# More fascinating facts about stars

**Posted:**1 April 2017

**Filed under:**Drawing, Education, Entertainment, Mathematics, Uncategorized |

**Tags:**mathematics, Prime Numbers, Stars, Symmetry 1 Comment

I’ve had a few thoughts and discussions since I wrote this article about drawing stars. I thought it was worth sharing.

## Stars within stars

The first observation is that stars, drawn in the way I described, contain *other* stars, nested within one another like a set of Russian dolls. Recall that we use the term ‘*n-k star*‘ to indicate a star with n points such that, if we draw a circle through all the points and consider the boundary of that circle as split up into n curve segments bounded by the points, then the straight line from one point to another traverses k of those curve segments. Like this, for a 7-3 star:

Yuriy made an interesting observation about the stars in my last article, that the sum of the angles of all the points of an n-k star is 180^{∘} × (n-2k). In the course of thinking of ways to prove that formula true, I came upon the realisation that a n-k star contains a n-(k-1)star, which contains a n-(k-2) star and so on down to the n-1 star, which is a regular, n-sided polygon.

Here’s a picture that shows this for an 11-5 star.

The red, outer 11-5 star contains a green 11-4 star, which contains a red 11-3 star, which contains a green 11-2 star, which contains a red 11-1 star (polygon). The points of each inner star are the *innermost* vertices of the star that immediately contains it. Since we will be referring to those vertices again later, let’s make up a name for them. We’ll call such a vertex a ‘**tniop**‘, since it is in a sense the opposite of a point. The above diagram shows a *point *and a *tniop*. We’ll call the stars inside a star ‘*sub-stars*‘.

We saw in my last essay that, when a star cannot be drawn without taking the pencil off the paper, it is made of a number of ‘component stars’, that are rotated copies of one another. Here is a picture of a 16-6 star, which uses different colours to highlight the two component stars. We have two 8-3 stars, one light blue and one red.

And here is a picture that uses colour variation to show the sub-stars of each of the two components.

The 8-3 light blue star contains an 8-2 star which is made up of two components that are 4-1 stars (also known as ‘squares’) coloured blue and pink. Similarly, the 8-3 red star contains an 8-2 star which is made up of two components that are 4-1 stars (also known as ‘squares’) coloured yellow and green.

The 8-2 stars each contain an 8-1 star (octagon) as the intersection of two squares -pink and dark blue for one octagon and yellow and green for the other.

Finally, those two octagons between them, bound a hexadecagon (16-sided polygon or 16-1 star). So altogether, in the one picture, we have:

- one 16-6 star (red and light blue)
- one 16-5 star (also red and light blue) inside that
- one 16-4 star (pink, green, dark blue and yellow) inside that
- one 16-3 star (also pink, green, dark blue and yellow) inside that
- one 16-2 star (also pink, green, dark blue and yellow) inside that
- one 16-1 star (also pink, green, dark blue and yellow) inside that
- two 8-3 stars (one red, one blue) making up the 16-6 star
- two 8-2 stars (one pink and dark blue, one yellow and green), one inside each of the 8-3 stars
- two 8-1 stars (octagon: one pink and dark blue, one yellow and green), one inside each of the 8-2 stars
- four 4-1 stars (squares: coloured pink, dark blue, yellow and green) which, in pairs, make up the 8-2 stars.

That’s sixteen stars altogether. What a lot of stars in one drawing! Can you see them all?

Here’s a different colouring that makes it easy to see all five 16-point stars:

Although the stars get smaller as *k* reduces, they do not shrink away to nearly nothing. In fact they get closer together as they go inwards, as if they are asymptotically approaching a circle of some fixed, minimum size.

To investigate this, I drew a 101-50 star:

You’ve probably noticed by now that I’m no longer drawing these by hand. My drawing is much too wobbly to capture the intricacies of stars-within-stars. So I wrote a computer program to draw them for me. I’ll try to remember to attach it at the end of the article, so that those of you who like mucking about with computers can muck about with it.

Anyway, that 101-50 star pretty well killed my hypothesis that the inner stars can’t get very small. In this one they almost disappear out of sight. I like the swirly patterns. I haven’t yet worked out whether they are really features of this very spiky, very complex, star, or whether they are just artefacts of the crudeness introduced by the computer’s need to pixillate.

Here’s a zoomed-in image of the interior of that star. Cool, eh?

This is a low-resolution image. I have saved a moderately high-resolution image of this star here. Zooming in and out is fun. It seems almost fractal as more patterns emerge from the inside when we zoom in. Also, the stars give the illusion that they are rotating as we zoom. To get the best effect you need to download the file (a .png image file) and then open it up, so that zooming is not limited by your internet connection’s speed.

## Ratio of Outer to Inner radius

Let me briefly pick up on that idea above about whether there is some minimum inner radius for these stars. For each n, the spikiest n-k star is where k is the largest integer less than n/2, and this contains another k-1 stars, nested one within the other, down to the innermost, which is a n-sided regular polygon. We can work out the ratios of each star to the one immediately inside it, and use that to work out the ratio of the outermost, n-k, star, to the innermost, n-1, star. The attached computer program contains trigonometric formulas to do that. Here are the ratios of the radii of the innermost to the outermost star for each n from 1 to 109:

We observe that the ratios generally go down as n increases, but the decline is not steady. It bumps up and down. I have highlighted the prime numbers with asterisks. Notice how the ratio for those is always lower than for the numbers around them. The two drivers of the ratio seem to be:

- the size of n: the ratio generally declines as n increases; and
- the number of different factors n has. Note how 16 (divisible by 2, 4, 8) has a higher ratio than 15 (divisible by 3, 5) and 56 (divisible by 2, 4, 7, 8, 14) has a higher ratio than 55 (divisible by 5, 11).

This would be interesting to look into further, and to see if there is some neat, sweet, compact formula for the ratio that highlights the relationship to size and number of factors (if there really is one). But I have to stop thinking about that now or I’ll never post this essay.

## The most general form of symmetrical stars

We can make an awful lot of stars using the above approach. For an integer n the number of different n-pointed stars is the biggest integer less than n/2.

But in fact there is an infinite number of different n-pointed stars, without having to loosen our standards by allowing asymmetry. After a bit of thought, I realised that the most general form of n-pointed star can be specified by a single number, which is the ratio of its *inner radius *to its *outer radius*. The outer radius is the radius of the circle on which the points sit. The inner radius is the radius of the circle on which all the *tniops* sit. Given n and that ratio – call it θ, we can draw a star as follows:

- Draw two concentric circles with ratio of the inner to the outer radius being θ.
- Mark
*n*equally-spaced dots around the outer circle and draw faint lines connecting each of these to the centre. These will be the points of our star. - Mark a dot on the inner circle halfway between each of the faint, radial lines drawn in the previous step. These dots will the the
*tniops*of our star. - Working consistently in one direction around the circle, draw a zig-zag line fro point to tniop to point to tniop and so on, always connecting to the nearest dot.

We will describe such a star as a n/θ star. We use a slash rather than a dash in order not to mix it up with the former type of star. Here is a sequence of six-point stars, with the ratio of the inner to outer radius going from 0.2 up to 0.8:

It is nice that this gives us more options for stars – infinitely many different kinds of star for each *n* in fact. But they are not as much fun to draw as the n-k stars, and it is harder to make them come out right without geometric instruments – which rules them out as an effective doodling pastime.

Note how, unlike with the n-k stars, the line leading away from a point does not intersect any other point. Nor do we get any inner stars for free. The price of gaining more variety is a loss of *structure*. The inner structure of a n-k star provides a great richness by enforcing all sorts of relationships between the vertices.

Only very specific values of the radius ratio θ give us n-k stars. I worked out the formula for the ratio by the way, with a bit of trigonometry. The ratio of the tniop radius to the point radius for a n-k star is:

2 sin(π(k-1)/n) sin(πk/n) sin(π(1-(2k-1)/n)) / (sin(2π/2)+sin(π(1-(2k-1)/n)) )

– sin(π((2k-1)/n – ½) )

For a n/θ star, if there are no integers n and k that give values of that formula equal to θ, the star will not have the array of inner stars that a n-k star has.

## Computer Program to draw pretty stars

This whole diversion started as an exercise in drawing stars by hand. But there’s a limit to how intricate those stars can get without getting too messy with smeared ink or graphite. For those that like to look at pretty, intricate geometrical pictures, here’s my computer program in R that can draw stars of any of the types discussed.

## Sums of angles

For those that like mathematical proofs, there are outlines of proofs here of Yuriy’s observation about the sum of the internal angles at points of a n-k star.

Andrew Kirk

Bondi Junction, April 2017

# Drawing Stars. Number Two in a Series of Adult Amusements

**Posted:**2 January 2017

**Filed under:**Drawing, Entertainment, Mathematics, Uncategorized |

**Tags:**Drawing, Entertainment, mathematics Leave a comment

It has become apparent to me that the world needs another instalment in my series of suggestions for Adult Amusements. There have been complaints. Some are from pedants, who insist that a single monograph about standing on one leg does not constitute a series. Others, more gravely, have expressed concern about the occupational health and safety implications of people trying to balance on one leg while their mind is distracted by other things, like budgets, work-shopping and brain-storming, not to mention trying to be Pro-Active, Customer-focused, Agile, Continuously Improving and Outside the Box all at the same time.

So, belatedly, here it is. I hope that this will be considered less dangerous, being a mostly sedentary activity.

When in business meetings that do not hold us riveted with fascination, **we should draw stars!**

But not just any old stars. Special stars. Mathematical ones. Stars with prime numbers in them.

It is the dearest wish of every little child, after that of being a firefighter or an astronaut, to draw excellent stars in their pictures. But a wish is one thing, and its fulfilment is another. When as a child I tried to draw stars, the only technique I could think of was to draw a spiky circle. Start anywhere, and draw a perimeter that goes around an imaginary centre, that is a series of spikes. Maybe this works OK for others, but for me it typically produced a result like this (Figure 1):

It invariably goes wonky, because it’s hard to keep track of where the centre is supposed to be, and to make the points point away from that centre. Mine looks like a confused kookaburra.

When one gets a little bit older and more sophisticated, one learns – by instruction or by observation of others – the two standard techniques for drawing stars. These are the six-pointed star, which is made by drawing an upside-down triangle slightly above a right-way up one (Figure 2):

and the five-pointed Pentacle, which requires a little more coordination, but can be done without taking the pencil off the paper (which I call a ‘single pencil stroke’), by following the arrows as shown (Figure 3):

Learning to draw either of these stars is on a par with learning to ride a bike, in terms of the sense of achievement, wonder and progress. All of a sudden, one can construct an image of symmetry and elegance with the stroke of a pencil – or two strokes, in the case of the six-pointer.

I was very happy with this advance in technology for a long time, but then came the day when I hankered after drawing more bristly stars, with seven, ten or twelve points. I tried, but found I was just reverting back to the method of figure 1, and my bent stars just did not satisfy me.

One could of course take out a protractor and compass and, with a bit of preliminary calculation, measure out the exact angles needed for each point, and draw the star using that. But firstly that’s cheating, and against the Spirit of Doodling, and second it might cause others to notice that one is not paying attention to whatever the meeting is discussing.

I thought I was destined to be forever that object of public ridicule – the man with the two-star repertoire. But just as I was starting to come to terms with this being my fate, a discovery came to me in a blinding flash: instead of trying to draw spikes in a circle, I needed to generalise the methods used for the five and six-pointer. Well, to cut a long story short, I tried that, it worked, and now I can draw stars with any number of points up to about fifty.

Here is the method that generalises the way we draw five-pointed stars:

## Drawing a star with a single pencil stroke

- Step 1: pick the number of points N, and draw that number of points, as evenly spaced as you can, around the perimeter of an imaginary circle. If there is a large number of points it’s best to first draw points at the 12, 3, 6 and 9 o’clock locations and then put one quarter of the remaining points into each of the four quadrants. To be precise, divide N by 4 to get a quotient Q and a remainder R. Then draw Q points in each of R quadrants of the circle, and Q-1 points in the other quadrants. Ideally, if R=2, adjacent quadrants should not contain the same number of points, but it doesn’t matter very much if that is forgotten.
- Step 2: pick a number K, greater than 1, that has no common factors with N. To make the spikiest possible star (ie with the thinnest spikes), choose K as the largest whole number less than N/2 that has no common factors with N. For instance if N=12 that number is 5. If N=13 it is 6. If N=6, 4 or 3 there is no possible K, and this method cannot be used. I’m pretty sure that, for any N greater than 6, there is at least one K for which this method will work, but I have not proved that yet.
- Step 3, choose your favourite direction in which you want to draw. Unless you are a pan-dimensional creature drawing on paper with three or more dimensions, your only possible choices are clockwise or anti-clockwise.
- Step 4 starting at any point, draw a straight line from that point to the point that is K steps away from it, hopping from point to point around the circumference in the chosen direction. We can call K the ‘side length’, since it is the length of the line that connects one point to another.
- Step 5: repeat step 4 until you get back to the starting point.

If this process is executed carefully, you will have drawn a star that has a point at every one of the points you drew in step 1. And, if you want, you can do all the actual line drawing in steps 4 and 5 without ever taking your pencil off the paper.

Here is a depiction of that process for an eleven-pointed star with side length 5:

And here is a depiction of this process for a sixteen-pointed star with side length 5:

Why do we not allow the side length K to be 1? That’s because if we do that, we just get a N-sided shape which, ignoring any irregularities in our drawing, is a regular polygon, like this, for N=12 (a ‘dodecagon’):

Now the thing about stars is that they are not *convex*, while regular polygons *are*. Using the word ‘vertex’ for a place where two edges of a shape meet, an N-pointed star has 2N vertices, of which N are points – the outermost part of a peninsula (if we imagine the shape as an island in an ocean) and the other N are the innermost part of a bay. As we go around the vertices of a star they alternate between inlet and bay. So a regular polygon is not a star because it has no bays, and that’s why K must be more than 1.

## Stars with more than one pencil stroke

We observed that the above method does not work for N=6. But we know we can draw a six-pointed star, using two pencil strokes to draw two overlapping triangles. We can use the approach taken there to invent many more stars. In fact, for an N-pointed star there are M different types we can draw, where M is (N+1)/2-2, rounded down to a whole number. Each of these shapes corresponds to using a different value of K, from 2 up to the biggest whole number below N/2.

Here is how we do it:

- Step 1, for picking N and drawing the points around an imaginary circle, is the same as above.
- Step 2. We pick any K as any whole number greater than 1 and less than N/2.
- Do steps 4 and 5 from above. This will draw a shape that is either a star or a polygon. Now comes the tricky bit.

If the shape you drew has not touched all the N points around the circle, repeat the process starting on a point that has not been touched yet. I like doing this with a different colour pencil, as it helps me see the pattern and avoid getting confused.

Repeat that process, using a different colour pencil each time, until all points have been touched.

You will now have a N-pointed star, made up of a number of identical overlapping shapes, which are either all polygons or all stars.

For those that like mathsy stuff, the number of overlapping shapes – the number of pencil strokes required – will be the greatest common factor of N and K. It’s fun to try to work out why that is.

The traditional six-pointed star in figure 2 above is what you get under this method when you use N=6 and K=2. Here are a couple of others:

If we are going to draw a lot of different stars, we need names for them. We could call the star drawn with N points and side length K a ‘N-K star’, so that the pentacle is a 5-2 star and the traditional six-pointer is a 6-2 star.

## Thin stars

If we wanted to, where N is even, we could let K be N/2. What we get then is this sort of thing:

The shape we have drawn with each pencil stroke is a single line between a point and the point directly opposite it. Strictly speaking, this too is a star, but I mostly leave it out because it’s not as interesting as the others because (1) everybody knows how to draw a star like that; (2) as any five-year old would tell us, that’s not what stars look like in pictures of things in the night; and (3) it has no inside, so we can’t colour it in all yellow (well, actually the one I drew has a tiny little inside in the middle, because it’s not perfectly symmetrical. But a more accurate drawing would have all the lines going exactly through the middle of the circle, so that there’s no inside at all).

## Other things

So now you know how to do lots of great stars. You need never be bored in a meeting again. Imagine if you started drawing all the possible stars, starting at the smallest number of points and going up in side-lengths and points until the meeting finished. Leaving out the too-easy ‘thin stars’, you would draw the following stars:

5-2, 6-2, 7-2, 7-3, 8-2, 8-3, 9-2, 9-3, 9-4, 10-2, 10-3, 10-4, 11-2, 11-3, 11-4, 11-5, 12-2, and so on.

Just drawing those, given a due amount of tongue-stuck-into-side-of-mouth-concentration, should be enough to get you through at least a half hour of Death By Powerpoint.

But let’s not forget our roots. With a very few exceptions, we all started off drawing stars like Figure 1. There is a touching ingenuousness about such stars, and I think it’s good to draw them as well. Often really interesting shapes arise when we do, looking like monsters or funny animals. And one good thing about that way is that you don’t have to decide how many points it will have before you begin. You just draw spikes around a circle until you get back to the start. I’ll sign off by doing that for a star with LOTS of points (it ended up being 21), and following it up by a series of the nine different stars with the same number of points drawn by the above method.

I think that each has a certain appeal, in a different way.

# Mastering maths monsters

**Posted:**22 October 2016

**Filed under:**Education, Mathematics, Science, Uncategorized |

**Tags:**mathematics, science Leave a comment

‘Partial differentiation’ is an important mathematical technique which, although I have used it for decades, always confused me until a few years ago. When I finally had the blinding insight that de-confused me, I vowed to share that insight so that others could be spared the same trouble (or was it just me that was confused?). It took a while to get around to it, but here it is:

https://www.physicsforums.com/insights/partial-differentiation-without-tears/

My daughter Eleanor make a drawing for it, of a maths monster (or *partial differentiation monster*, to be specific) terrorising a hapless student. The picture only displays in a small frame at the linked site, so I’m reproducing it in all its glory here.

Andrew Kirk

Bondi Junction, October 2016

# Acoustic ‘beats’ from mismatched frequencies

**Posted:**1 August 2016

**Filed under:**Acoustics, Mathematics, Music, Physics, Uncategorized |

**Tags:**Acoustics, mathematics, Music, physics, science Leave a comment

Here’s a piece I wrote explaining the mathematics behind the peculiar phenomenon of acoustic ‘beats’.

https://www.physicsforums.com/insights/acoustic-beats-mismatched-musical-frequencies/

It’s a bit *maths-*y. But for those that don’t love maths quite as much as I do, it also has some interesting graphics and a few rather strange sound clips.

Andrew Kirk

Bondi Junction, August 2016

# Why mathematics is like plumbing

**Posted:**14 December 2014

**Filed under:**Humour, Mathematics, Science |

**Tags:**Homology Theory, mathematics, Plumbing, Topology Leave a comment

Mathematics is a bit like plumbing, don’t you think?

Not that I’ve ever done any proper plumbing. Electrical work I can manage, having paid careful attention when doing electricity in our high school physics classes. But plumbing is another matter altogether. Getting those joints water-tight requires just a bit more dexterity than I can manage.

That’s why it’s like maths. It’s those joints that are the problem. Let me explain.

Often in maths, when one is trying to prove some important result, one creates a series of theorems that build on one another to reach the final conclusion. If a proven result along the way is substantial enough – say if it takes more than half a page to prove – we call it a ‘theorem’, otherwise we call it a ‘lemma’. But a lemma is really just another theorem, only a bit smaller than usual.

Now a theorem is something that reads like this:

IF <bunch of premises> is true THEN <conclusion> is true.

Mathematicians try to make their premises as few and weak as possible, and their conclusion as strong as possible. For instance, we could prove a theorem that

IF x=2 THEN x^{2}>0

This theorem is true, and easy to prove. But we can either make the conclusion stronger (ie more specific), turning it into say x^{2}=4, or we can make the premises weaker, turning them into say ‘x is a real number’. We can’t do both however. We could, if we wanted, make the premises weaker and the conclusion stronger, but not by as much as what we just did. We could prove the theorem:

IF x>1 THEN x^{2}>1

Now our premise is weaker (less specific) than before, and our conclusion is stronger (more specific) than before.

How this is like plumbing is as follows. Say we want to prove theorem D and, to do that we need to first prove theorems A, B and C. This might be because we need A to prove B, B to prove C and then C to prove D. So the conclusion of A becomes a premise for B, the conclusion of B becomes a premise for C and so on.

Now unless D is very easy to prove (eg like proving IF x=2 THEN x^{2}>0), we won’t have much room for manouevre. If we make the premises of D too weak then we won’t be able to prove the conclusion, which is our ultimate goal. On the other hand, if we make the premises of D too strong we won’t be able to satisfy them with the conclusion of C.

This is like connecting a series of pipes (got you! And just in time. You thought I’d never get around to the plumbing didn’t you?). Each theorem is a pipe. Each pipe is a straight cylinder except that, at its inflow end, it flares out like a trumpet bell and, at its outflow end, it narrows. This enables pipes of the same diameter to snugly fit together. The end of each pipe has to fit *inside* the end of the pipe immediately downstream from it if the fit is to be tight enough to avoid a leak.

The inflow end represents the theorem’s premises. The stronger the premises, the narrower the inlet, and the stronger the conclusion the narrower the outlet. Now what happens if we make the premises of pipe D too strong? Then the downstream end (conclusions) of pipe C may not be able to fit inside the premise end of pipe D. If that happens then we need to narrow pipe C, which means making its premises stronger – hence a narrower inlet. That may then create a problem with fitting B into C, and so on. So if we are too ambitious in what we are trying to prove at D, that can create problems all the way back up the stream so that our premises at A are so narrow and restrictive as to make the whole combined theorem of no practical use (because the premises at A will hardly ever be satisfied). The theorem might only apply to left-handed, bearded Scottish taxidermists whose first and last names both begin with ‘X’.

Or, starting at the upstream end, if we start with premises that are too weak – non-specific – at A, our whole series of pipes will have to be so wide that the eventual conclusion at A will be as generalised as an astrologer’s forecast (‘Today will be a good day for working hard and believing in yourself!’). How can I relate this diffuse outcome to plumbing? Hmm, well if the pipe is too wide at the outlet then perhaps there won’t be enough pressure. Yes that’s it, no pressure. It would be like trying to water the garden with a big drainage pipe instead of a hose.

What one finds oneself doing then, as one tries to make the connection from A all the way to D, is going back and forth, loosening a premise here, tightening a conclusion there, then finding that with the premise we just loosened, the conclusion coming out of that pipe is too loose (wide) to fit into the next pipe so we tighten the premise again or alternatively look at the following pipe to see if we can get away with loosening the premises for that.

And now for one last bit that is completely unrelated, except that it also is like plumbing. Recently I’ve been working on something called Homology Theory, which is what you need to be able to tell the difference between a sphere and a donut. This discipline uses techniques of algebra to identify differences between different shapes. It often makes use of chains of things called *modules*, connected by *functions*, which are like arrows leading from one module to another. It’s a bit like how you can make a model of a molecule using plasticine for the atoms and straws for the chemical bonds (the modules are the plasticine and the functions are the straws, except it’s important to remember that the functions have a *direction*, as if water were flowing through the straw). An important part of a function going out of a module is something called the ‘Kernel’, which is a bit like the nucleus of the atom. Each function in a sense ‘projects’ the module from which it starts onto the module at which it ends, and the projection it makes is called the ‘Image’ of the function, which will be part or all of the target module. We call a chain of functions and modules ‘Exact’ if the image of the function coming in is identical to the Kernel of the function going out. Most chains are not Exact, but when a chain is Exact we can prove all sorts of useful things.

So this is more plumbing. If the pipe/straw (function) coming into a joint (module) is too wide (has too big an Image) it won’t fit the Kernel of the pipe/straw (function) going out. Then the liquid will spill out, we’ll have a big mess and we’ll never be able to tell our donuts from our spheres, let alone our circles and Klein Bottles. But if it all fits neatly, we’ll have an Exact Sequence, there’ll be no spillage, and we’ll be able to prove all sorts of useful things without needing a change of clothes.

I’m rather pleased with how many different metaphors I managed to mix together in that last couple of paragraphs. It leaves ‘to take arms against a sea of troubles’ far behind. I think the clarity may have suffered somewhat as a consequence though. Never mind.

Andrew Kirk

Bondi Junction, December 2014