The Teapot of Sagittarius

The Milky Way, that superb river of stars, dust and gas that delineates the spiral nature of our home galaxy was probably first suspected as harbouring many unresolved stars by the Greeks. It wasn’t identified as such until Galileo turned his newly completed telescope towards it during 1609 to 1610.

“I have observed the nature and material of the Milky Way. With the aid of the telescope this area has been scrutinised so directly and with such ocular certainty that all the disputes which have vexed philosophers through so many ages have been resolved, and we are at last freed from wordy debates about it. The Galaxy is, in fact, nothing but a congeries of innumerable stars grouped together in clusters”.

Many cultures have seen the Milky Way as a pathway to their Gods and a place of rest for the souls of their dead. The Chinese called it the ‘River of Heaven’ and also the ‘Great Path of Life’. To the American Indians it was the ‘Path to the Land of the Hereafter.’ The Algonquins imagined the campfires of their departed warriors whilst the Norsemen described it as the ‘Path of the slain warriors on their way to Valhalla’.

The months of July through to September show this panoramic ‘real estate’ to perfection. At this time, the celestial archer is well placed high overhead for both binocular and telescopic observation, whether from a dark site or perhaps under the cover of a light polluted sky. Many of the objects are superb under any conditions and a delight in the smallest of viewing aids.

The central portion of Sagittarius is sometimes called the ‘Teapot’. The Chinese saw this asterism as a ‘Ladle’ (the handle) and a ‘Sieve’ (the spout). They complained,

“In the south is the Sieve

Idly showing its mouth…

But it is of no use to sift;

In the north is the Ladle

Raising its handle to the west…

But it ladles out no liquor!”

Those observers blessed with great imagination can join the ancient Arabians in seeing the teapot’s spout as the ‘Going Ostriches’ whilst the teapot’s handle was the ‘Returning Ostriches’. The ‘Keeper’ was represented by lambda Sagittarii, the top star of the teapot’s lid.

This article will take us on a tour around the stars that form the celestial teapot, visiting those objects that lie near to this easily identified asterism. Beginning with lambda (Kaus Borealis), an orange giant of magnitude 2.8, move around 2.5° to the north east to spot one of the jewels in all of the heavens, M22. This stunning globular cluster was first recognised as a cluster of stars by none other than Sir William Herschel on July 04, 1783. Discovery is usually credited to Abraham Ihle in 1665. Herschel recorded it as:

“ With a small 20-feet Newtonian telescope, power 200, it is all resolved into stars, that are very small and close. There must be some hundreds of them. With 350, I see the stars very plainly; but the nebula is too low in this latitude for such a power”.

A secondary look by Herschel on July 12, 1784 brought forth:

“A most beautiful extensive cluster of stars, of various magnitudes, very compressed in the middle, and about 8’ in diameter, besides the scattered ones, which do fill the extent of the field of view…the large stars are red…”

Admiral Smyth made considerable reference to it in 1835:

“A fine globular cluster, outlying that astral stream, the Via Lactea [Milky Way], in the space between the Archer's head and bow, not far from the point of the winter solstice, and midway between Mu and Sigma Sagittarii. It consists of very minute and thicky condensed particles of light, with a group of small stars preceding by 3m, somewhat in a crucial form. Halley ascribes the discovery of this in 1665, to Abraham Ihle, the German; but it has been thought this name should have been Abraham Hill, who was one of the first council of the Royal Society, and was wont to dabble with astronomy [This thought is probably spurious]. Hevelius, however, appears to have noticed it previous to 1665, so that neither Ihle nor Hill can be supported [Evidence for this appearance is low, in particular as Ihle and Hevelius were probably in good contact].

In August, 1747, it was carefully drawn by Le Gentil, as seen with an 18-foot telescope, which drawing appears in the Mémoires de l'Académie for 1759. In this figure three stars accompany the cluster, and he remarks that two years afterwards he did not see the preceding and central one: I, however, saw it very plainly in 1835. In the description he says, "Elle m'a toujours parue tres-irrégulière dans sa figure, chevelue, et rependant des espèces de rayons de lumière tout autout de son diamètre." This passage, I quote, "as in duty bound;" but from familiarity with the object itself, I cannot say that I clearly understand how or why his telescope exhibited these "espèces de rayons." Messier, who registered it in 1764, says nothing about them, merely observing that it is a nebula without a star, of a round form; and Sir William Herschel, who first resolved it, merely describes it as a circular cluster, with an estimated profundity of the 344th order. Sir John Herschel recommends it as a capital test for trying the space-penetrating power of a telescope.

This object is a fine specimen of the compression on which the nebula-theory is built. The globular systems of stars appear thicker in the middle than they would do if these stars were all at equal distances from each other; they must, therefore, be condensed toward the centre. That the stars should be accidentally disposed is too improbable a supposition to be admitted; whence Sir William Herschel supposes that they are thus brought together by their mutual attractions, and that the gradual condensation towards the centre must be received as proof of a central power of such kind.”

Referring to my observing notes, one of the my best views of M22 occurred on the evening of 25 July 1990 with a 20cm f 6.7 dobsonian reflector from the light polluted sky of Mount Eden in Auckland. Employing an eyepiece of lower power (48x), M22 appeared as a large and very bright irregular cluster with chains of bright stars radiating outwards to the south west.

A step up in power to 90x magnification showed ‘fingers’ of bright stars overlaid upon a milky white background of fainter unresolved stars, giving this cluster an obvious 3D effect. Further observations over the years have not dimmed my enthusiasm for this delightful cluster, truly one of the best of its type available to telescopes.

NGC 6642 is another globular cluster lying around 1° to the north west of M22. Discovered by Sir William Herschel on August 7, 1784, he recorded it as:

“globular, pB, R, gpmbM, 2’, resolved into visible but vS stars 15…16m”. NGC 6642 appears as a small and faint irregularly round glow, unimpressive to the eye. Large mirrors will show brightening toward the central region and add a prominent stellar nucleus.

Moving back to lambda, look around 1° to the north west to see M28, another globular cluster but of far more presence than our latest target. Discovered by Messier in July 1764, he recorded it as:

“ Nebula discovered in the upper part of the bow of Sagittarius at about one degree from the star Lambda & little distant from the beautiful nebula which is between the head and the bow [M22]. It contains no star; it is round, it can only be seen difficultly with an ordinary telescope of 3.5-foot.”

Modern instruments improve immensely upon Messier’s view, the cluster appearing large and bright with a brighter central region. Large telescopes will resolve most of this compressed cluster though small instruments make do with the outer edges. A very nice object.

NGC 6638 is a globular cluster lying around 0.75° to the east of lambda. A 20cm mirror under a city sky will show a small and rather faint glow, rising to a brighter centre. A 30cm mirror may glimpse a few resolved stars but overall not an impressive view.

Directly south of lambda lies one of my favourites, a superb asterism of bright white stars forming a beautiful triangle. Burnham 133A and 133B lies on the western edge of a 20.0’ field of view. It is a close binary of 1.3” separation in position angle (PA) 251° but needs high power to resolve. Stars C and D lie 8.9’ south and slightly east, separated by 54.5” in PA 136° but are not listed as a bound pair. Stars E and F (WNO 6) of our asterism lie 15.5’ to the north east and are listed as a pair, separated by 41.9” in PA 182°. Measurements since 1890 have shown no change in E and F’s separation or PA. The chart below gives measurements from the Hipparcos database.