OMI-2 CYG (Omicron-2 = 32 Cygni). Stellar astronomy is filled with both confusion and coincidences, the "Omicrons" of Cygnus providing a full measure of each. Fourth magnitude (3.79) Omicron-1 (31 Cygni in Flamsteed parlance) and also-fourth magnitude (3.98) Omicron-2 (32 Cygni) make a line-of- sight wide "double" to the east of Deneb. Tucked next to Omicron-1 is fainter fifth magnitude (4.83) 30 Cygni. Unfortunately, some authors have referred to the close pair 30 and 31 as Omicron-1 and Omicron-2, while others have called 30, 31, and 32 Omi-1, Omi-2, and Omi-3. Worse, since 30 Cygni seems to be related to 31, it is sometimes also called 31 Cygni D, even though its motion through space shows that the two do not physically belong together. We stick to the most common definition of 31 and 32 as Omi-1 and Omi- 2. More intriguing Omi-1 and Omi-2 (31 and 32), though not related
Omicron 1 (the lower of the two bright stars) and Omicron 2 (upper) show the orange colors of class K giants. Just barely up and to the right of Omicron 1 lies contrasting white (almost blue in the photo) 30 Cygni, a class A5 dwarf. To the left and a bit up from Omicron 2 is a reddish star, U Cygni, a class R (carbon-type) Mira (long-period) variable.
at all, are almost identical kinds of stars, both long-period "Algol-type" eclipsing binaries with super- large class K primaries orbited by hot, dimmer class B dwarf secondaries much like Haedus I (Zeta Aurigae). Every 3.143 years, Omi-2, 1100 light years away, takes a slight dip as the class K (K2) supergiant just barely grazes a class B (B3) dwarf, producing a dimming of a mere 0.06 magnitudes (about six percent). In fact, for this type of system, Omi-2 sets a record for the weakest eclipse. Only if the primary star is huge can such long-period eclipses have any probability of being seen. The temperature of the supergiant is problematic, the measures ranging from 3900 to 3500 Kelvin. Adopting the more realistic upper value to account for infrared radiation (and ignoring the fainter B star) gives a luminosity of 11,100 times that of the Sun and a diameter of 230 times solar, close to two Astronomical Units. Direct measure of angular diameter agrees. Analysis of the orbit gives a mass of 5 solar for the class B star and 9.7 for the prominent supergiant. Less than 20 million years old, its fate is either to explode or to create a rare neon- magnesium white dwarf. Slightly metal-deficient compared to the Sun, this ponderously rotating star spins with a projected equatorial speed of just 3.4 kilometers per second. If that is the true rotational velocity (that is, if the rotation axis is perpendicular to the line of sight), the star would take an amazing nine years to make a full rotation. Stars like Almaaz, Omi-1 Cygni, and Omi-2 Cygni are of great importance, as during much of the eclipse, the light from the class B star shines through the large star's hugely distended "chromosphere," the tenuous layer above its actual surface, backlighting it and allowing its detailed study.
Written by Jim Kaler. Return to STARS.