Starwatch: Teapot in Sagittarius

Mars remains the most conspicuous object low in Britain’s S sky at nightfall where it stands below the Moon on Thursday. Saturn, 17° to Mars’s left and fainter, is 6° above Antares in Scorpius.

By midnight BST it is the quaintly named Teapot that hovers just above our S horizon. The Teapot is an asterism, being the most prominent part of the constellation of Sagittarius the Archer which is also identified as a Centaur – a human/horse hybrid.

Graphic by Paddy Allen

Our chart plots a 30°-wide area around the Teapot, but the lower third of this window remains forever below Britain’s horizon and we must travel to the latitude of S Europe or below to see it all.

The Teapot appears to be pouring towards the W – or right – with its spout represented by the stars Alnasl and Kaus Australis. The latter is named for the southern end of the Archer’s bow and, at mag 1.8, is the brightest star in the Teapot and, indeed, the whole of Sagittarius. The second brightest, mag 2.0 Nunki in the Teapot’s handle, is one of the hottest of our brightest stars, with a surface some four times hotter than our Sun.

Above-left of Nunki is another asterism, the Teaspoon, with its star Pi. The dwarf planet Pluto was a dim 14th-mag telescopic object when it crept only 3 arcmin S of Pi on 26 June.

The rest of Sagittarius strays off-chart to the left before returning to end at the wide naked-eye double star Beta. The curl of stars between Beta and Kaus Australis is Corona Australis, the pale southern counterpart to Corona Borealis, the Northern Crown, that featured here last month. An extension to the Teapot/spoon leitmotif makes Corona Australis the lemon to go with our tea.

The centre of our galaxy, the Milky Way, lies W of the Teapot’s spout and is labelled GC. Some 27,000 light years (ly) away, it is hidden from optical view by intervening clouds of gas and dust and is best “seen” in the infrared.

Once the current moonlight subsides, the band of the Milky Way may be seen slanting through the galactic centre before it flows through the Summer Triangle high in our S sky. Peppered along it are numerous star clusters and nebulae, the brighter ones dimly visible to the unaided eye and first catalogued by Charles Messier in the 18th century.

Energised by the young stars within it, M8, the Lagoon Nebula, is a glowing cloud of gas, mainly hydrogen, and dust that lies about 5,200ly away and appears three times wider than the Moon. Close by is the smaller but similar Trifid Nebula, M20, which also reflects the light from a new-born star cluster within it.

M7, almost 1,000ly away and further S towards the Stinger of Scorpius the Scorpion, is a superb star cluster that is also called Ptolemy’s Cluster after the Greco-Roman astronomer who first recorded it. M23 and M25 are similar in nature but more distant, while M22 is a globular cluster of many thousands of stars more than 10,000ly from us.