who decribed the constellation for the first time?
Goes back to prehistory.
Introduction
The origins of the earliest constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. 
what is hydrus? Small constellation in the deep southern sky. what are its characteristics? Irregular in shape and its tail has been shortened to make space for this last constellation he had drawn up. name a few stars in the constellation? Beta Hydri, Alpha Hydri, etc. where did the second star get its name from? Chinese system
Petrus Plancius Pieter Dirkszoon Keyser and Frederick de Houtman Johann Bayer
Hydrus Hydrus Hydrus
History History Introduction
Hydrus was one of the twelve constellations established by the Dutch astronomer Petrus Plancius from the observations of the southern sky by the Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman, Hydrus was one of the twelve constellations established by the Dutch astronomer Petrus Plancius from the observations of the southern sky by the Dutch explorers Pieter Dirkszoon Keyser and Frederick de Houtman, who had sailed on the first Dutch trading expedition, known as the "Eerste Schipvaart", to the East Indies.  Johann Bayer
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Constellation [SEP] Introduction
A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. The recognition of constellations has changed significantly over time. Many have changed in size or shape. Some became popular, only to drop into obscurity.