The Atlantic Star.

A six pointed star, in the centre the royal cypher GRI VI , within a circlet bearing the title "THE ATLANTIC STAR". A crown is positioned on the circlet at 12 o'clock.
Bars issued: two; Air Crew Europe,   France and Germany.
Ribbon emblem issued: one; a silver rosette denoted the award of a bar. (for when the ribbon alone was worn)

     Awarded for service in the Atlantic and home waters including the north Russian convoys. The qualification period was generally 6 months¹ for the navy and army, and two months for RAF aircrews². Time spent as a prisoner of war³ counted, personnel whose service was cut short due to death, wounds or sickness would qualify, those who won an award /decoration, mention in despatches etc. in the relevant theatre were also automatically entitled to the star.

Notes.
1. Between the 3rd September 1939 and the 8th May 1945.
2. Provided the 1939-1945 Star had been earned first.
3. Provided The 1939-1945 Star had been earned at the time of capture.

Also see official 1945 Government statement.

The Ribbon

The Atlantic Star ribbon: three stripes of shaded and watered dark blue, white and sea green, symbolising the Atlantic Ocean