Why do they read as Ichibanboshi? Isn't the Kunyomi of 星 is hoshi? Moreover, why do 星 is read with Kunyomi? Isn't supposed to be read with it's Onyomi considering that 一番星 is a bunch of kanji without any okurigana?
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2For your first question, look up rendaku, e.g. japanese.stackexchange.com/q/2526/1628 – Earthliŋ♦ Jun 16 '18 at 21:00
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2I don't understand your second question — "bunch of kanji without any okurigana" doesn't necessarily imply it should be read with on'yomi. – Earthliŋ♦ Jun 16 '18 at 21:01
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Why? Unless it's "special kanji(s)", it's should be read with Onyomi, right? – Lika Jun 16 '18 at 21:41
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2^ examples of three kanji compounds that contain kunyomi: ja.wikipedia.org/wiki/三字熟語#訓読みと三字熟語 – Chocolate♦ Jun 17 '18 at 3:45
It's been explained a bit in the comment replies already, but...
Why do they read as Ichibanboshi? Isn't the Kunyomi of 星 is hoshi?
As mentioned, this is "rendaku" in action. In compound words (such as 一番星), the later words sometimes have their first kana become accented. There are many such words. There are some rules that guide whether a word is rendaku'd or not, but it's hard to be sure. At the end of the day, you have to just get a feel for it and memorize the words.
why do 星 is read with Kunyomi? Isn't supposed to be read with it's Onyomi considering that 一番星 is a bunch of kanji without any okurigana?
The guideline that a "bunch of kanji without any okurigana" should be read using onyomis is not a bad rule of thumb, but there are many many many exceptions. Again, at the end of the day, you just have to get a feel for it, and memorize the words.
Sorry for possibly disappointing answer. Basically, you just have to accept it.