it has something sticking out of it

[GoldfishLord]

You can refer to a shape or an object as a star when it has four, five, or more points sticking out of it in a regular pattern.

Source: https://www.collinsdictionary.com/dictionary/english/star

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It would seem to me that "sticking" could not be substituted with "which stick" because it would make the sentence odd.
Is this right?

 

[Tarheel]

That sentence is not odd at all.

 

[Tarheel]

You can refer to a shape or an object as a star when it has four, five, or more points sticking out of it in a regular pattern.

Source: https://www.collinsdictionary.com/dictionary/english/star

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It would seem to me that "sticking" could not be substituted FOR "which stick" because it would make the sentence odd.
Is this right?

No, you are not right.

 

[GoldfishLord]

You substitute the new for the old, but substitute the old with the new. The easiest way to get it right is to think of sport, where, if someone is injured, you bring on a substitute.
I think that "sticking" could not be substituted WITH "which stick"" is correct.

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You can refer to a shape or an object as a star when it has four, five, or more points sticking out of it in a regular pattern.

Could "sticking out of it in a regular pattern" be seen as a reduced relative clause?

 

[Tarheel]

Two things. One, thanks for the lecture, but I'll stick with my way of doing things. Two, in the original sentence nothing was substituted for anything, so there's that.

 

[teechar]

It would seem to me that "sticking" could not be substituted with "which stick"

That is not correct. Either would work in that sentence.
Both structures (the present participle clause "sticking out of it", and the relative clause "which stick out of it") do the same job here (give extra information).

because it would make the sentence odd.
Is this right?

No.

Participle clauses

Do you know how to use participle clauses to say information in a more economical way? Test what you know with interactive exercises and read the explanation to help you.

learnenglish.britishcouncil.org

Could "sticking out of it in a regular pattern" be seen as a reduced relative clause?

Yes, you can think of the full version as "which are sticking out of it".