The far southern sky

Seen from Hanoi on a spring evening, the whole of the Southern Cross rises above the southern horizon, low but complete, with its brightest star Acrux reaching only about 6° up. From Ankara it never rises at all, and from Sydney it never sets. The Southern Cross, Crux, is the smallest of the 88 constellations, yet it is the most famous pattern of the southern sky, and it appears on the flags of Australia, New Zealand, Brazil, Papua New Guinea and Samoa. Beside it shine the two Pointers, α and β Centauri, and round them lies a part of the sky that observers in Europe and North America never see. It includes the brightest star after Sirius, the nearest star system to the Sun, and two companion galaxies of our own that the naked eye sees as patches of light. The south celestial pole itself has no bright star to mark it, so in this section we learn the Cross and the Pointers first, then three ways to find the pole, and then the rest of the far southern sky.

O1.6.1The Southern Cross and the Pointers

The four main stars of the Cross are Acrux (α Cru) at the foot, magnitude 0.77, Mimosa (β Cru) on the eastern arm, magnitude 1.25, Gacrux (γ Cru) at the top, magnitude 1.59, and Imai (δ Cru) on the western arm, magnitude 2.79. A fainter fifth star, Ginan (ε Cru), lies between Acrux and Imai. Gacrux looks orange next to the blue-white stars of the rest of the Cross, because it is a cool red giant. Just east of Acrux lies the Coalsack, a dark cloud of dust that blots out the Milky Way behind it and looks like a hole in the sky, and about 1° from Mimosa lies the Jewel Box, a small open cluster that binoculars show well.

East of the Cross shine the Pointers, Rigil Kentaurus (α Cen) and Hadar (β Cen), magnitude 0.61, 4.4° apart. Rigil Kentaurus is a pair of stars too close together for the naked eye to split, of magnitudes −0.01 and 1.35, and their combined light, of magnitude −0.28, makes it the third brightest star of the night sky. It is the nearest star system visible to the naked eye, only about 4.4 light-years away. The line from Rigil Kentaurus through Hadar, continued about three times as far, points to Gacrux at the top of the Cross, which is how the Pointers got their name. Seen from Sydney, the Cross and the Pointers never set, and on evenings in May and June they stand high in the south.

O1.6.2Finding the south celestial pole

Since no bright star marks the south celestial pole, southern observers find it from patterns round it. Knowing where it is matters for the planetarium round as much as outdoors, because the pole stands at an altitude equal to the observer's latitude (Section 1.4), and the whole southern sky turns round it.

The first and best-known way uses the Cross. The long axis of the Cross, from Gacrux through Acrux, continued about four and a half times its own length, leads to within a few degrees of the pole, 26.9° from Acrux. The second way adds the Pointers. The line that runs midway between Rigil Kentaurus and Hadar at right angles to the line joining them, which is their perpendicular bisector, crosses the line from the Cross about 3° from the pole, so the two lines together fix it to within about 3°. The third way uses the two brightest stars on the other side of the pole. Canopus and Achernar make a rough triangle with the pole, with sides of 37° from the pole to Canopus, 33° from the pole to Achernar and 39° between the two stars, so the pole lies a little closer to Achernar.

O1.6.3Carina, Vela and the False Cross

The False Cross is an asterism of four stars of magnitude 1.9 to 2.5, Avior (ε Car) and Aspidiske (ι Car) in Carina and Alsephina (δ Vel) and Markeb (κ Vel) in Vela. Its two bars are 9.2° and 6.3° long, larger than the 6.0° and 4.3° of the real Cross. Carina, the Keel, and Vela, the Sails, are two parts of a great ship that older star maps drew as a single constellation, Argo Navis, together with Puppis, the Stern. Carina holds Canopus, magnitude −0.62, the second brightest star of the night sky, and Miaplacidus (β Car), magnitude 1.67, only 20° from the south celestial pole. Between the False Cross and the real one lies the Carina Nebula, a large, bright cloud of glowing gas round the unstable giant star η Carinae, easy to see with the naked eye as a brightening of the Milky Way.

O1.6.4The Magellanic Clouds and other southern sights

Two hazy patches of light, which look like detached pieces of the Milky Way, lie about 20° from the south celestial pole. They are the Large and Small Magellanic Clouds, two small companion galaxies of our own, about 160 000 and 200 000 light-years away. With the pole they make a nearly equilateral triangle, with sides of about 17° to 21°. The Large Cloud lies in Dorado, the Swordfish, and the Small Cloud in Tucana, the Toucan. The Large Cloud lies 18° from Canopus and the Small Cloud 16° from Achernar, so both are easy to find once those two stars are known. Close to the Small Cloud, 2.3° away, lies 47 Tucanae, the second brightest globular cluster in the sky. The brightest of all is Omega Centauri, which lies 14° from Hadar and looks to the naked eye like a fuzzy star. It contains several million stars.

Further from the pole the southern sky has a few more bright stars to learn. Atria (α TrA), magnitude 1.91, is the brightest of the three stars of Triangulum Australe, the Southern Triangle, east of the Pointers. Peacock (α Pav), magnitude 1.94, is the brightest star of Pavo, and Alnair (α Gru), magnitude 1.73, the brightest of Grus, the Crane, lies 20° from Fomalhaut (Section O1.5). Achernar, magnitude 0.45, at the end of Eridanus, completes the bright stars round the pole.