100 Her—A Matching Double Star

by Bill Pellerin

Originally published in the July 2015 Guidestar

Star 100 Hercules

Object: 100 Her, SAO 85753, STF2280
Class: Double Star
Magnitude: 5.8, 5.8, combined 5.79
R.A.:  18 h, 07 m, 50 s
Dec:  26 degrees, 05 minutes, 51 seconds
Distance:  230 ly
Constellation:  Hercules
Spectral: Matching A3V stars
Optics needed: Small Telescope

Why this object is interesting:

One of the challenges of double stars is figuring out what catalog they’re in, what their catalog number is, and what catalog numbers are represented on my map (paper or computer). SkyTools lists eleven catalog designations for this star. TheSky recognizes 100 Her by at least three designations. You should be able to find it by one of the designations given here, and if that fails, you can find it by its RA and DEC.

The STF (think ‘Struve the Father’) designation is interesting because it identifies this star as being in the Friedrich Georg Wilhelm (von) Struve (1793-1864) catalog. The catalogs for FGW Struve are written with a Sigma and then a catalog number (example Σ2280) or with the STF designation. Those cataloged by his son, Otto Wilhelm Struve (1819-1905), have the letter O preceding the Sigma character (example OΣ18) or use STT preceding the catalog number.

The grandson of Otto Wilhelm Struve, also named Otto (1897-1963), was the first Director of the UT McDonald Observatory (1932-1947).

What you’ll see when you look at this double star is a pair of stars (nearly) perfectly matched in brightness and color. When I see a pair like this I think of headlights approaching on a long dark road.

The color designation indicates that each of these stars is white, with a ‘surface’ temperature between 7500 and 10,000 degrees Kelvin. They are low mass stars and a spectral analysis would show a strong hydrogen absorption line (mean that there’s a lot of hydrogen in the outer layers of the star).

The August, 2007 GuideStar has another double star in Hercules, 95 Her.

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