The history of Stars

Stars has a family tree that runs back to the chessboard. Its rules are a cousin of one of the most famous puzzles in mathematics, and its modern form was born in puzzle competitions.

Queens and kings

In 1848 the chess composer Max Bezzel asked how to place eight queens on a chessboard so that none could attack another. Queens attack along rows, columns and diagonals, so the answer needs one queen per row and per column, spread so that no two share a diagonal. The puzzle went on to be studied by Gauss and countless others, and it's still a standard programming exercise today.

A star in our puzzle behaves more like a king: it only rules out the eight squares touching it. One star per row and column, none touching, is a classic counting problem of its own, known as Hertzsprung's problem. There are just 90 ways to do it on a 6×6 board, and 5,242 on an 8×8. Add regions that each need a star, and a well-made puzzle cuts those thousands of possibilities down to one.

From competitions to phones

The two-star form of the puzzle, on a 10×10 grid, became a favourite at international puzzle championships in the early 2000s under the name Star Battle, and it's usually credited to the Dutch puzzle designer Hans Eendebak. Competitive solvers loved it for the same reason casual ones do now: the rules fit in a line, but the reasoning runs deep.

In recent years, one-star versions on smaller grids have become a popular daily puzzle on phones. Ours keeps to the traditional look: regions marked by walls, not colours, and two-star grids for the Hard tier.

Variations

• More stars: three-star puzzles on 14×14 grids and up, for experts.
• Irregular shapes: some puzzles cover only part of the grid with regions, or use non-square boards.
• Other pieces: puzzles where the objects are bigger than one cell, or attack like a different chess piece.