Sun in Merriam-Webster means
1 a often capitalized : the luminous celestial body around which the earth and other planets revolve, from which they receive heat and light, which is composed mainly of hydrogen and helium, and which has a mean distance from earth of about 93,000,000 miles (150,000,000 kilometers), a linear diameter of 864,000 miles (1,390,000 kilometers), and a mass 332,000 times greater than earth
b : a celestial body like the sun : STAR
Under sense 1b it can mean any star and so "a sun" is fine.
Sense 3 is the same dictionary is
3 : one resembling the sun (as in warmth or brilliance)
So the quotation makes sense (no pun intended).