EPS CAP (Epsilon Capricorni). A couple degrees southwest of brighter Gamma and Delta Capricorni (of Capricornus, the Zodiac's Water Goat) lies more or less obscure Epsilon Capricorni, which was clearly given the fifth letter of the Greek alphabet by Bayer because of its proximity to Gamma and Delta rather than for its dim fifth magnitude (4.68) brightness. The star is a surprising gem. It's a hot class B (B2.5) emission line dwarf (a "Be" star similar to Gamma Cassiopeiae and Delta Scorpii) with a surrounding disk and also a close double. Five degrees south of the ecliptic, Eps Cap can be occulted by the Moon, which reveals the presence of two stars, the brighter (Eps Cap Aa) at magnitude 5.0, the other (Eps Cap Ab) at 6.3. At a substantial distance of 1055 light years (plus or minus 62) and not that far off the Milky Way in Sagittarius to the west, Epsilon Cap is dimmed about 0.2 magnitudes by interstellar dust. The temperature of the brighter, Eps Cap Aa, is measured at 19,780 Kelvin, just right for a B2.5 dwarf. From its absolute visual brightness, the fainter should be a B4 dwarf with a temperature of 17,000 Kelvin. After correction for a lot of ultraviolet light, we find the respective luminosities to be 5600 and 1235 times that of the Sun, which give the stars radii of 6.4 and 4.1 times solar. Theory then yields masses of 8.5 and 5.8 Suns. The angular separation found from the occultation yields a physical distance between the two of 1.5 Astronomical Units, about the distance of Mars from the Sun, which because of foreshortening could be a serious underestimate. Kepler's laws then give an orbital period of at least half a year.

The separation between Aa and Ab is sufficient to allow a rotating disk around the brighter star that is somehow formed by the rapid observed rotation speed of 218 kilometers per second, which gives Eps Aa a rotation period of under 1.48 days. Radiating emission lines of hydrogen, the disk varies in brightness with a period of 0.99 days. If the disk and star are rotating with the same period (which seems unlikely), the stellar rotation axis would be tilted by 42 degrees to the line of sight, giving the star an upper limit to the true rotation speed of 325 km/s. Given the high rotation velocity, which mixes the stellar gases, it's rather odd that Epsilon Capricorni is a "silicon star" in which silicon's abundance is raised by elemental separation. Not far from its hydrogen-fusing lifetime of 30 million years, the brighter of the close pair is near the limit at which it could blow up as a supernova, perhaps ejecting the fainter "Ab" as a runaway star. If it doesn't, Eps Cap Aa will evolve into a massive white dwarf. If "Ab" is drawn close enough by evolutionary processes to the dying "Aa, the latter could exceed the white dwarf limit and still blow up as a classic "Type Ia" supernova. Off in the distance, at separations of 66 and 63 seconds of arc are tenth and fourteenth magnitude companions, Eps Cap B and C, which are moving far too fast relative to far brighter Eps Cap proper to be gravitationally bound, and are thus just line of sight coincidences.
Written byJim Kaler 10/09/15. Return to STARS.