Sky Guide September 2023

For millennia the sky has been divided into groups, or constellations, that have been used as tools to share significant cultural stories, calendrical events and as positional markers. Today, the 88 Western constellations that trace their roots to the ancient Middle East are used to help astronomers map the sky and search for astronomical objects. In September these constellations dominate the sky.

Virgo the maiden, very low in the west is the second-largest constellation in the sky and one of the two in which the ecliptic, (path of the Sun, Moon and planets) and the celestial equator cross. This crossing marks the current position of the spring equinox for those in the Southern Hemisphere. Virgo is home to a cluster of galaxies of which M87 is the largest at about54 million light-years with a central black hole more than 6billion times the mass of the Sun. Virgo’s brightest star, the 16th brightest in the night sky, Spica, a spectroscopic double star, is about 250 light-years away. The primary of the pair is about 7 times the diameter of the Sun and 11 times more massive, making it a prime candidate to one day explode as a Type II supernova. The hot surface temperature of both at between 20,000 to 25,000 kelvins makes them appear blue-white in colour.

Scorpius higher in the western sky, with its hooked tail and red supergiant Antares as the heart, the constellation is easily seen to look like its namesake. In Greek mythology, the scorpion plays a role in many stories, however it is best known for its pursuit of Orion through the night sky.

Sagittarius also known as the Archer, can be found 15° to the east of Scorpius’s stinger and directly overhead an hour or so after sunset. In Greek mythology, the archer is a centaur, pointing his arrow towards the heart of Scorpius. The centre of our Milky Way galaxy appears in this constellation at about 26,000 light-years and is home to a 4 million solar-mass black hole called Sagittarius-A-Star (Sag A*).