Hard Star Battle answer — March 9, 2025

Full written solution

Every deduction from the walkthrough above, in order. Each technique is explained in the Star Battle strategy guide.

1. Starting position — the grid is split into regions (bold outlines). Each row, column, and region needs exactly two stars, and no two stars may touch.
2. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 5, column 8).
3. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 6, column 8).
4. Three candidates with two adjacent: Isolated candidate must hold a star (row 5, column 7).
5. Mark 6 cells adjacent to stars as X (row 4, column 6).
6. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 2, column 8).
7. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 2, column 9).
8. Three candidates with two adjacent: Isolated candidate must hold a star (row 3, column 9).
9. Mark 2 cells adjacent to stars as X (row 3, column 8).
10. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 8, column 1).
11. Clump analysis (Row 4): candidate clumps can only just supply the remaining stars (row 3, column 2).
12. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 3, column 6).
13. Clump analysis (Region 1): candidate clumps can only just supply the remaining stars (row 2, column 7).
14. Clump analysis (Region 2): candidate clumps can only just supply the remaining stars (row 1, column 9).
15. Region completion: 1 star needed, 1 candidate (row 1, column 8).
16. Mark 1 cell adjacent to stars as X (row 1, column 7).
17. Clump analysis (Column 8): candidate clumps can only just supply the remaining stars (row 8, column 7).
18. Clump analysis (Region 7): candidate clumps can only just supply the remaining stars (row 2, column 2).
19. Clump analysis (Region 9): candidate clumps can only just supply the remaining stars (row 6, column 1).
20. Clump analysis (Region 6): candidate clumps can only just supply the remaining stars (row 8, column 6).
21. Clump analysis (Region 8): candidate clumps can only just supply the remaining stars (row 5, column 5).
22. Region completion: 1 star needed, 1 candidate (row 6, column 9).
23. Mark 1 cell adjacent to stars as X (row 7, column 8).
24. Two last candidates (row 5): Eliminate 1 cell adjacent to both (row 4, column 2).
25. Two last candidates (column 8): Eliminate 1 cell adjacent to both (row 9, column 7).
26. Clump analysis (Row 6): candidate clumps can only just supply the remaining stars (row 7, column 4).
27. Clump analysis (Region 8): each clump must supply its full share of stars (row 5, column 1).
28. Mark 1 cell adjacent to stars as X (row 4, column 1).
29. Row completion: 2 stars needed, 2 candidates (row 4, column 3).
30. Mark 2 cells adjacent to stars as X (row 3, column 3).
31. Row completion: 1 star needed, 1 candidate (row 4, column 5).
32. Mark 1 cell adjacent to stars as X (row 3, column 5).
33. Two-option rule: Cell adjacent to both candidates (row 7, column 3).
34. Clump analysis (Region 7): candidate clumps can only just supply the remaining stars (row 1, column 4).
35. Region completion: 2 stars needed, 2 candidates (row 7, column 2).
36. Mark 2 cells adjacent to stars as X (row 6, column 3).
37. Region completion: 1 star needed, 1 candidate (row 6, column 4).
38. Mark 2 cells adjacent to stars as X (row 6, column 5).
39. Region completion: 1 star needed, 1 candidate (row 9, column 2).
40. Mark 1 cell adjacent to stars as X (row 9, column 3).
41. Column 2 complete: Mark 1 remaining cell as X (row 1, column 2).
42. Blocking rule: a star here would touch every other open cell in its region, leaving no room for the region's second star (row 8, column 5).
43. Row completion: 2 stars needed, 2 candidates (row 8, column 4).
44. Mark 2 cells adjacent to stars as X (row 9, column 4).
45. Region completion: 1 star needed, 1 candidate (row 9, column 6).
46. Row completion: 1 star needed, 1 candidate (row 8, column 8).
47. Mark 2 cells adjacent to stars as X (row 7, column 7).
48. Region completion: 1 star needed, 1 candidate (row 7, column 6).
49. Column 4 complete: Mark 1 remaining cell as X (row 2, column 4).
50. Column completion: 1 star needed, 1 candidate (row 2, column 5).
51. Mark 1 cell adjacent to stars as X (row 2, column 6).
52. Region completion: 1 star needed, 1 candidate (row 3, column 7).
53. Region complete: Mark 1 remaining cell as X (row 2, column 3).
54. Row completion: 1 star needed, 1 candidate (row 2, column 1).
55. Mark 2 cells adjacent to stars as X (row 1, column 1).
56. Region completion: 1 star needed, 1 candidate (row 1, column 3).
57. Solved! Every row, column, and region has its two stars, none touching.