[per-ee-as-truhn, -tron]
noun, plural periastra [per-ee-as-truh] . Astronomy.
the point at which the stars of a binary system are closest (opposed to apastron ).

1850–55; < peri- + Greek ástron star; modeled on perihelion

periastral, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
periastron (ˌpɛrɪˈæstrɒn)
astronomy the point in the orbit of a body around a star when it is nearest to the star, esp applied to double-star systems

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
periastron   (pěr'ē-ās'trən, -trŏn)  Pronunciation Key 
Plural periastra
  1. The point at which two stars orbiting each other in a binary star system are closest together.

  2. The point at which an object, such as a planet or comet, is closest to the star it is orbiting. Compare apastron.

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