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The explanation of the 7 stars, as with many of the symbols in Revelation, is given within the context of the passage in which it occurs. Here are the passages in which this phrase occurs in the book of Revelation.
Revelation 1:16 And he had in his right hand seven stars: and out of his mouth went a sharp two-edged sword: and his countenance was as the sun shineth in his strength.
Revelation 1:20 The mystery of the seven stars which thou sawest in my right hand, and the seven golden candlesticks. The seven stars are the angels of the seven churches: and the seven candlesticks which thou sawest are the seven churches.
Revelation 2:1 Unto the angel of the church of Ephesus write; These things saith he that holdeth the seven stars in his right hand, who walketh in the midst of the seven golden candlesticks;
Revelation 3:1 And unto the angel of the church in Sardis write; These things saith he that hath the seven Spirits of God, and the seven stars; I know thy works, that thou hast a name that thou livest, and art dead.
In Revelation 1:20 the 7 stars are the "angels" of the 7 churches to whom he is writing. The word "angel" in the New Testament simply means "messenger". It can denote a divine messenger, which is what we usually think the term "angel" means, but it can also refer to a human messenger.
In this case it would seem that the letters are sent to the 7 churches to the one who is the leader of the congregation or by the hand of a particular messenger.
The most likely explanation is that it refers to the leader of the church to whom the letter was addressed. Just as the "11 stars" in Joseph's dream in Genesis 37:9,10 refer to the patriarchs of Israel so these stars refer to the leaders of the churches to whom John was directed to write by the Lord Jesus Christ.
Here's how Young's Literal Translation puts Revelation 1:20:
...the secret of the seven stars that thou hast seen upon my right hand, and the seven golden lamp-stands: the seven stars are messengers of the seven assemblies, and the seven lamp-stands that thou hast seen are seven assemblies.
This verse explains not only the meaning of the 7 stars but also the meaning of the symbol of the lampstand. The stars then are the messengers or leaders of the congregations to whom the book is addressed and the lampstands symbolize the actual congregations.
I hope you have found this helpful.