# Star (graph theory)

Star | |
---|---|

The star S. (Some authors index this as _{7}S.)_{8} | |

Vertices | k+1 |

Edges | k |

Diameter | minimum of (2, k) |

Girth | ∞ |

Chromatic number | minimum of (2, k + 1) |

Chromatic index | k |

Properties | Edge-transitive Tree Unit distance Bipartite |

Notation | S_{k} |

Table of graphs and parameters |

In graph theory, a **star** *S*_{k} is the complete bipartite graph *K*_{1,k}: a tree with one internal node and *k* leaves (but, no internal nodes and *k* + 1 leaves when *k* ≤ 1). Alternatively, some authors define *S*_{k} to be the tree of order *k* with maximum diameter 2; in which case a star of *k* > 2 has *k* − 1 leaves.

A star with 3 edges is called a **claw**.

The star *S*_{k} is edge-graceful when *k* is even and not when *k* is odd. It is an edge-transitive matchstick graph, and has diameter 2 (when *k* > 1), girth ∞ (it has no cycles), chromatic index *k*, and chromatic number 2 (when *k* > 0). Additionally, the star has large automorphism group, namely, the symmetric group on k letters.

Stars may also be described as the only connected graphs in which at most one vertex has degree greater than one.

## Relation to other graph families[edit]

Claws are notable in the definition of claw-free graphs, graphs that do not have any claw as an induced subgraph.^{[1]}^{[2]} They are also one of the exceptional cases of the Whitney graph isomorphism theorem: in general, graphs with isomorphic line graphs are themselves isomorphic, with the exception of the claw and the triangle *K*_{3}.^{[3]}

A star is a special kind of tree. As with any tree, stars may be encoded by a Prüfer sequence; the Prüfer sequence for a star *K*_{1,k} consists of *k* − 1 copies of the center vertex.^{[4]}

Several graph invariants are defined in terms of stars. Star arboricity is the minimum number of forests that a graph can be partitioned into such that each tree in each forest is a star,^{[5]} and the star chromatic number of a graph is the minimum number of colors needed to color its vertices in such a way that every two color classes together form a subgraph in which all connected components are stars.^{[6]} The graphs of branchwidth 1 are exactly the graphs in which each connected component is a star.^{[7]}

## Other applications[edit]

The set of distances between the vertices of a claw provides an example of a finite metric space that cannot be embedded isometrically into a Euclidean space of any dimension.^{[8]}

The star network, a computer network modeled after the star graph, is important in distributed computing.

A geometric realization of the star graph, formed by identifying the edges with intervals of some fixed length, is used as a local model of curves in tropical geometry. A tropical curve is defined to be a metric space that is locally isomorphic to a star shaped metric graph.

## References[edit]

Wikimedia Commons has media related to .Star graphs |

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