Messier 21 is an open star cluster located in the constellation Sagittarius. It was discovered by the French astronomer Charles Messier in 1764 and is approximately 4,000 light-years away from Earth. The cluster contains around 57 stars, with its brightest member being a magnitude 7.5 star. The stars in Messier 21 are relatively young, estimated to be about 6 million years old. This cluster is part of the larger Sagittarius star cloud, which is rich in gas and dust, making it an interesting area for studying stellar formation and evolution.