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 (1.4 and 1.9) only 4.4 arcseconds apart. Crux is the smallest constellation in the sky covering only 68 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 (500 light years) dark nebula and is formed by a cloud of interstellar dust which absorbs the radiation emitted by the stars in this area.