Hard Star Battle answer — February 7, 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 2, 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 2, column 9).
4. 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).
5. 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 9).
6. Three candidates with two adjacent: Isolated candidate must hold a star (row 7, column 9).
7. Mark 3 cells adjacent to stars as X (row 7, column 8).
8. Two-option rule: Cell adjacent to both candidates (row 4, column 8).
9. Two-option rule: Cell adjacent to both candidates (row 4, column 9).
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 1, column 3).
11. 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 3).
12. Clump analysis (Region 4): candidate clumps can only just supply the remaining stars (row 5, column 2).
13. Clump analysis (Region 6): candidate clumps can only just supply the remaining stars (row 1, column 5).
14. Clump analysis (Region 7): candidate clumps can only just supply the remaining stars (row 7, column 4).
15. Clump analysis (Region 8): candidate clumps can only just supply the remaining stars (row 3, column 3).
16. Clump analysis (Column 3): candidate clumps can only just supply the remaining stars (row 8, column 4).
17. Clump analysis (Column 4): candidate clumps can only just supply the remaining stars (row 4, column 5).
18. Clump analysis (Region 7): each clump must supply its full share of stars (row 8, column 5).
19. Mark 3 cells adjacent to stars as X (row 7, column 6).
20. Crowding (Row 4): a star here would leave too little room for its 2 remaining stars (row 5, column 3).
21. Clump analysis (Column 3): candidate clumps can only just supply the remaining stars (row 9, column 2).
22. Clump analysis (Region 1): candidate clumps can only just supply the remaining stars (row 3, column 5).
23. Two last candidates (column 5): Eliminate 3 cells adjacent to both (row 5, column 4).
24. Clump analysis (Column 6): candidate clumps can only just supply the remaining stars (row 1, column 7).
25. Crowding (Row 4): a star here would leave too little room for its 2 remaining stars (row 5, column 5).
26. Column completion: 1 star needed, 1 candidate (row 6, column 5).
27. Three candidates with two adjacent: Isolated candidate must hold a star (row 4, column 6).
28. Mark 2 cells adjacent to stars as X (row 3, column 6).
29. Three candidates with two adjacent: Isolated candidate must hold a star (row 6, column 7).
30. Mark 2 cells adjacent to stars as X (row 5, column 8).
31. Region completion: 1 star needed, 1 candidate (row 5, column 9).
32. Row completion: 1 star needed, 1 candidate (row 5, column 1).
33. Mark 2 cells adjacent to stars as X (row 4, column 2).
34. Row completion: 1 star needed, 1 candidate (row 4, column 4).
35. Mark 1 cell adjacent to stars as X (row 3, column 4).
36. Row 6 complete: Mark 1 remaining cell as X (row 6, column 3).
37. Region completion: 2 stars needed, 2 candidates (row 3, column 2).
38. Mark 1 cell adjacent to stars as X (row 2, column 2).
39. Region completion: 1 star needed, 1 candidate (row 7, column 3).
40. Mark 1 cell adjacent to stars as X (row 8, column 3).
41. Region completion: 1 star needed, 1 candidate (row 9, column 3).
42. Row completion: 2 stars needed, 2 candidates (row 2, column 4).
43. Mark 1 cell adjacent to stars as X (row 1, column 4).
44. Region completion: 1 star needed, 1 candidate (row 1, column 2).
45. Row completion: 1 star needed, 1 candidate (row 2, column 6).
46. Mark 1 cell adjacent to stars as X (row 1, column 6).
47. Row 7 complete: Mark 1 remaining cell as X (row 7, column 1).
48. Region completion: 1 star needed, 1 candidate (row 8, column 1).
49. Row 8 complete: Mark 1 remaining cell as X (row 8, column 7).
50. Column completion: 1 star needed, 1 candidate (row 9, column 7).
51. Mark 1 cell adjacent to stars as X (row 9, column 8).
52. Region complete: Mark 1 remaining cell as X (row 9, column 9).
53. Column completion: 2 stars needed, 2 candidates (row 1, column 8).
54. Mark 1 cell adjacent to stars as X (row 1, column 9).
55. Column completion: 1 star needed, 1 candidate (row 3, column 8).
56. Mark 1 cell adjacent to stars as X (row 3, column 9).
57. Solved! Every row, column, and region has its two stars, none touching.