# Is there a proper term for the 'arms' of a star?

I have the need to describe, in a technical document, the particular portion of a star here highlighted red (or any of the other like areas):

For example:

The width of each `…?` shall be eight units at the point where they meet.

While in the title of this question I describe it as an arm, given the formal context of my intended usage, I'm loath to use such, as it seems an informal or slang term. Likewise, as I will be describing the dimensions of this section, using a term such as point seems inappropriate, as I would typically view that as referring to the vertex itself, not the area between the convex and concave vertices.

I've done some searches for arm and lobe, but can't seem to find any examples of their usage in this context. Is there a formal, proper, or technical term for this particular area of a star?

• @aparente001: "Point" could indeed be appropriate, except that it's also commonly used with a different meaning in geometry. In particular, it could be very confusing to speak of e.g. "the three corner points of the point of the star" or something like that. Dec 5, 2017 at 11:58
• I think the problem with point is that while you would say "a five-pointed star" to describe a star with five of the "things" the OP wants to refer to, the word "point" – probably both technically and in everyday understanding – means the outer vertex (or "point") where the "thing" ends. Personally, I'd be happy with arm (or possibly spoke). Dec 5, 2017 at 13:01
• For a moment, I thought that specifications of national flags would be a good place to look for this term, especially given the example desired usage in the question. However, national flags tend to just specify the diameter of the star's circumscribing circle (fixing that fixes the width of the _____s). Dec 5, 2017 at 16:26
• I suspect the reason why stars in flags etc. are often described in terms of inner and outer circles is that if you're drawing (or sewing) the flag with a compass and a ruler, it's convenient to start by tracing the circles first. Of course, depending on just how irregular your star is, you might be able to use some other enclosing shape (e.g. a triangle) instead of a circle, or perhaps even different enclosing shapes for different points (e.g. if you want the even-numbered points to be longer than the odd-numbered ones). Dec 6, 2017 at 11:52

In heraldry they are referred to as "rays".

I think I've heard them referred to as limbs. The Oxford English Dictionary (subscription required) gives the following definition which I think applies well to this situation:

4b) In various uses, chiefly of material things and more or less technical: A projecting section of a building, e.g. the outworks of a castle; one of the four branches composing a cross; ... a spur of a mountain range; ...

Admittedly, neither the definition nor the usage examples mention stars explicitly. Other dictionaries give similar definitions.

Also, starfish are described as having limbs and, while this is obviously by analogy with arms and legs, it seems reasonable to use the same word for non-living stars.

Note that astronomers use the term "limb" to refer to the whole edge of the visible round disc of an actual star such as the Sun – thanks to @Michael Seifert for pointing this out. I don't think this would be likely to cause confusion.

• In astronomical circles, the limb of a star denotes the whole edge, as in limb darkening. (Of course, stars don't have "points" when you look at them through a telescope.) Dec 5, 2017 at 19:32
• @MichaelSeifert: Actually, quite often they do, especially if it's a bright star and if the telescope happens to be an axial reflector (e.g. Newton or Cassegrain) design. Of course, five is a rather odd number (pun semi-intended) of diffraction spikes, since they usually come in symmetric pairs. Dec 6, 2017 at 11:37
• @Ilmari Karonen: There can be five. (A technique based on this can be used to observe Sirius B using relatively small telescopes - although usually a hexagonal diagphram is used.) Dec 6, 2017 at 21:56