The Constellation Eridanus

The southern constellation Eridanus is one of the largest of the modern 88, ranking 6th after Hydra, Virgo, Ursa Major, Cetus and Hercules. At 1,138 sq deg area, Eridanus is the third-largest southern constellation, and the sixth-largest overall. Eridanus starts below Taurus and Orion then snakes through the southern sky between Lepus and Cetus, Caelum and Fornax, through Horologium and Phoenix, before finishing with its brightest star (alpha Eridani - Achernar) next to the border with Hydrus.

Eridanus is one of Greek astronomer Ptolemy's original 48 constellations. Homer called it the 'Ocean Stream' but the river it is thought to represent on Earth has been lost in the mist of time. It may have been the Sumerian Strong River, also known as Ariadan, and the rivers the Tigris, the Euphrates, the Po, the Ganges or the Nile have been named by various cultures.

Objects of interest include: the 'Eridanus Supervoid' which is the largest chunk of barren space yet discovered; epsilon Eridani (Sadira) which is a popular science-fiction topic due to its similarity with our own Sun, and four extrasolar planetary systems.

Mythology

The constellation Eridanus represents the erratic passage taken by the sun chariot one day when Helios (the sun god) allowed his son Phaëton to take the reins. Phaëton convinced his father that he was strong enough to control the horses, but he wasn't. The result was that the sun swung so high that the earth almost froze, then so low that parts of Africa, which had been lush, were frazzled and the skins of the populace was scorched. This is how the ancient Greeks explained the ebony skin of Ethiopians.

Obviously this calamity caused mass panic and to prevent the destruction of the earth, the great god Zeus hurled a thunderbolt at the sun chariot. Phaëton was killed by the blow and his body tipped out, falling into the river Eridanus below. Phaëton's grieving sisters, who had encouraged their brother in his misadventure, were morphed into poplar trees, which supposedly still stand along its banks.

Stars

The scientific star names are simple to understand (if you know your Greek alphabet). For example: 'alpha Eridani' means that it is the brightest star in the constellation Eridanus. The next brightest is designated 'beta', etc. Some stars have proper names as well; for example, alpha Eridani is Achernar; others are known by their catalogue numbers or 'Bayer designation'.

Achernar (alpha Eridani) is a first magnitude star, the ninth brightest star in the sky. It's a blue main sequence dwarf, with an unusual spectrum. This star rotates so fast it bulges in the middle, images of it look like a blue jellybean. Achernar lies at the southernmost end of the constellation, barely a degree separating it from Eridanus' neighbour Hydrus the water snake.

Epsilon Eridani, Sadira, has a dusk disc and an extrasolar planet system. This star, along with beta Pictoris, Vega (alpha Lyrae) and Fomalhaut (alpha Piscis Austrini), are dubbed the 'Fabulous Four' debris stars discovered by the IRAS (Infrared Astronomical Satellite). Situated just ten light years away, and thanks to its similarities to our own Sun, Sadira is often a topic of sci-fi novels.

The orange dwarf star Keid (40 Eridani) is worthy of a mention purely because it's one of the rare stars to have a common name but no Greek letter designation. Gene Roddenberry chose this stellar area to be the origin of one of the most famous sci-fi aliens, Mr Spock (played by actor Leonard Nimoy), of Star Trek. If it really existed, Spock's homeworld, the planet Vulcan, would boast a spectacular view from its vantage point because the star it orbits is not a single star, it has two companions. The white star and a red dwarf star would shine silver white and brilliant red, much like we view Venus and Mars.

New General Catalogue (NGC)

The NGC catalogue was compiled by John Louis Emil Dreyer (the director of the Armagh Observatory from 1882 to 1916).

NGC Objects of Eridanus

Hickson 21 is a group of five galaxies all registered separately on the NGC table. They are in such close proximity that they are interacting, and may eventually collide, providing a cosmic trainwreck of intergalactic proportions:

The Eridanus Supervoid

The Eridanus Supervoid was discovered by a combination of NASA's Microwave Anisotropy Probe and data from the Very Large Array. It is at least six billion light years away and is the largest absence of galaxies discovered so far. At a billion light years in diameter², the scale of this supercool gap has so far escaped explanation by the scientific community. However, several ideas have been put forward which may be of interest, some a little wackier than others:

• Quantum entanglement between our Universe and a parallel Universe.
• Dark energy: Scientists know so little about it but it is thought to be accelerating the expansion of the Universe.
• Big Bang: This is where it happened, and the entire universe accelerated from here.
• Heaven/the Afterlife: Over 90% of the population of the Earth believe in some form of religion including the possibility of life after death. Is this where all the souls go after earthly life has ceased? Or maybe it's the storage facility of all the souls yet to be born.
• It's Bad News. After all, nothing wants to hang around bad news.
• A cloaked military base camp run by hostile aliens.
• It's the last known frontier for Star Trek to 'boldly go where no-one has gone before'.
• It's a space that has been cleared for an intergalactic bypass, still awaiting planning permission (with apologies for the inconvenience).
• It's the graveyard of the Great Grey Ghost Elephant, from the legend behind the song 'Ging Gang Goolie'³.
• As it's the coolest spot in the known universe that has to be where Elvis Presley is, that's the 'real' Elvis of course, no impersonators, and those fans who have since followed him.
• It is where all the unrealised dreams, lapsed friendships, regrets and unrequited love feelings go.
• It's a rather small (solar system sized) reflector array, pointed away from us, put in place mumblety billion years ago during the early times of a treaty/conglomerate of beings who were running out of conventional renewable energy sources. Basically, they put together an array of small(er) mirrors, surrounded by steerable reflectors, that catch all the light from that sector and send it back to themselves - including the light reflected back from themselves. Imagine, if you will, the complex technologies involved. Unfortunately, being designed under the auspices of their government, a small error occurred. We are left with the dead remains. We (well, just a few of us) know this because a probe has been recovered and its contents translated (using the 'Itsarose' tablet, recently found in Mariana Trench). It gives a brief account of the artifact but then, part way through, fades out saying 'Don't ... ...' That probe and its message is held secret by certain authorities here ... until now, that is.

It's a very hot topic in the cosmology world right now.

- Professor Lawrence Rudnick of the University of Minnesota.

Extrasolar Planets

The Holy Grail of astronomers is to find the right size (rocky) planet orbiting its parent star at the correct distance for life to be viable and sustainable. The orbit would need to be non-eccentric, and the parent star should be stable. This 'just right' set of circumstances has earned the nickname the 'Goldilocks zone' after the children's story Goldilocks and the Three Bears. As of December 2007, no such planets have been found.

Methods used for detection have greatly expanded since the 1990s, and the techniques are being fine-tuned so Earth-like planets can be discovered and studied. Some methods include:

• The 'Wobble' technique: the planet's gravitational pull on its parent star produces changes in the star's light spectrum.
• MOA: Microlensing Observations in Astrophysics is a Japan/New Zealand collaboration using the gravitational microlensing technique at the Mt John Observatory in New Zealand.
• OGLE: Optical Gravitational Lensing Experiment makes observations at the Las Campanas Observatory, Chile, using a second generation CCD 8kMOSAIC camera. OGLE regularly monitors 130 million stars in the galactic bulge of the Milky Way.
• RoboNet: Optimised robotic monitoring of galactic microlens events at a UK national facility, the two metre robotic telescope at the Telescope Management Centre at Liverpool JMU (John Moores University). Although mainly concerned with delivering school-age educational programmes, the technique is being utilised to assist in the search for rocky Earth-like extrasolar planets.
  

Extrasolar Planets in Eridanus

There have been four extrasolar planetary systems found in the constellation Eridanus up to 2007; the first was discovered in 2000. Figures given in the table below are the length of the planet's orbital period around its parent star, which we know of as a year. The size of the extrasolar planet is compared to the mass of Jupiter, our Solar System's largest planet, known by astronomers as the 'Jovian scale'.

Down to Earth

• Eridanus Giffin is a self-representing Californian artist who likes to paint flowers, particularly poppies, tulips and daffodils.