MU ERI (Mu Eridani). Eridanus, the River, winds to the west of Orion. Springing from bright-third-magnitude Cursa (Beta Eridani), it flows to the west and then far to the south to end in first magnitude Achernar (Alpha), the ninth brightest star in the sky. Excepting rather faint Omega (which is usually not part of the classic pattern), Mu Eri is the first major stepping stone in the River's course, Nu Eri the second. Both are hot class B stars, Mu a B5 subgiant (implying that the star is giving up, or has just given up, core hydrogen fusion), Nu a B2 giant. Mu Eri, 530 light years away, has an appropriately high temperature gauged at 14,800 Kelvin (from old measures that could use improvement). A small bit of interstellar dust dims it by a mere six percent. Temperature (to account for ultraviolet radiation) and distance give a substantial luminosity of 1675 Suns and a radius of 6.2 times solar. In league with most stars of its class, it is a rather fast rotator, its equatorial spin velocity at least 145 kilometers per second, which yields a rotation period of under two days -- not fast or short enough apparently to make it a B- emission star like Zeta Tauri. The star is indeed an evolutionary subgiant, with a mass between 5.1 and 5.5 times that of the Sun (depending on its exact state) and an age of between 70 and 90 million years. After ejecting its outer envelope when it becomes a true giant, it will die as a massive white dwarf. Not alone, Mu Eri is a spectroscopic double (one known through velocity shifts in its spectrum) with a companion that takes only 7.36 days to orbit. Ignoring the mass of the companion, the two are separated on average by only 0.13 Astronomical Units (33 percent Mercury's distance from the Sun). While many doubles with this age and separation have circularized their orbits through their gravitational interactions, this one still has a fairly high eccentricity that takes the two between 0.10 and 0.16 AU apart. In a study of Nu Eri, wherein Mu was used as a comparison, Mu was discovered to be very subtle "slowly pulsating B star" (like 53 Persei) with a principal period of 1.63 days (as well as several other periods). It was also found to be an eclipsing variable in which the small companion gets in front of its larger mate every 7.4 days to produce a dip of a couple hundredths of a magnitude, showing the orbital plane to be nearly edge-on (and the above rotation speed likely to be the true value). The hottest stars, those of classes O and B, are supposed to stick closely to the Milky Way, the disk of our Galaxy, where stars are born. Mu Eri, Nu Eri, Rigel, those of Orion's Belt and Sword, and many others of the kind, though, are rather far off it (Mu some 300 light years south of the Galaxy's plane). It and the others are part of the famous "Gould Belt," named after B. A. Gould, in which the distribution of massive stars has been locally bent and distorted as compared with their more distant brethren in the Milky Way. The distortion was also noted by John Herschel from his observations in the southern hemisphere. It is caused by a huge region of sequential star formation that the Sun just now happens to be passing through.
Written by Jim Kaler 2/02/07. Return to STARS.