EPS PER (Epsilon Persei). Set with bright Jewel-like stars, Perseus oddly has relatively few with well- known proper names. Among those missing them are bright third magnitude (2.89) Epsilon Persei, which lies in the middle of the eastern of the streams of stars that make the Hero's figure. Luminous, blue, beautiful, and possibly multiple, this class B (B0.5) hydrogen fusing dwarf is (at 27,600 Kelvin) among the hottest stars of the naked-eye sky. At a distance of 538 light years, and set within the Milky Way, it is subject to a bit of dimming by interstellar dust. Were there none in the way, Eps Per would make it almost to second magnitude. This correction, plus a goodly allowance for ultraviolet radiation from the hot surface, gives a great luminosity of 24,900 times that of the Sun, from which we derive a radius 7 times solar. The large mass, 14 times that of the Sun, puts Eps Per well over the limit of stars that are destined to explode as supernovae. Less than 10 million years old, it has only a few million years left before the grand event takes place. Though young, the star is slightly evolved, and has entered the realm of the subtly-unstable "Beta Cephei stars," which pulsate with multiple periods, the instability driven by layers deep within the star. Epsilon Persei is also famed among stellar astronomers as one of the most extreme "spectrum variables," in which the shapes of the absorptions seen in the spectrum vary rapidly over multiple periods between 2.27 and 8.46 hours (and are related to the small brightness variations that the star undergoes). The minimum equatorial rotation velocity of 134 kilometers per second implies a period less than 2.6 days, while the spectral oscillations suggest a consistent 1.7 days, leading to an axial tilt relative to us of about 50 degrees (though the rotation speed is vigorously argued). Spectral variations also suggest that Epsilon Per has a close companion with an eccentric orbit and a 14 day period, which would imply a separation of around 0.3 Astronomical Units (AU). It seems possible, however, that the pulsations merely fool us into thinking the companion is there. More real is a class A2 dwarf companion (Epsilon Persei B) separated from Eps Per proper by about 10 seconds of arc, or at least 1600 AU, implying a period of at least 16,000 years. The smaller star's apparent brightness shows that it lies at a distance similar to Eps Per A, so it is probably a real companion. Farther out, 78 seconds of arc (at least 13,000 AU) away is Eps Per C. If a real companion, which seems unlikely, its brightness shows that it is a class K (K7) dwarf with a period of at least 370,000 years. It is probably line a line of sight coincidence, the best guess making Eps Per a visual double.
Written by Jim Kaler. Return to STARS.