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.