The full moon is a starting point to locate two of the brightest stars in the sky – Arcturus in Boötes and Spica in Virgo
The moon becomes full this week and on 18 April it sits squarely in the constellation Virgo, the Virgin. The chart shows its location at 2200BST that night. With the moon as a guide, two of the brightest stars can be located and compared. Higher in the sky to the east is Arcturus in the constellation Boötes, the herdsman. Arcturus is the fourth brightest star in the sky. It shines with an orange light and is roughly 37 light years from Earth. It contains about the same mass as the sun but has expanded to about 25 times the Sun’s diameter. This is the behaviour of a star nearing the end of its life. In the expansion, the surface temperature has dropped to roughly 4,000C (about 2,000 degrees lower than the sun’s surface temperature). Lower in the sky, slightly to the west is Spica, the brightest star in Virgo and the seventeenth brightest star in the sky. It is 250 light years from Earth, and actually comprises two stars in orbit around each other. Both blaze with a brilliant white light because they have surface temperatures around 25,000C and 20,000C. They contain seven and eleven times more mass than the Sun, and are seven and four times larger than our star.