November 13, 2021: Hydra, the Snake, slithers into the morning sky. Look for its stars in the southern sky before the morning sky brightens.
by Jeffrey L. Hunt
The celestial snake, Hydra, climbs into the morning sky at this season. Its head follows the star Procyon and precedes Regulus into the sky, seemingly following the great stellar bundle of stars in the Orion region of the sky, that includes Sirius, Betelgeuse, Rigel, and others.
The constellation is narrow and twisty, like that of a snake. It takes over seven hours to rise from head to tail.
The head can be found below an imaginary line that extends from Regulus to Procyon.
The constellation is without any noticeably bright star. The only named star is Alphard – “the solitary one,” It is about the brightness of stars in the Big Dipper. It is about halfway from the south-southeast horizon to Regulus.
The star is distinctly reddish, indicating that it is not as hot as our sun. Located at less than 200 light years, the star shines with a brightness of less than 1,000 suns. Alphard’s diameter is at least 100 times that of our sun.
Such stars are known as red giants. They are theorized to be near the end stages of their lives as stars. No longer able to fuse hydrogen at their fiery cores, the stellar interiors collapse to increase their interior temperatures to fuse heavier elements. In reaction, the exterior layers expand, brighten, and cool. It’s an interesting notion that a cool, red star can be bright. These red giants and red super giants are unusual and they shine brightly from long distances.
It should be noted that some red stars are smaller and intrinsically dimmer than our central star. They are still fusing hydrogen at their cores, but at a very efficient manner. These “regular” stars are frequent members of the stellar population. Over 70% of the sun’s nearest neighbors are the dim reddish stars.
From Alphard, the snake then wiggles eastward toward the southeast horizon.
The snake protects a cup of water (Crater) and a raven (Corvus) that ride on its back. They are shown on the chart.
From mythology the trio were placed in the sky by Apollo who summoned a raven to bring him a cup of water from a distant well. The tardy raven also returned with a deadly snake that was supposed to be the reason for the bird’s delayed return. Apollo, angered because the raven stopped to feast on fruit and used the snake as the reason for the late return, placed the trio in the sky.
Hydra is also in one of the Herculean tasks. In this myth, Hercules slew a multi-headed water snake.
As the nights continue to lengthen, take a look for Hydra’s stars, as the starry creature slithers across the morning sky.
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