Object: Struve 2816 (A, C, and D components)

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Cepheus
: 21:39:13 / +57.31
: 5.6 / 7.7 / 7.8
: 11.7” / 19.9”
: 121° / 339°
: O6.5 V((f)) / B1.5V / B0V
: Yellow / White / White
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 19/08/09 / 03:00
: Landgraaf
: 3 / 3
: Orion Optics UK 300mm
: 17mm Nagler+2x Powermate
: 184x / 26'
Struve 2816
Stuve 2816

Move mouse pointer over image to see individual stars labeled

Observing Report

In the 12-inch Dobson, the multiple star Struve 2816 is easily split into its three visual components at a magnification of 46x. The field of view is packed with field stars, too much to include in the sketch. Struve 2816 must be part of a star cluster, because of the large number of stars which are visible. The optimum magnification is 184x (Nagler 17 + 2x Powermate), which shows Struve 2816 as a beautiful triple star: a yellowish A component and two, much dimmer, white companions.

The C component lies to the southeast of the A component. The D component lies to the northwest. I notice two other interesting groups of stars, both lying to the northeast of Struve 2816. First of all, near the edge of the field of view I definitely see another double star. Both stars look white, and after a quick look in my star atlas, I find out that it is Struve 2819. So that's a nice catch, a Struve triple and double together in the eyepiece. Then I notice that between Struve 2816 and Struve 2819, there is a small asterism of four dimmer stars. It looks a bit like a hook or a trapezium.

I do not detect any other colored stars in the field of view. I suspect a lot of dimmer stars with averted vision, just visible as a background glow. However, I am not sure if this is the glow of unresolved stars, or maybe some nebulosity.


Admiral Smyth observed Struve 2816 as well. In his Cycle of Celestial Objects, he describes the colors as pale yellow for the primary and grey for both the companions. O' Meara also sees the primary as yellow in his book "hidden Treasures" (Hidden Treasure 105: Trumpler 37).

However, the color is not what you would expect. If you look at the spectra of the three components, the primary is a very hot O6 Vf star, and the two companions C and D ar both type B stars. So you would think all three would look more ore less blue-white, especially the O6 A component. I have no plausible explanation for this. Maybe there is some absorption of blue light due to interstellar dust, which would make the star appear yellowish. But why would the other B stars of Struve 2816 still look white? All three components lie at the same distance, but apparently only the blue light of the A component absorbed.

Struve 2816, also known as the multiple star system HD206267, lies at the heart of a big star cluster, Trumpler 37. This young and large open cluster is also known as a subgroup of the Cepheus OB2 association, Cepheus OB2a, which again is associated with the HII region IC1396. This giant HII complex has a diameter of three degrees, or six full moons. Amazingly, the yellowish looking O6 star in my sketch, HD206267A, is the main source of illumination of IC1396. On the beautiful image to the right, shot by Robert Gendler, we see IC1396 with the bright HD206267A at the centre of the image. The bright star at the left of the image is Herschel's Garnet Star, Mu Cepheus.

Struve 2816

IC 1396 by Robert Gendler
(Click on image to enlarge)

On the image below, which was taken with the Spitzer MIP (Multiband Imaging Photometer) in the far infrared, we see HD206267A, in a very different light. The hot massive star near the right edge of the image is HD206267A. At the centre of the image is a smaller star visible with a comet-like tail pointing away from HD 206267. So what is it we see on this image?

The star with the comet-like "tail" is actually a solar system in the making, that is being destroyed by the O-star's powerful wind and intense ultraviolet light. Through the process of photo evaporation, the immense output of the O-star heats up the protoplanetary disk so much that the dust and gas boil off. The consequence of this boiling off is that the disk can no longer hold together. Light-blasts (photon-pressure) from the O-star blow away the evaporated material. The small star looses its protoplanetary disk. This is what you see happening in the comet like structure of the smaller star.
Struve 2816

NASA/JPL-Caltech/Z. Balog (Univ. of Ariz./Univ. of Szeged)