a big gold star
an upset/underdog victory; giant killing
金星 means the same thing but is less common than 大金星. It is often used as 大金星を挙(あ)げる meaning “achieve an upset victory”. It originates from the sumo term that describes a surprising victory of an inexperienced sumo wrestler against an experienced one.
Japan defeated Germany in FIFA World Cup 2022 and achieved an upset victory.