What are the rules of Star Battle?
Star Battle is played on a square grid divided into outlined regions, one region per row. Place one star in every row, one in every column and one in every region. No two stars may touch, not even diagonally. There is nothing to guess: the shape of the regions is enough to find exactly one place for every star.
Choose a 5 × 5, 7 × 7 or 8 × 8 grid. Tap a cell once to put a dot there, your own note that no star can go in it, tap again for a star, and once more to clear it. Stars that touch, or two in one row, column or region, turn red as soon as they are placed.
How is your Star Battle time calculated?
Your result is the time from Start until all the stars are placed and none of them breaks a rule. Next to it are the times you pressed Check, which marks any star that shouldn't be there and any dot on a cell that needs a star, and Hint, which places one star (or removes a wrong one). Dots are only notes and never count against you.
There is no rating, because no published typical time exists for puzzles built like these. A bigger grid takes much longer, and some region shapes need more careful reasoning than others.
How do you solve Star Battle?
- Start with the smallest regions. A region of one or two cells leaves almost no choice, and its star rules out its whole row, column and the eight cells around it.
- Look for regions inside one row. If every cell of a region lies in the same row, that row's star is in that region, so the rest of the row can be dotted. The same goes for columns.
- Count in pairs. Two regions that fit inside the same two rows take both of those rows' stars, so every other cell in those rows is empty.
- Test a cell against its neighbours. A star in a cell rules out the cells around it. If that would leave some region with nowhere to go, the cell can't hold a star.
- Dot as you go. Marking the cells you have ruled out makes the last open cell in a row or region easy to see.
Does every Star Battle puzzle have exactly one answer?
Yes. The 90 puzzles here were made ahead of time. Each started from stars placed at random, with a region grown around each one, and the regions were then reshaped until a solver using only the moves above could find every star. The site's tests solve every stored puzzle again and count its answers by trying every placement: exactly one each time.
Is Star Battle the same as Two Not Touch?
Two Not Touch is the name The New York Times uses for a Star Battle with two stars in every row, column and region on a larger grid. The puzzles here have one star per row, column and region, on grids small enough to tap comfortably on a phone, so they take minutes rather than a long sitting. The no-touching rule is the same.
For another puzzle of placing marks by rules, try the Binary Puzzle; for joining islands, the Bridges Puzzle.