Figure 1
Palomar Sky Survey
15x15 arcminute field centered on Cyg OB2 #12

Table I: Data for Cyg OB2-12
log Teff Mv Mbol log L/Lsun Mass
log age
D (kpc) spectral
Cyg OB2 #12 Cyg OB2 Association 4.270 -10.6 -12.2 6.78 92 6.43 1.7 B5 Ie Massey et al., 2001

Cygnus OB2 #12

Located some distance from the center of the Cygnus OB2 association, Schulte's star number 12 sits at RA 23h30m53.38s, DEC +41°04'12.9" (B1950). Its absolute visual magnitude of -10.6 (Massey et al., 2001) makes Cyg OB2 #12 the visually brightest star known, in the Milky Way. But it also suffers the highest visual extinction (Av=10.31, Massey & Thompson, 1991) of any star in the Cyg OB2 association. The picture in figure 1, a 15x15 arcminute field from the first Palomar Sky Survey (retrieved from MAST DSS), is centered on the location of Cyg OB2 #12, and demonstrates the affect of the extinction. It's there, but it is unspectacular, considering that it's the brightest visual star in the galaxy. The absolute bolometric magnitude of Cyg OB2 #12, -12.2, also makes it the most intrinsically luminous star known, in the Milky Way, barring the high temperature model of the Pistol star.

Why is Cyg OB2 #12 so highly reddened? According to Massey & Thompson, 1991, it is because the star is surrounded by material lost in an earlier eruptive event, typical of a luminous blue variable (LBV). Cyg OB2 #12 is considered an LBV candidate, and sits at the 85 solar mass spot of the color magnitude diagram in Massey & Thompson, 1991, thoughj Massey et al., 2001 assign it a slightly higher 92 solar masses. Also, Massey & Thompson, 1991, give a spectral type classification B5 Ie, where the "e" denotes H-beta emission. Earlier observers have classified it as B8 Ia+ or B8 Ia. Waldron ,et al., 1998 use B8 Ia. Humphreys & Davidson, 1994, classified it as an A-type hypergiant. So it seems that, perhaps due to the heavy extinction, it is very hard to discern the proper spectral type for this star.

The Cygnus OB2 Association

Figure 2
Palomar Sky Survey
15x15 arcminute field at the center of Cyg OB2

An OB association is a grouping of young stars, usually of about the same age (or, as in Orion OB1, in subgroups of the same age). Cyg OB2 is centered about RA 20h33m10s and DEC +41°12' (J2000), and has an angular diameter of about 2°, which translates into 60 pc at a distance of about 1.7 kpc. There are about 8600 stars earlier than F3V, and about 2600 OB stars. Its total mass is about 4,000 to 10,000 solar masses (Knödlseder, 2000). Since Cyg OB2 extends for 2°, the 15'x15' image in figure 2 only shows the centermost portion of the association. Because it is so close, appearance can be deceptive. According to Knödlseder, 2000, Cyg OB2 is in reality a young globular cluster. We are so used to looking at globulars that are 10 kpc in the distance, can we overlook one that is a mere 1.7 kpc distant? Maybe. In any case, Cyg OB2 has about 2600 OB class stars, and about 120 O stars, which clearly makes it a hot, bright, and probably young association.

Figure 3
Palomar Sky Survey
Cyg OB2 #8

Near the center of Cyg OB2 is the trapezium-like system Cyg OB2 #8, shown above in figure 3 (a 5x5 arcminute field). The bright star at the center is Cyg OB2 #8A, classified O5.5 I(f) by Massey & Thompson, 1991, but earlier works classified it as O6 Ib(n)(f), O6 I(f) and O6f. Herrero et al., 2000, use O5.5 I(f). Its effective temperature is a high 44,000 K, and its bolometric luminosity of 2,750,000 suns certainly puts it in the bright camp, even if it is a tad dim to be on the list of candidates for brightest. Cyg OB2 #8B is hidden in the glow of A, its image merging with A, is below and to the right. Cyg OB2 #8C is the other bright star, just to the left and below A, while Cyg OB2 #8D is above, and to the left of A, almost touching the glow of A. Cyg OB2 #8C is the 2nd brightest of the 8-cluster, O5 If, and is even hotter than 8A, 48,000 K. But is is also much smaller, and so shines only about 850,000 times brighter than the sun.


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