A star number counts the dots in a centered six-pointed star (a hexagram, like the Star of David), given by \(S_k = 6k(k-1) + 1\). The sequence: 1, 13, 37, 73, 121, 181, 253, 337, 433, 541.
Star numbers are a centered figurate family: each is built by surrounding the previous star with another pointed ring. Some are also square (1, 121, 11881, …) and some are prime (the "star primes": 13, 37, 73, 181, 337, 433, …). They differ from the ordinary [[hexagonal]] numbers, which are not centered.