The Southern Cross Crux constellation The four brightest stars seen here identify the familiar asterism of the Southern Cross Alpha Crucis lower bottom right the brightest star in the constellation is a double system formed by two stars with similar magnitudes and only arcseconds apart Crux is the smallest constellation in the sky covering only square degrees The Coal Sack Nebula round dark at lower centre left is one of the main features in Crux It is a relatively nearby light years dark nebula and is formed by a cloud of interstellar dust which absorbs the radiation emitted by the stars in this area