While making a three dimensional sculpture out of 2140 elements would be a little much for a series where she makes a new piece of art every other day, it did seem reasonable to make a much smaller cubic star number shape out of 120 marbles. So she did.
What you're seeing here is (as far as I can tell) the first picture of a cubic star number searchable on the internet. While diagrams of these may exist in yet-to-be-indexed books, I could not find such a picture in anything that refers to cubic star numbers (such as Gulliver's 2002 article Sequences from Arrays of Integers).
The star number polygon shown here consists of a hexagon with triangles on each side (i.e., a hexagram). But you don't have to use a hexagon on the inside. You could just as easily use a square, with four triangles on each side of that square. This square star number might not look as pleasing as the hexagram does, but it has interesting properties all on its own.
But I think things get even more interesting once you move into the third dimension. Instead of a square, you can use a cube; and, rather than making a stellated shape where a pyramid exists on every face of the square, you can merely place the pyramids on four of the sides, so that the front and back of the cube remain flat. In this way, you're extrapolating out what a two dimensional star number might look like if you literally pulled it out into a new dimension, but turned the triangles into pyramids while allowing the square to fill out a cube.blogger referencing house numbers, which, to be fair, has a more distinctive shape to them. House numbers are closely related to cubic star numbers; rather than four pyramids, they exhibit just one. But it's easy to see how you can get to a cubic star number from the corresponding house number: just add three more pyramids and stick 'em on the sides.
Katherine chose to use the fourth cubic star number, 120. It consists of a 4x4x4 cube with four pyramids that each have a 3x3 base. This small cubic star number was created entirely out of marbles, using liquid silicone to connect them. It stands as a symbol of the much larger tenth cubic star number, 2140, which consists of a 10x10x10 cube with four pyramids that each have a 9x9 base.
I'm looking forward to seeing what else Katherine comes up with. I was fascinated by her prime factorization series, and this current series on the integers of covid cases seems just as good. I just wish we didn't have to keep spreading covid in order to generate these depressing numbers and associated fascinating art.