The Milk Dipper

The are various 'dipper' formations in the sky - groups of stars that form a rough quadrangle with one or more associated stars forming a 'handle'. Probably best known among these are the Big Dipper in Ursa Major and the Little Dipper of Ursa Minor, but a group of five stars in the heart of Sagittarius forms another 'dipper', the Milk Dipper. Unlike the Big and Little Dippers are in the northern sky, the Milk Dipper extends into the densest parts of the Milky Way, a fact from which it takes its name.

As well as forming a distinctive shape within Sagittarius, the constellation of the Archer, the stars of the Milk Dipper also belong to various other star-shapes in the sky. The handle of the Dipper, Kaus Borealis, also marks the northern end of Sagittarius' bow, and all five stars together define the handle of a larger asterism known as the Teapot.

The Stars of the Milk Dipper

In descending order of brightness, the five stars that form the Milk Dipper are Nunki, Ascella, Kaus Borealis, Phi Sagittarii and Tau Sagittarii. These stars are unrelated to one another, having different spectral types and being widely distributed through space. The Dipper formation, then, is a purely a line-of-sight effect due to the Sun's position relative to these five stars.

The closest of the Milk Dipper's stars to Earth is Kaus Borealis, an orange giant some 78 light years distant. The most distant is Phi Sagittarii, a blue star more than three times farther away than Kaus Borealis at a distance of some 239 light years. Nunki, though the brightest of the five stars, is actually tiny and distant (lying nearly as far from the Sun as Phi Sagittarii) but it is also intensely luminous, so that it outshines the other four stars of the Milk Dipper despite being a distant dwarf.

Deep Sky Objects

The constellation of Sagittarius marks the direction of the centre of the Milky Way Galaxy, and so the band of the Milky Way is particularly dense in this part of the sky. The Milk Dipper also lies in this dense direction, and there are several prominent star clusters in and around the formation. In particular, three globular clusters stand out, with two of these, M22 and M28, lying close to the handle star Kaus Borealis. The third, M54, is a little to the south of the 'bowl' of the Dipper.

All three of these clusters are immensely distant, but M54 is a special case: it lies beyond the Galaxy's Core at a distance of about 86,000 light years. In fact, it forms the heart of a larger and more distant object that extends into the Milk Dipper's bowl, an object only recently discovered. This is the Sagittarius Dwarf Galaxy, variously designated Sgr DSph or Sgr DEG, a small elliptical galaxy with a diameter of about 10,000 light years (about a tenth that of the Milky Way Galaxy itself).

This dwarf galaxy is one of several small galaxies in close orbits around the Milky Way. It follows an approximately polar orbit around the Galaxy (that is, in a plane perpendicular to the Galaxy's spiral arms). Its orbit at present places it on the opposite side of the Milky Way from the Sun, so that it lies beyond the Galactic Nucleus and the obscuring material of the Galactic disc. This positioning explains why such a relatively large and nearby galaxy remained undiscovered until recently.