February 26, 2020

Astro-Challenge: The Changing (?) Colors of 95 Herculis.

I love double stars. While many a wispy nebulae are elusive and vanish at the first hint of light pollution, double stars tend to stand up well under the less than optimal viewing circumstances that plague most of us. I’ve pulled off many a moonless star party from light-polluted urban areas by adding bright double stars to the menu, a handy item to have in your repertoire when a decent planet is nowhere to be found. But YOU want to be the one observer that ventures beyond Albireo, right? Well, this week, we’re turning our scopes towards an often over-looked double star that’s currently near the zenith for mid-northern latitude observers on September evenings; 95 Herculis.  At a distance of 470 light years, this pair of +5 magnitude stars is separated by about 6.3” seconds, and the color contrast is a real stunner to anyone sweeping the field. First measured by F.G.W. Struve in 1829, this pair has changed little in orientation since discovery and is estimated to have an actual physical separation of 900 Astronomical Units and an orbit of about 11,000 years. Remember all this “Wow” factor stuff as you’re showing off any given double star; it’s part of the effective showmanship that comes with a successful star party.

…and here’s where the mystery and the controversy surrounding 95 Herculis deepens. Component A is a white, type A5 giant of 6.8 solar diameters, while B is a yellow-white class G star of 19.4 solar diameters. Observers, however, have assigned the pair a wide variation of colors, from both white by Piazzi Smith (1856), to green and red for William Dawes  (1857), to bluish-white and white for William Herschel in 1780 and bluish-white and reddish for his son John in 1824. Struve himself assigned them the colors of greenish-yellow and reddish yellow…

So, what gives? This wide range of color observation has led some to speculate that 95 Herculis has undergone some sort of physical change not normally associated with these types of stars, but I believe the answer to be more prosaic. The frequent reports of a greenish tint contrasted with the red companion star is rather telling. No star is truly ‘green’ in appearance, although they may emit peak energy at green wavelengths, as does our Sun. I believe our old friend the Purkinje Effect is coming into play, causing a slight contrast shift towards the blue end of the spectrum when seen paired with a slightly redder companion star. What’s truly interesting is that neither of the stars in the 95 Herculis would appear exceptionally red on their own! But, like many celebrity couples, put ‘em together and you’ve got you’ve got a pretty pair, one that can confound generations of scientists. Coordinates for 95 Herculis are;

R.A.: 18H 01.5’

Declination: +21° 36’

Position Angle: 256°

Let us know what color(s) YOU see. Adding a new double star pair can make for a unique star party experience… and it’s a great feeling to be the one telescope down the row that’s NOT aimed at Saturn!

The Astronomy words for this week are Color Index. Our eyes are notoriously bad when it comes to guess-timating color under low light conditions. A more high fidelity method in measuring the true spectral energy output of a star is to compare and subtract its magnitude variation as viewed through successive filters. Generally this is a pair comparison, the filters being either U(Ultraviolet), B(Blue), or V (Green-yellow). This is why you generally see the color index also stated as the “U-B” or “B-V” color index of a star. Color index is also luminosity class dependant, and color indices can go a long way towards pegging down the class of the particular stars along a color magnitude diagram, especially in the case of stars that defy visual consensus such as 95 Herculis. White hot B and O stars run a color index of around -1, (think Rigel) while  a color index of +1.5 would tip the scale at the other end of the stellar classification spectrum at class M and N. And our own humble Sol? It has a B-V index right around +0.656, proof that its energy does indeed peak in the green!

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