The Celestial River

Copyright © 2007 by Bob Moler

The constellation Eridanus from StellariumIt might be the largest constellation if you stretched it out  It's the river Eridanus.  We can see a bit more than half of it above the southern horizon.
The head waters of the river rise near the bright blue-white star Rigel, Orion's foot in the Stellarium chart on the left.  It flows west, down, then east before flowing southeast again to the bright blue-white star Achernar at the mouth of the river, only 32 degrees north of the south pole of the sky.  Rigel is the 7th brightest night-time star, while Achernar is the 8th or 9th, depending on who you ask.
Except for Achernar, Eridanus has no other star brighter than third magnitude.  The mythical connections to real and imagined terrestrial and sub-terrestrial rivers are varied.  It was related the the fabled earth girdling River of Ocean of the flat earth days.  Of course in actuality the continents of the earth are but islands in the great salt water earth girdling ocean.  On the other end of size, at least, it was associated with the small brook the flows under the Acropolis in Athens, or the Po in Italy. Maybe we can say its the heavenly counterpart to the Boardman River.  It's a catchier name.
Achernar is a very unusual star.  It rotates so fast that its equatorial diameter is more than 1 ½ times its polar diameter.  The rotation rate at the equator is around 250 kilometers per second or 155 miles a second.  The orientation of the axis is in some doubt. As stars go, it isn't too far away at 144 light years.
Eridanus also provides us with a much closer star.  Its Epsilon Eridani, shown in the chart above marked with the Greek letter e.  It is at magnitude 3.7.  But it is the third nearest naked eye star to the sun. If you count telescopic stars and stellar companions, it's the 10th nearest star to the sun at 10.5 light years.  Epsilon is so inconspicuous that it has no proper name.
Epsilon is somewhat like the sun.  It's mass is 15 percent less than the sun, and about the sun's size, though its brightness is only 28% that of the sun because it's surface temperature is cooler than the sun's.  It is a K type star compared to the sun's G type in the O-B-A-F-G-K-M hot to cool spectral type sequence.  It's color is decidedly orange compared the the sun's yellow.  (If you're keeping track, both blue-white Rigel and Achernar are spectral type B.)
Epsilon is interesting for more than its nearness. Spectroscopic measurements show that Epsilon has less metals in its spectrum than you'd expect in a K type star, even less than the sun.  In astronomy metals are elements heavier than hydrogen and helium, not necessarily what we'd normally think of as metal.  
Low metal stars tend to be older stars, presumably born when heavier elements hadn't been around, having been created in older stars that went supernovae to 'contaminate' the clouds of gas from which later stars were born.  However Epsilon is a young star, less than a billion years.  It has a debris disk around it at about the same distance from it as the Kuiper belt is from the sun.
In 1973 Peter van de Kamp (1901–1995) announced the detection of a planet around Epsilon with a period of 25 years at a distance of 7.6 astronomical units (AU) from the star.  Note an AU is the mean distance of the earth from the sun, and is the basis of all astronomical distance measurement past the moon.  Van de Kamp based his findings on the tiny wobble he perceived in the position of Epsilon in 860 photographs made of the star.  This observation of the stars position is called astrometry.    Unfortunately Van de Kamp's planet wasn't real.
However in 2000 astronomers were able to detect a planet about the one and a half times mass of Jupiter orbiting the star in an elongated orbit in about 7 years. Detection was made by studying the change in the star's radial velocity as the star and planet mutually revolve about their common center of gravity.  This is a much more sensitive method than van de Kamp's astrometric observations.  The planet is designated Epsilon b, and has a mean distance of 3.4 AU from the star.  
In 2002 another planet about twice the mass of Neptune was found in the dust ring that orbits the star in 280 years, this by observing the clumpiness of the dust ring.  This will be Epsilon c if verified, and lies about 60 AU from the star.  In October 2006, it was reported that the Hubble Space Telescope was able to observe the astrometric wobble of the innermost planet.  Astronomers hope to image the planet this year when Epsilon b reaches its periapsis, or closest approach to the star of 2.4 AU.
On a fictional note, Epsilon Eridani is the home of Star Trek's planet Vulcan.  
Happy New Year and Live long and prosper!

Questions? Send Email to me at bob@bjmoler.org

Updated:  01/01/07