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if i shot 15 in foreground pic + 15 pic in background in same place at night photographywith 20 sec for each pic... would it be supposed to have slight "star trail" effect on the star?? how did you remove the "star trail" effect while you can maintain the perfect exposure for the background during editing?

Look up a technique called "exposure stacking for noise reduction". You can learn all about it.
But basically one of the steps is to align all of the sky layers. That way you get pin sharp stars :)