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NASA Hubble Space Telescope Deep Field Image of Galaxies ( when stars are viewed through a telescope, they appear as bright points of light without any apparent size or structure.  However, there are some objects in the sky that, viewed through a telescope, look like "fuzzy" clouds.  Some of these are the star-forming regions called nebulae.  Others, like those shown in the Hubble Space Telescope image to the right, are actually islands of stars that are much farther from us than the individual stars we see in the night sky.  Although Immanual Kant had first advanced the idea of "island universes" to explain the observed compact clouds during the eighteenth century, is wasn’t until this century that astronomers began to develop an understanding of the nature of galaxies. 

Hubble’s Classification SchemeEdwin Hubble's Tunning Fork Galaxy Classification Scheme Edwin Hubble developed a galaxy classification scheme consisting of four types: elliptical, spiral, barred spiral, and irregular.  Three of these types are represented in his famous "tuning fork" diagram below.  Hubble thought that this scheme might well represent an evolutionary sequence for galaxies.  Today, we know that this is not the case.  Nonetheless, his classification scheme is still used by astronomers today. 

Elliptical Galaxies
An elliptical galaxy shows no spiral structure and can vary from almost round (which Hubble called E0) to almost cigar shaped (E7).  This classification is based on our perspective from Earth and not on the actual shape. 
elliptical galaxy M87
Spiral Galaxies
As their name implies, spiral galaxies have outstretched, curving arms suggestive of a whirlpool or pinwheel.  Hubble distinguished different sub-classes according to the tightness of the arms and the size of the nucleus.  He called these Sa, Sb, and Sc.  In terms of the arms, Sa is the tightest wound while Sc is the most open.  In terms of the size of the nucleus, Sa has the largest while Sc has the smallest.  The galaxies that appear to have a spiral disc, but no visible arms, are called S0.
spiral galaxxy NGC2997
Barred Spirals
Barred spirals show the same spiral structure as normal spirals, and also a prominent bar through the nucleus.  The spiral arms emerge from the end of the bar.  The sub-classifications are the same as for normal spirals.
barred spiral galaxy
Certain galaxies lack either an obvious spiral structure or nuclear bulge, appearing instead as a random collection of stars with no obvious order.  They are distinguished from ellipticals by their lack of symmetry. 
irregular galaxy

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