The Water-Serpent is the largest constellation. Nevertheless, it is difficult to observe – on the one hand it consists almost entirely of faint stars, on the other hand it meanders close to the horizon. Only α Hydrae is striking with 2.0m. It is called either Alphard (the solitary one) or Cor Hydrae (heart of the snake). As the constellation is both far away from the galactic plane and far away from the galaxy-rich region of the Virgo Cluster, it contains relatively few interesting objects even for larger telescopes.
The Serpent’s Head is an asterism of six brighter stars. Although they are reminiscent of a star cluster, they just happen to stand in the same line of sight. The individual stars are between 100 and 400 light-years away. With a diameter of 5° they still fit well into the field of a binocular. With magnitudes between 3.1 and 4.4m, it can be clearly seen even with the naked eye. ε Hydrae is the northernmost star of this asterism and a multiple star with five components.
27 Hydrae is an optical double star, which is separated by a distance of nearly four minutes of arc – an easy target even at lower magnifications. The main star is a 244 light-years distant yellow sun with 4.9m, its companion is a 7.0m bright, pure white star at 202 light-years distance. 27 Hydrae is easy to find if you look for it about 2° southwest of Alphard.
R Hydrae has been observed since 1704, since then the period of this Mira-star has shortened from 500 to 389 days. The star is 330 light-years away and shines with 3.5 to 10.9m, so at the maximum it is five hundred times brighter than our Sun. When it was discovered in 1704, only two other long-period variables were known – ο Ceti (Mira) and χ Cygni. It is located about 12° south of Spica, you can find it best if you turn 2.5° almost exactly to the east from the 3.0m bright γ Hydrae.
U Hydrae is together with Y Canum Venaticorum in the Hunting Dogs one of the brightest carbon stars. It appears very red because the high carbon content of the stellar atmosphere absorbs much of the blue light. In the course of 450 days the brightness of this 530 light-years distant star varies between 4.7 and 6.2m. U Hydrae is located 4° northwest of ν Hydrae, close to the constellation Sextant.
The slightly triangular shape of M 48 is visible to the naked eye under really good seeing conditions. The 5.8m bright open cluster has nearly twice the size of the full Moon and forms with Sirius (-1.1m) and ε Hydrae (3.4m) a nearly equilateral triangle with an edge length of 13° to 15°. The 300 million year old star cluster contains 80 stars, of which about a dozen can be well resolved, the rest blurs to a shimmer. Whether this object really is M 48 is uncertain – at the coordinates that Messier wrote down in 1771, no object that matches his description can be found. M 48 is located 4° south of the original coordinates and is 2,000 to 3,000 light-years away.
M 68 is a 30,000 light-years distant globular cluster, which is not an easy object despite a brightness of 7.8m. It appears only as a blurred patch of light. Some observers claim to have seen it from southern latitudes with the naked eye. Given enough time, its visibility could improve as it approaches our region of the Milky Way at 73 m/s (117 km/s). It is easiest to find if you follow the line between δ and β Corvi southward. M 68 is located on this line about 3.5° south of β Corvi.
M 83 is a spiral galaxy with an unusually bright core, which can be seen in a pair of 10 × 50 binoculars. The spiral arms of the galaxy require about a 6” (150 mm) aperture, so that they remain targets for a telescope. The brightness is between 7.6 to 8.4m – depending on the source where you look it up. Anyway, it is one of the brightest galaxies in the southern sky. It is 12 million light-years away and part of the Centaurus Group of galaxies.
Jupiter’s Ghost (NGC 3242) is with 8.6m the brightest planetary nebula in the spring sky and can be seen at 7x magnification as a small disk about the size of Jupiter. The central star remains invisible at 12.1m, although it is very hot – about 108,000° F (60,000° C) on the surface. Only two planetary nebulae are brighter than Jupiter‘s Ghost – The Dumbbell Nebula M 27 in Vulpecula and the Helix Nebula NGC 7293 in Aquarius. Since it is very far in the south, it is difficult to observe from northern latitudes.
For about 200 years, Noctua or Night Owl is the nickname of a small group of five stars in the eastern Water-Serpent. Noctua is only an asterism, so the name is not used very often today. The stars form a 2° long arc and can be found 3° west of σ Librae (4.4m). In addition to four stars of the Water-Serpent, the star 4 Librae in the constellation Libra is also part of it. The stars shine at 5 to 6m.