Star | |
---|---|
The star S7. (Some authors index this as S8.) |
|
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 | Sk |
In graph theory, a star Sk is the complete bipartite graph K1,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 Sk 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 Sk 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).
Stars may also be described as the only connected graphs in which at most one vertex has degree greater than one.
Claws are notable in the definition of claw-free graphs, graphs that do not have any claw as an induced subgraph.[1][2]
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 K1,k consists of k − 1 copies of the center vertex.[3]
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,[4] 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.[5] The graphs of branchwidth 1 are exactly the graphs in which each connected component is a star.[6]
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.[7]
The star network, a computer network modeled after the star graph, is important in distributed computing.