Hard Star Battle answer — February 16, 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 10, column 8).
3. Three candidates with two adjacent: Isolated candidate must hold a star (row 10, column 9).
4. Mark 4 cells adjacent to stars as X (row 9, column 8).
5. Two-option rule: Cell adjacent to both candidates (row 9, column 6).
6. Two-option rule: Cell adjacent to both candidates (row 10, column 6).
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 2).
8. 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 2).
9. Three candidates with two adjacent: Isolated candidate must hold a star (row 2, column 3).
10. Mark 6 cells adjacent to stars as X (row 1, column 2).
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 8, column 8).
12. Three candidates with two adjacent: Isolated candidate must hold a star (row 8, column 9).
13. Mark 4 cells adjacent to stars as X (row 7, column 8).
14. Column 9 complete: Mark 6 remaining cells as X (row 1, column 9).
15. Three candidates with two adjacent: Isolated candidate must hold a star (row 6, column 10).
16. Mark 1 cell adjacent to stars as X (row 5, column 10).
17. Two-option rule: Cell adjacent to both candidates (row 5, column 7).
18. Two-option rule: Cell adjacent to both candidates (row 6, column 7).
19. Two-option rule: Cell adjacent to both candidates (row 7, column 6).
20. Two-option rule: Cell adjacent to both candidates (row 8, column 6).
21. Clump analysis (Region 10): candidate clumps can only just supply the remaining stars (row 5, column 1).
22. Star claim: Region 7 must place a star in row 10; cells touching all its options are eliminated (row 9, column 4).
23. Star claim: Region 7 must place a star in column 5; cells touching all its options are eliminated (row 9, column 5).
24. Clump analysis (Region 9): candidate clumps can only just supply the remaining stars (row 6, column 3).
25. Crowding (Row 9): a star here would leave too little room for its 2 remaining stars (row 8, column 2).
26. Row constraint forcing: Row 10 can only take 1 star, region needs 2 (row 8, column 5).
27. Mark 3 cells adjacent to stars as X (row 7, column 4).
28. Row 8 complete: Mark 3 remaining cells as X (row 8, column 1).
29. Region completion: 1 star needed, 1 candidate (row 7, column 7).
30. Mark 2 cells adjacent to stars as X (row 6, column 6).
31. Region completion: 1 star needed, 1 candidate (row 5, column 8).
32. Mark 2 cells adjacent to stars as X (row 4, column 7).
33. Three candidates with two adjacent: Isolated candidate must hold a star (row 9, column 3).
34. Mark 3 cells adjacent to stars as X (row 9, column 2).
35. Region completion: 1 star needed, 1 candidate (row 10, column 5).
36. Row 10 complete: Mark 2 remaining cells as X (row 10, column 1).
37. Region completion: 1 star needed, 1 candidate (row 9, column 7).
38. Row 9 complete: Mark 1 remaining cell as X (row 9, column 1).
39. Column 3 complete: Mark 2 remaining cells as X (row 4, column 3).
40. Region completion: 1 star needed, 1 candidate (row 6, column 4).
41. Mark 2 cells adjacent to stars as X (row 5, column 5).
42. Row 6 complete: Mark 2 remaining cells as X (row 6, column 1).
43. Column completion: 1 star needed, 1 candidate (row 4, column 4).
44. Mark 2 cells adjacent to stars as X (row 3, column 5).
45. Column 5 complete: Mark 2 remaining cells as X (row 1, column 5).
46. Column 7 complete: Mark 3 remaining cells as X (row 1, column 7).
47. Clump analysis (Column 2): candidate clumps can only just supply the remaining stars (row 4, column 1).
48. Row completion: 1 star needed, 1 candidate (row 7, column 2).
49. Three candidates in a column: Middle cell can't hold a star (row 2, column 1).
50. Region completion: 1 star needed, 1 candidate (row 3, column 1).
51. Mark 1 cell adjacent to stars as X (row 4, column 2).
52. Region completion: 1 star needed, 1 candidate (row 5, column 2).
53. Row 5 complete: Mark 1 remaining cell as X (row 5, column 6).
54. Region completion: 1 star needed, 1 candidate (row 4, column 6).
55. Mark 1 cell adjacent to stars as X (row 3, column 6).
56. Row 4 complete: Mark 1 remaining cell as X (row 4, column 10).
57. Column completion: 1 star needed, 1 candidate (row 1, column 1).
58. Clump analysis (Column 6): candidate clumps can only just supply the remaining stars (row 1, column 8).
59. Row completion: 1 star needed, 1 candidate (row 3, column 10).
60. Mark 1 cell adjacent to stars as X (row 2, column 10).
61. Column completion: 1 star needed, 1 candidate (row 2, column 8).
62. Region complete: Mark 1 remaining cell as X (row 1, column 10).
63. Row completion: 1 star needed, 1 candidate (row 1, column 6).
64. Mark 1 cell adjacent to stars as X (row 2, column 6).
65. Solved! Every row, column, and region has its two stars, none touching.