Play Galaxies Online

What Is Galaxies?

Galaxies — also known as Tentai Show (天体ショー) or Spiral Galaxies — is a logic puzzle originally published by the Japanese puzzle company Nikoli. The puzzle is played on a rectangular grid containing a number of circle markers called galaxy centres. Your task is to divide every cell in the grid into regions (galaxies) such that each region contains exactly one centre and each region is 180° rotationally symmetric (point symmetric) around its centre.

The elegance of Galaxies lies in its single, intuitive constraint: symmetry. Because every region must look the same when rotated half a turn around its dot, a single cell placement immediately forces its mirror partner into the same region. This creates satisfying chains of deductions, where working out the boundary of one galaxy ripples through the entire grid. No arithmetic is required — only spatial reasoning and logical elimination.

Rules of Galaxies

1. Divide the grid. Every cell must belong to exactly one region (galaxy). There are no leftover or shared cells.
2. One centre per region. Each region contains exactly one galaxy centre (circle marker). The centre may sit on a cell centre, a cell edge, or a cell corner.
3. Rotational symmetry. Every region must be 180° rotationally symmetric (point symmetric) around its galaxy centre. If a cell is in the region, the cell at the opposite side of the centre (rotated 180°) must also be in the same region.
4. Contiguous regions. Each region must be a single connected group of cells — no disconnected parts.

How to Play Online

1. Choose a grid size and difficulty. Pick from 5×5 to 10×10 grids, each with Easy, Medium, or Hard settings.
2. Study the centres. Galaxy centres (circles) can appear on cell midpoints, edges between cells, or grid-line intersections. The position of the centre tells you a lot about the region's shape.
3. Click or tap edges. Click between two cells to toggle a wall on or off. Walls mark the boundaries between different galaxies.
4. Use Check. Press ✓ Check to see if any region violates the rules — incorrect edges will highlight briefly.
5. Solve it! When every edge is correctly placed and all regions are valid, the puzzle auto-completes and shows your time.

Strategy Tips

1. Start with Corner and Edge Centres

Galaxy centres near the corners or edges of the grid are the most constrained. A centre on a corner must always produce a region that fits against two walls, severely limiting possible shapes. Work these out first — they often provide anchor points that ripple inwards.

2. Use the Symmetry Constraint

Every cell you assign to a galaxy automatically forces its rotational partner into the same region. If adding a cell would make the partner fall outside the grid or into another galaxy's territory, that cell cannot belong to this galaxy. This two-for-one deduction is the backbone of the puzzle.

3. Look for Forced Walls

Two adjacent cells that belong to different galaxies must be separated by a wall. If you can determine that two neighbouring cells cannot share a galaxy (because their symmetry partners conflict), place a wall between them immediately.

4. Mind Centre Positions

A centre on a cell midpoint means the region has an odd number of cells (the centre cell plus symmetric pairs). A centre on an edge between two cells guarantees those two cells are in the same region. A centre on a corner (grid-line intersection) means the four cells around it all belong to the same region.

5. Eliminate by Neighbour Galaxies

If a cell can only belong to one galaxy (all other neighbouring galaxies' symmetry constraints would be violated), assign it immediately. Similarly, if a galaxy would become disconnected without a certain cell, that cell must belong to it.

6. Work Incrementally

Don't try to solve one galaxy completely before moving on. Place a few certain cells in one galaxy, then switch to a neighbour where the new boundaries create fresh deductions. This interleaved approach prevents dead ends and keeps the logic flowing.

History of Galaxies

Galaxies was created by Nikoli, the renowned Japanese puzzle publisher that also brought the world Sudoku, Slitherlink, and Nurikabe. The puzzle first appeared in Nikoli's Puzzle Communication magazine in 2001 under the name Tentai Show (天体ショー, literally "Celestial Show"). It was later adopted internationally under the name Spiral Galaxies or simply Galaxies.

The puzzle gained broader recognition through the World Puzzle Championship, where it has appeared as a competition puzzle multiple times. Its visual appeal and the unique focus on rotational symmetry set it apart from other grid-based logic puzzles. In computer science, Galaxies has been studied as an NP-complete problem, meaning that while solutions can be verified quickly, finding them is computationally hard in the general case — which is part of what makes it such a satisfying challenge for human solvers.

Unlike pure number-placement puzzles like Sudoku or Kakuro, Galaxies is fundamentally a spatial partitioning puzzle. This has made it popular among puzzle enthusiasts who enjoy geometric reasoning. It has appeared in collections alongside Shikaku, Nurikabe, and Fillomino — all of which share the theme of dividing a grid into constrained regions.

Galaxies vs Other Grid Puzzles

Galaxies shares DNA with several other partition-based logic puzzles:

• Shikaku (Rectangles) also divides a grid into regions, but every region must be a rectangle containing exactly one number equal to its area. Galaxies allows any shape, with the symmetry constraint replacing the rectangle requirement.
• Nurikabe partitions a grid into numbered islands and a connected sea. Both puzzles require spatial reasoning, but Nurikabe focuses on connectivity and the 2×2 pool rule rather than symmetry.
• Fillomino divides a grid into polyomino regions where each region's size matches a number inside it. Like Galaxies, the shapes can be irregular, but the constraint is numerical rather than geometric.
• Slitherlink (Fences) defines regions implicitly through a single closed loop. Both puzzles involve boundary deduction, but Slitherlink uses edge-count clues instead of symmetry centres.

Frequently Asked Questions

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