View source for Small stellated dodecahedron

{{Short description|A Kepler-Poinsot polyhedron}}
{{infobox polyhedron
 | name = Small stellated dodecahedron
 | image = SmallStellatedDodecahedron.gif
 | type = [[Kepler&ndash;Poinsot polyhedron]]
 | faces = 12
 | edges = 30
 | vertices = 12
 | dual = [[great dodecahedron]]
}}
[[File:Small stellated dodecahedron.stl|thumb|3D model of a small stellated dodecahedron]]
In [[geometry]], the '''small stellated dodecahedron''' is a [[Kepler–Poinsot polyhedron]], named by [[Arthur Cayley]], and with [[Schläfli symbol]] {{{frac|5|2}},5}. It is one of four [[nonconvex]] [[List of regular polytopes#Non-convex 2|regular polyhedra]]. It is composed of 12 [[pentagram]]mic faces, with five pentagrams meeting at each vertex.

It shares the same [[vertex arrangement]] as the convex regular [[icosahedron]]. It also shares the same [[edge arrangement]] with the [[great icosahedron]], with which it forms [[Great complex icosidodecahedron|a degenerate uniform compound figure]].

It is the [[List of Wenninger polyhedron models#Stellations of dodecahedron|second of four stellations of the dodecahedron]] (including the original dodecahedron itself).

The small stellated dodecahedron can be constructed analogously to the pentagram, its two-dimensional analogue, via the extension of the edges (1-faces) of the core polytope until a point is reached where they intersect.

== Properties ==
If the [[pentagram]]mic faces are considered as 5 triangular faces, it shares the same surface topology as the [[pentakis dodecahedron]], but with much taller [[isosceles]] triangle faces, with the height of the pentagonal pyramids adjusted so that the five triangles in the pentagram become coplanar. The critical angle is atan(2) above the dodecahedron face.

If we regard it as having 12 pentagrams as faces, with these pentagrams meeting at 30 edges and 12 vertices, we can compute its [[Genus (mathematics)|genus]] using [[Euler characteristic|Euler's formula]]
<math display="block"> V - E + F = 2 - 2g</math>
and conclude that the small stellated dodecahedron has genus 4. This observation, made by [[Louis Poinsot]], was initially confusing, but [[Felix Klein]] showed in 1877 that the small stellated dodecahedron could be seen as a [[branched covering]] of the [[Riemann sphere]] by a [[Riemann surface]] of genus 4, with [[branch points]] at the center of each pentagram. This Riemann surface, called [[Bring's curve]], has the greatest number of symmetries of any Riemann surface of genus 4: the [[symmetric group]] <math>S_5</math> acts as automorphisms<ref>{{cite news | first=Matthias | last=Weber | title= Kepler's small stellated dodecahedron as a Riemann surface| journal = Pacific J. Math.| volume = 220| year=2005 | pages=167–182 }} [http://msp.org/pjm/2005/220-1/p09.xhtml pdf]</ref>

== In art ==
[[File:Marble floor mosaic Basilica of St Mark Vencice.jpg|thumb|Floor mosaic by Paolo Uccello, 1430]]
A small stellated dodecahedron can be seen in a floor [[mosaic]] in [[St Mark's Basilica]], [[Venice]] by [[Paolo Uccello]] {{circa|1430}}.<ref>{{cite book|contribution=Regular and semiregular polyhedra|first=H. S. M.|last=Coxeter|author-link= Harold Scott MacDonald Coxeter|pages=41–52|title=Shaping Space: Exploring Polyhedra in Nature, Art, and the Geometrical Imagination|edition=2nd|editor-first=Marjorie|editor-last=Senechal|editor-link=Marjorie Senechal|publisher=Springer|year=2013|doi=10.1007/978-0-387-92714-5_3|isbn=978-0-387-92713-8 }} See in particular p. 42.</ref> The same shape is central to two [[lithograph]]s by [[M. C. Escher]]: ''Contrast (Order and Chaos)'' (1950) and ''[[Gravitation (M. C. Escher)|Gravitation]]'' (1952).<ref>{{cite book|title=Gems of Geometry|edition=2nd|first=John|last=Barnes|publisher=Springer|year=2012|page=46}}</ref>

== Formulas ==

For a small stellated dodecahedron with edge length E,
* Inradius = <math>\frac{\text{E}\sqrt{\frac{5}{2}+\frac{11}{10}\sqrt{5}}}{2}</math>
* Midradius = <math>\frac{\text{E}(3+\sqrt{5})}{4}</math>
* Circumradius = <math>\frac{\text{E}\sqrt{50+22\sqrt{5}}}{4}</math>
* Surface area = <math>15\sqrt{5+2\sqrt{5}}\text{E}^2</math>
* Volume = <math>{\tfrac{5}{4}}(7+3\sqrt{5})\text{E}^3</math>

== Related polyhedra ==
[[File:Small stellated dodecahedron truncations.gif|thumb|290x290px|Animated truncation sequence from {{{frac|5|2}}, 5} to {5, {{frac|5|2}}}|alt=]]
Its convex hull is the regular convex [[icosahedron]]. It also shares its edges with the [[great icosahedron]]; the compound with both is the [[great complex icosidodecahedron]].

There are four related uniform polyhedra, constructed as degrees of truncation. The dual is a [[great dodecahedron]]. The [[dodecadodecahedron]] is a rectification, where edges are truncated down to points.

The ''[[Truncation (geometry)|truncated]] small stellated dodecahedron'' can be considered a [[Degeneracy (mathematics)|degenerate uniform polyhedron]] since edges and vertices coincide, but it is included for completeness. Visually, it looks like a [[dodecahedron|regular dodecahedron]] on the surface, but it has 24 faces in overlapping pairs. The spikes are truncated until they reach the plane of the pentagram beneath them. The 24 faces are 12 [[pentagons]] from the truncated vertices and 12 decagons taking the form of doubly-wound pentagons overlapping the first 12 pentagons.  The latter faces are formed by truncating the original pentagrams.  When an {{mset|{{frac|''n''|''d''}}}}-gon is truncated, it becomes a {{mset|{{frac|2''n''|''d''}}}}-gon.  For example, a truncated pentagon {{mset|{{frac|5|1}}}} becomes a decagon {{mset|{{frac|10|1}}}}, so truncating a pentagram {{mset|{{frac|5|2}}}} becomes a doubly-wound pentagon {{mset|{{frac|10|2}}}} (the common factor between 10 and 2 mean we visit each vertex twice to complete the polygon).{{Dodecahedron stellations}}

{| class="wikitable" width=500
!Name
!Small stellated dodecahedron
!Truncated small stellated dodecahedron
![[Dodecadodecahedron]]
![[Truncated great dodecahedron|Truncated<br>great<br>dodecahedron]]
![[Great dodecahedron|Great<br>dodecahedron]]
|- align=center
![[Coxeter-Dynkin diagram|Coxeter-Dynkin<br>diagram]]
|{{CDD|node|5|node|5|rat|d2|node_1}}
|{{CDD|node|5|node_1|5|rat|d2|node_1}}
|{{CDD|node|5|node_1|5|rat|d2|node}}
|{{CDD|node_1|5|node_1|5|rat|d2|node}}
|{{CDD|node_1|5|node|5|rat|d2|node}}
|- align=center
!Picture
|[[Image:Small stellated dodecahedron.png|100px]]
|[[Image:Dodecahedron.png|100px]]
|[[Image:Dodecadodecahedron.png|100px]]
|[[Image:Great truncated dodecahedron.png|100px]]
|[[Image:Great dodecahedron.png|100px]]
|}

==See also==
* [[Compound of small stellated dodecahedron and great dodecahedron]]

== References ==
{{Reflist}}

==Further reading==
* {{cite book | first=Magnus | last=Wenninger | author-link=Magnus Wenninger  | title=Polyhedron Models | publisher=[[Cambridge University Press]] | year=1974 | isbn=0-521-09859-9 }}
* {{citation | first=Matthias | last=Weber | title= Kepler's small stellated dodecahedron as a Riemann surface| journal = Pacific J. Math.| volume = 220| year=2005 | pages=167–182 |url=http://msp.org/pjm/2005/220-1/p09.xhtml | doi=10.2140/pjm.2005.220.167| doi-access=free }}

== External links ==
* {{mathworld2 | urlname = SmallStellatedDodecahedron| title =Small stellated dodecahedron | urlname2 = UniformPolyhedron | title2 = Uniform polyhedron}}

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[[Category:Polyhedral stellation]]
[[Category:Regular polyhedra]]
[[Category:Kepler–Poinsot polyhedra]]