Eridanus is the 6th largest constellation in the sky, but it is often overlooked due to the lack of bright stars. Or at least certainly from the northern hemisphere. Its brightest star Achernar is not visible from the UK. The constellations starts close to the bright star Rigel in Orion and then meanders south and below the horizon.
To the Greeks Eridanus was sometimes called the River of
Orion due its closeness to Rigel. It is often associated with the river into
which Phaethon fell after trying to use the chariot of his father, the sun god.
Some Greeks also just referred to it as the river.
It was also seen as a river by the Euphratean astronomers
who used the name of Erib-me-gali.
Keeping up this theme of a river the Arab astronomers called
this constellation Al Nahr which again means the river.
The brightest star in Eridanus is alpha or Achernar which in Arabic is Al Anir al Nahr or the End of the River. An interesting point to note is that this name was first given to the star theta. This was because it was the southernmost bright star to be seen from Greece. When European explorers travelled to the southern hemisphere, they saw a bright star that could not be seen from Europe.
In 1603 the German astronomer Johannes Bayer extended the
length of Eridanus and named it Achernar the End of the river. The star theta
was renamed Acamar which in Arabic means the Root.
Achernar is a B class star with a magnitude of 0.5 and is the 9th brightest star in the sky, Achernar is139 light years away. We normally
think of a star as a sphere of hot spinning gas but Achernar spins so quickly
that it is pushed out of shape, so it appears more like an oblate spheroid. It
is quite possibly the least known spherical star in our galaxy. It is not a
single star it has a companion star. Achernar is around 10 times the diameter
bigger than our Sun. The average temperature is about 15,000 degrees much
hotter than the Sun; however, this varies because at the poles it is 17,000
degrees while at the equator its 12,000 degrees.
Theta or Acamar has a magnitude of 2.9 and is 164 light
years away. Acamar is a double star, and both stars are A class stars
indicating they are hotter than our Sun.
Beta or as it is known to the Arab astronomers as Cursa
which means The chair or the Footstall of the Central One is located just above
the bright star Rigel in the constellation of Orion. To the Chinese this was
called Yuh Tsing or The Golden well. It has a magnitude of 2.8 and is an A
class star lying 90 light years away.
Gamma was known to the Arab astronomers as either Zaurac or
Zaurak which means the Bright Star of the Boat. The star is a red M class giant
star around 192 light years away and has a magnitude or brightness of 2.9. As
it is an M class star it will be cooler than the Sun with a surface temperature
of about 3,500 degrees compared to the 5,800 degrees of the Sun.
Gamma or Azha as it
was known to the Arab astronomer Al Sufi was chief among the stars of the
Ostrich’s Nest which the word Azha means. Its an K class star again cooler than
the Sun with a magnitude of 3.9 and is 137 light years away.
There are no bright deep sky objects to mention.
www.theramblingastronomer.co.uk
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