April 5, 2020

Astro-Challenge: Haunting the “Ghost Double.”

The “Ghost of Gamma…” (Created by the Author in Starry Night).

Sometimes, the new and the unexpected lies just inside the field of view of the familiar. This week, we’d like to turn your attention to a hidden double star in the field of a star party favorite.  Halloween means sidewalk astronomy season, as we show off the delights of the universe to high-fructose corn syrup-filled suburbanites. Hey, it’s wonderful that a pagan Cross Quarter tie-in holiday (as in a celebration approximately midway between the equinox and the solstice) gets some play in this day and age.

The trouble is, what do you show ‘em when city lights are burning bright and planetary detail is nowhere to be seen in prime time skies, such as Halloween 2012?

We give em’ double stars, and one of the finest examples is Gamma Delphini in the constellation Delphinus the Dolphin. The pair is an easy find on Fall evenings, being located at the eastern “tip” of the “diamond” asterism. If thoughts of happy frolicking dolphins aren’t ghoulish enough for Halloween viewing, the asterism also sometimes goes by the obscure name of “Job’s Coffin.”

The constellation of Delphinus the Dolphin… or do you say “Job’s Coffin?” (Photo by Author).

Gamma Delphinus is an easy split with a small telescope of about 9” arc seconds. The pair is within one magnitude brightness at +5.0 & +4.4, and has a physical separation of about 300 astronomical units and an estimated orbital period of about 3,249 years. First recorded by F.G.W. Struve in 1830, colors at the eyepiece range from white to yellow to violet. A pretty pair, to be sure… but did you know a visual twin lurks nearby?

James Mullaney notes in the October 2011 issue of Sky & Telescope that a “Ghost Double” in the form of Struve 2725 is apparent in the same 30’ arc minute field as Gamma Delphinus. At magnitudes +8.2 and +7.5 respectively, the pair is an easy grab, once you know to look for them. Struve 2725 is only a slight bit closer in visual separation at 6.1” arc seconds and about 256° degrees out of position angle alignment with Gamma Delphini.

Though the pair lies in the same direction, that’s only a happy circumstance of our vantage point in space and time. Both systems have different proper motions and are moving apart from one another. Struve 2725 is 125 light years distant, versus Gamma Delphini’s slightly closer 102 light years. Struve 2725 lies 15’ arc minutes south-southwest of Gamma Delphinus.

There has also been some controversy over the 1999 claim of an exoplanet for Gamma Delphini B; estimates from a study carried out by the McDonald Observatory place a lower limit on the unseen planet at 0.7x Jupiter masses and a distance of 1.5 A.U.s. The total mass for the Gamma Delphini system weighs in at 3.3x Sols. Together, Gamma Delphini and Struve 2725 are also sometimes referred to as “the Dolphin’s Double-Double” in reference to Epsilon Lyrae, but I like the Halloween meme of “The Ghost Double” as a system that seems to be stalking the famous pair.

The current positions are as follows;

Gamma Delphini:

R.A.: 20 Hours 47’

Declination: +16° 8’

Struve 2725:

R.A.: 20 Hours 46’

Declination: +15° 54’

Happy Halloween!


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