- Star polyhedron
In
geometry , a star polyhedron is apolyhedron which has some repetitive quality of nonconvexity giving it a star-like visual quality.There are two general kinds of star polyhedron:
*Polyhedra which self-intersect in a repetitive way.
*Concave polyhedra of a particular kind which alternate convex and concave or saddle vertices in a repetitive way.Studies of "star polyhedra" are usually concerned with regular, uniform polyhedra, or the duals to the uniform polyhedra. All these stars are of the self-intersecting kind. So some authoritiesWho|date=August 2008 might argue that the concave kind are not proper stars. But the latter usage is sufficiently widespread that it cannot be ignored. The important thing is to be clear which kind you mean.
Regular star polyhedra
The regular star polyhedra, are self-intersecting polyhedra. They may either have self-intersecting faces, or self-intersecting
vertex figure s.There are four regular star polyhedra, known as the
Kepler-Poinsot polyhedra . TheSchläfli symbol {p,q} implies faces with "p" sides, and vertex figures with "q" sides. Two of them havepentagram mic {5/2} faces and two have pentagrammic vertex figures.
These images show each form with a single face colored yellow to show the visible portion of that face.Uniform and dual uniform star polyhedra
There are many uniform star polyhedra including two infinite series, of prisms and of antiprisms, and their duals.
The uniform and dual uniform star polyhedra, are also self-intersecting polyhedra. They may either have self-intersecting faces, or self-intersecting
vertex figure s or both.The uniform star polyhedra have regular faces or regular
star polygon faces. The dual uniform star polyhedra have regular faces or regularstar polygon vertex figures.Examples
Other star polyhedra
Beyond the forms above, there are unlimited classes of self-intersecting (star) polyhedra.
One class is the
isohedral figure s, which are like the uniform figures, but don't require regular faces.For example, the
complete icosahedron can be interpreted as a self-intersecting polyhedron composed of 12 identical faces, each a (9/4) wound polygon. Below is an illustration of this polyhedron with one face drawn in yellow to help visualize the face within the complex figure.Star polytopes
Higher dimensional intersecting
polytope s are called star polytopes.A regular polytope {p,q,r,...,s,t} is a star-polytope if either its facets {p,q,...s}, or its vertex figure {q,r,...,s,t} is a star polytope.
In four dimensions, the 10 regular star polychora, called the Schläfli-Hess polychora. Like the regular star polyhedra, these 10 are all composed of facets which are either one of the five regular
Platonic solid s or one of the four regular starKepler-Poinsot solid s.For example, the
great grand stellated 120-cell , projected orthogonally into 3-space looks like this::See also
*
Star polygon
*Stellation
*Polyhedral compound
*List of uniform polyhedra Notes
References
*Coxeter, H.S.M., M.S. Longuet-Higgins and J.C.P Miller, Uniform Polyhedra, "Phil. Trans." 246 A (1954) pp. 401-450.
*Coxeter, H.S.M., "Regular Polytopes", 3rd. ed., Dover Publications, 1973. ISBN 0-486-61480-8. (VI. Star-polyhedra, XIV. Star-polytopes)
* John H. Conway, Heidi Burgiel, Chaim Goodman-Strass, "The Symmetry of Things" 2008, ISBN 978-1-56881-220-5 (Chapter 26. pp. 404: Regular star-polytopes Dimension 3)External links
*Mathworld | urlname=StarPolyhedron | title=Star Polyhedron
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