54 ERI (54 Eridani). Immensely long, Eridanus, the River, winds from just
northwest of Rigel, the seventh brightest
star of the sky, deep into the southern hemisphere, where it ends
at Achernar, number nine, both stars of
magnitude zero. Not all of Eridanus's more modest stars are on the
winding stream, however. On the far southern bank lies 53 Eri, a
cool class K fourth magnitude (3.87) giant 110 light years away, and
farther south our star, 54 Eri, an even cooler also fourth magnitude
(4.37) class M (M4) giant 368 light years distant (give or take 25).
54 Eri (a Flamsteed number)
is also a semi-regular variable that goes by the alternative name
DM Eridani, the star
wandering slightly between magnitudes 4.28 and 4.36 (unnoticeable
to the naked eye) over a rough 30-day period. It presents something
of a mystery. In 1877 and on two occasions in the 1920s it was
observed as double, the
companion seen to be almost as bright as 54 Eri proper. On the other
hand, lots of other astronomers never saw it, even in modern times
using Hipparcos data and interferometry. So (from Bill Hartkopf,
US Naval Observatory) "the duplicity of 54 Eri is definitely not
definite (or certainly not certain) ... if it is double it has either
closed to under 0.03 seconds of arc, or the magnitude difference is
much larger, or some combination of the two...my feeling is that it's
single - perhaps they were seeing some reflection due to the
brightness." More telling, the spectrum shows no trace of another star,
just that of 54 Eri itself, the class M giant. So single it is, 54
Eri quite vividly revealing the problems astronomers can be faced
with. The temperature is as ill-defined as the duplicity. Three
values range between 3050 and 3660 K, averaging 3385 Kelvin.
Whatever the true temperature, a lot of the star's radiation falls
in the infrared. Including
that, the distance gives a luminosity 2430 times that of the Sun and a radius of 143 solar radii (0.67
Astronomical Units), which makes the star almost as big as the orbit
of Venus. Like most of 54 Eri's parameters, the mass is not
well-defined either, but may fall around double that of the Sun. No
surprise, the evolutionary status is also unclear. Possibly the star
is in the early phase of brightening and growing for the second time,
now with a dead carbon/oxygen core, but it could also be in one of
the stages of its first brightening with a dead or burning helium
core. Whatever the case, the star's outer envelope will eventually
be expelled, and the core will be left to cool forever as a white dwarf. Both 53 and
54 Eri are moving fairly quickly relative to the Sun, 53 at (oddly)
53 kilometers per second, 54 at 63 km/s, respectively away and towards
us at more than three and four times normal, the two obviously having
nothing to do with each other except for their contiguity in the
Flamsteed catalogue. (Thanks to Bill Hartkopf for detailed
commentary.)
Written byJim Kaler 2/27/15. Return to STARS.