In geometry, a star polygon is a type of non-convex polygon. Regular star polygons have been studied in depth; while star polygons in general appear not to have been formally defined, certain notable ones can arise through truncation operations on regular simple or star polygons. Branko Grünbaum identified two primary usages of this terminology by Johannes Kepler, one corresponding to the regular star polygons with intersecting edges that do not generate new vertices, and the other one to the isotoxal concave simple polygons. Polygrams include polygons like the pentagram, but also compound figures like the hexagram. One definition of a star polygon, used in turtle graphics, is a polygon having q ≥ 2 turns (q is called the turning number or density), like in spirolaterals.
Names Star polygon names combine a numeral prefix, such as penta-, with the Greek suffix -gram (in this case generating the word pentagram). The prefix is normally a Greek cardinal, but synonyms using other prefixes exist. For example, a nine-pointed polygon or enneagram is also known as a nonagram, using the ordinal nona from Latin. The -gram suffix derives from γραμμή (grammḗ), meaning a line. The name star polygon reflects the resemblance of these shapes to the diffraction spikes of real stars.
Regular star polygon
A regular star polygon is a self-intersecting, equilateral, and equiangular polygon. A regular star polygon is denoted by its Schläfli symbol {p/q}, where p (the number of vertices) and q (the density) are relatively prime (they share no factors) and where q ≥ 2. The density of a polygon can also be called its turning number: the sum of the turn angles of all the vertices, divided by 360°. The symmetry group of {p/q} is the dihedral group Dp, of order 2p, independent of q. Regular star polygons were first studied systematically by Thomas Bradwardine, and later Johannes Kepler.
Construction via vertex connection Regular star polygons can be created by connecting one vertex of a regular p-sided simple polygon to another vertex, non-adjacent to the first one, and continuing the process until the original vertex is reached again. Alternatively, for integers p and q, it can be considered as being constructed by connecting every qth point out of p points regularly spaced in a circular placement. For instance, in a regular pentagon, a five-pointed star can be obtained by drawing a line from the 1st to the 3rd vertex, from the 3rd to the 5th vertex, from the 5th to the 2nd vertex, from the 2nd to the 4th vertex, and from the 4th to the 1st vertex. If q ≥ p/2, then the construction of {p/q} will result in the same polygon as {p/(p − q)}; connecting every third vertex of the pentagon will yield an identical result to that of connecting every second vertex. However, the vertices will be reached in the opposite direction, which makes a difference when retrograde polygons are incorporated in higher-dimensional polytopes. For example, an antiprism formed from a prograde pentagram {5/2} results in a pentagrammic antiprism; the analogous construction from a retrograde "crossed pentagram" {5/3} results in a pentagrammic crossed-antiprism. Another example is the tetrahemihexahedron, which can be seen as a "crossed triangle" {3/2} cuploid.
Degenerate regular star polygons If p and q are not coprime, a degenerate polygon will result with coinciding vertices and edges. For example, {6/2} will appear as a triangle, but can be labeled with two sets of vertices: 1–3 and 4–6. This should be seen not as two overlapping triangles, but as a double-winding single unicursal hexagon.
Construction via stellation Alternatively, a regular star polygon can also be obtained as a sequence of stellations of a convex regular core polygon. Constructions based on stellation also allow regular polygonal compounds to be obtained in cases where the density q and amount p of vertices are not coprime. When constructing star polygons from stellation, however, if q > p/2, the lines will instead diverge infinitely, and if q = p/2, the lines will be parallel, with both resulting in no further intersection in Euclidean space. However, it may be possible to construct some such polygons in spherical space, similarly to the monogon and digon; such polygons do not yet appear to have been studied in detail.
Isotoxal star simple polygons When the intersecting line segments are removed from a regular star n-gon, the resulting figure is no longer regular, but can be seen as an isotoxal concave simple 2n-gon, alternating vertices at two different radii. Branko Grünbaum, in Tilings and patterns, represents such a star that matches the outline of a regular polygram {n/d} as |n/d|, or more generally with {n𝛼}, which denotes an isotoxal concave or convex simple 2n-gon with outer internal angle 𝛼.
For |n/d|, the outer internal angle 𝛼 = 180(1 − 2d/n) degrees, necessarily, and the inner (new) vertices have an external angle βext = 180[1 − 2(d − 1)/n] degrees, necessarily. For {n𝛼}, the outer internal and inner external angles, also denoted by 𝛼 and βext, do not have to match those of any regular polygram {n/d}; however, 𝛼 < 180(1 − 2/n) degrees and βext < 180°, necessarily (here, {n𝛼} is concave).
Examples in tilings
These polygons are often seen in tiling patterns. The parametric angle 𝛼 (in degrees or radians) can be chosen to match internal angles of neighboring polygons in a tessellation pattern. In his 1619 work Harmonice Mundi, among periodic tilings, Johannes Kepler includes nonperiodic tilings, like that with three regular pentagons and one regular star pentagon fitting around certain vertices, 5.5.5.5/2, and related to modern Penrose tilings.
Interiors The interior of a star polygon may be treated in different ways. Three such treatments are illustrated for a pentagram. Branko Grünbaum and Geoffrey Shephard consider two of them, as regular star n-gons and as isotoxal concave simple 2n-gons.
These three treatments are:
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