies. In his classification of galaxies published in 1926, Edwin Hubble delineated three classes of galaxies: ellipticals, spirals (G 4), and irregulars (G 5), and ten years later in his famous book The Realm of the Nebulae he referred to extreme ellipticals as “lenticulars,” now often recognized as a fourth class (G 3). In his famous “tuning fork” diagram of galaxy types (Fig. 14.1), ellipticals were placed on the handle because Hubble thought they were early objects before acquiring their more mature morphologies as spirals. However, today many astronomers believe that at least some ellipticals form when smaller disk galaxies collide and merge. Thus, while Hubble distinguished this class of objects, he did not understand its place in cosmic evolution. Even today, and despite a long history of observation, perhaps the only consensus is that “we have not yet determined with certainty the physical mechanisms that differentiate galaxies into classes.”

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  • Steven J. Dick
    • 1
  1. 1.AshburnUSA

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