The star and crescent is an iconographic symbol used in various historical contexts, including as a prominent symbol of the Ottoman Empire, with numerous modern countries still using it as a national symbol. It was developed in the Greek colony of Byzantium ca. 300 BCE, though it became more widely used as the royal emblem of Pontic king Mithradates VI Eupator after he incorporated Byzantium into his kingdom for a short period.

The Ottoman flag of 1844 with a white "ay-yıldız" (Turkish for "crescent-star") on a red background continues to be in use as the flag of the Republic of Turkey with minor modifications.

Other Ottoman successor states using the star and crescent design in their flag are Tunisia (1831), Libya (1951, re-introduced 2011) and Algeria (1958). The modern emblem of Turkey shows the star outside the arc of the crescent, as it were a "realistic" depiction of a conjunction of Moon and Venus.

The same symbol was used in other national flags introduced during the 20th century, including the flags of Azerbaijan (1918, re-introduced 1991), Pakistan (1947), Malaysia (1948), Mauritania (1959).

More Info: en.wikipedia.org