The night sky in spring

As spring gets into full swing, Brian Jones takes a look at the night sky in March and explains how to spot Hydra, the celestial water snake…

This month we are turning our attention to the long and sprawling constellation of Hydra (the water snake). The head of the water snake can be found a little to the south-west of the constellation Leo (the Lion), from where it winds its way eastwards, passing immediately to the south of Leo and Virgo (the virgin). Hydra is the largest of the 88 modern-day constellations and was one of the 48 star groups drawn up and catalogued by the Greek astronomer Ptolemy during the 2nd century AD. However, despite its size there is nothing about Hydra that stands out particularly well. Even its brightest star, Alphard, located a little to the south of Regulus in Leo, is not especially notable.

Northern sky in spring 

Where to find Hydra

To locate the tiny circlet of stars forming the head of Hydra, first look directly upwards and you should see the distinctive form of the Plough, which is located at or near the overhead point this time of year. Following an imaginary line roughly southwards from Dubhe through Merak, as shown on the chart, will eventually lead you to the equally prominent Leo, the distinctive shape of which stands out well in the spring night sky. From here, the head of the water snake can be found by carefully searching the area of sky a little to the west-southwest of Regulus, the brightest star in Leo. The head of Hydra is reasonably distinctive and you should have little trouble finding it, although a pair of binoculars may be useful.

Hydra

The constellation chart shows Hydra, along with two prominent stars – Spica (in Virgo) and Regulus (in Leo) – to guide you as you identify and follow the constellation along its length. The whole of Hydra can be seen from anywhere south of latitude 55°N and portions of it are visible from any location around the world.

When to look for Hydra

As far as backyard astronomers and stargazers located at mid-northern latitudes are concerned, the constellation is best sought out during the months of March and April, when its winding form can be seen straddling an extended region of sky above the southern horizon. The stars forming Hydra are not particularly bright and you will need clear, dark and preferably moonless skies in order to identify the entire constellation. Binoculars will help you to pick out the individual stars forming the group, working your way eastwards from the head of Hydra. Observers in more southerly latitudes will have less trouble finding Hydra, as the constellation will, of course, be located higher in the sky and away from the horizon glow.

The tiny but distinctive circlet of stars depicting the head of the water snake was alluded to by the English astronomer Joseph Henry Elgie, who was clearly impressed when he referred to them as '...a group of fairly bright stars form(ing) the head of the great sinuous constellation of Hydra, the water snake, a fearful monster, worthy to rank among the weird creatures seen by Coleridge’s Ancient Mariner, when that immortal seafarer lay becalmed in the tropics. When once this curve has been recognised the feeling of the observer is that it constitutes one of the most striking configurations in the heavens.’

The legend of Hydra

According to legend, Hydra represents the fearsome multi-headed monster, which lived near the town of Lerna in the Peloponnese region of southern Greece. The area it occupied was ideal territory for such a creature, containing as it did a lake, numerous springs and areas of swampland. From here, the monster spread fear and panic among the local inhabitants as it wandered around destroying the countryside and eating various livestock. Its reign of terror eventually came to an end when Hercules battled and killed the creature as the second of his twelve labours.

Understanding Hydra

The star at the western tip of the head of Hydra is the magnitude 4.44 Minchir, which, at a distance of a little over 350 light years, derives its name from the Arabic minkhir al-Shuja, meaning ‘the nostril of Hydra’ or ‘the Serpent’s nostril’, referring to the nose of the celestial water snake.

The brightest of the stars that form Hydra is the magnitude 1.99 Alphard, which can be found by following the line of stars south-eastwards from Zeta, a magnitude 3.11 yellow giant star located at a distance of over 160 light years and situated at the eastern end of the head of Hydra. Alphard is an orange giant star, shining from a distance of around 180 light years. Its name is derived from the Arabic al-fard meaning ‘the solitary one’, a fitting title taking into account its position in relation to the surrounding field of comparatively faint stars.

As we make our way along the length of Hydra, we come across a few more reasonably bright stars, one of which is the magnitude 3.11 orange giant Nu. The light we are seeing from Nu set off on its journey towards us around 145 years ago, putting it slightly more distant than Gamma, a magnitude 2.99 yellow giant star located a little over 130 light years away. Situated in the eastern regions of Hydra, Gamma can be found a short way to the south of the brilliant Spica in the neighbouring constellation Virgo, which can be used as a guide to help you track it down.

From Gamma, the winding form of Hydra continues through the magnitude 3.25 orange giant Pi and on to the faint 58 Hydrae, yet another orange giant star which, shining at a rather dim magnitude 4.42 from a distance of around 330 light years, marks the tail of the celestial water snake.

Take a trek out to a dark sky site, or even your own garden, and check out the obscure and meandering, but nonetheless interesting, line of faint stars that depicts Hydra, the celestial water snake. Happy stargazing! 

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