It might be the largest constellation if you stretched it out It's the river . 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 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 . It's a
catchier name. 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 , 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 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
is from the sun. In 1973 (1901–1995) announced the detection of a planet
around Epsilon with a period of 25 years at a distance of 7.6
(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
It might be the largest constellation if you stretched it out It's the river . We can see a bit more than half of it above the southern horizon.