Celestial Quadrant SQ4

The fourth southern quadrant of the celestial sphere, measured from right ascension 18h00 eastward to 24h00 (or 0h00), and from the celestial equator southward to the southern celestial pole. The quadrant therefore represents an area of one eighth of the full celestial sphere.

The most significant constellations in this area of the sky are three Ecliptic constellations that run across the northern parts of the quadrant. Each year, the Sun enters the quadrant in December, and passes eastward over the following months through Sagittarius, Capricornus and Aquarius. In late March it reaches the northeastern tip of Quadrant SQ4, at right ascension 0h00 and declination 0°. This is the First Point of Aries (actually currently in Pisces) and marks the point of the Vernal Equinox.

Running southward from these three major Ecliptic constellations, the quadrant is divided into a mosaic of smaller constellations, mainly belonging to the Bayer and La Caille constellation families. The largest of these are Pavo the Peacock and Grus the Crane, and alongside them are a selection of less prominent constellations such as Indus, Microscopium and Telescopium. At its southern tip, the quadrant comes together with the other three southern celestial quadrants within the polar constellation of Octans.

Stars

This is not a region of the sky populated by bright stars. The brightest star in the quadrant as seen from Earth is Fomalhaut in Piscis Austrinus, a relatively nearby white star with a visual magnitude of +1.2. Following an approximate southwestward line from Fomalhaut, two other stars stand out: blue Alnair in Grus (magnitude +1.8), and another blue star, Peacock, the Alpha star of the constellation Pavo, with a magnitude of +1.9.

A short section of the band of the Milky Way runs through the northwestern corner of the quadrant, and this area of the sky is more populated with distinctive stars than the rest of the region. Especially notable is the asterism known as the Teapot, within the central regions of Sagittarius. While none of these individual stars are as bright as Fomalhaut, collectively they stand out against the backdrop of the Milky Way.

The Teapot is made up from two smaller star-groups: the Milk Dipper formation and the three stars Kaus Borealis, Media and Australis that together form the bow of the Archer. Directly southward from the Teapot is another recognisable formation: a string of relatively faint stars that form the curving arc of Corona Australis, the Southern Crown.

Deep Sky Objects

The most significant clustering of deep sky objects within Quadrant SQ4 lies in Sagittarius in the northwest. This direction represents a view toward the approximate centre of the Milky Way Galaxy (though the actual core is just across the quadrant's border in neighbouring SQ3). This part of the Milky Way is dense with nebulae and star clusters, with notable examples including the Lagoon Nebula (M8), the Omega Nebula (M17) and the Trifid Nebula (M20). The Eagle Nebula (M16) is also within this area, northward from Sagittarius in the constellation of Serpens.

Galactic clusters also abound in this region, with a prominent example being the Wild Duck Cluster (M11) in Scutum. Sagittarius holds numerous examples, including M18, M21, M23 and many more besides. One interesting object is M24, known as the Sagittarius Star Cloud. This is not a star cluster as such, but a view of a distant starfield towards the Milky Way's core. This field consists of thousands of stars, which are partially obscured by clouds of dark material swarming through the intervening space.