Pole Star vs pole star

Pole Star

name
  • The star nearest to Earth's axis when extended above the North Pole: currently Polaris, formerly Thuban. 

pole star

noun
  • The star visible to the naked eye which was in the past, is now, or will be in the future nearest a celestial pole of a planet. 

  • A person, principle, or thing which acts as a guide; a guiding light; also, something which attracts much attention; a centre of attention. 

How often have the words Pole Star and pole star occurred in a corpus of books? (source: Google Ngram Viewer )