Some doubles around Gemini

[Moonshane]

Conditions were pretty poor the other night with seeing not great and a thing layer of high cloud. I thought I'd make use of the first clearish night in ages and use my Cambridge Double star Atlas for the first time since getting it at Christmas - can you believe that.........

Gemini was n a good position so I thought I'd have a mooch around that constellation and see if I could find some of the many doubles in the maps. My usual method is look at what I see and then try to reconcile that with what I'm supposed to see. I find this useful in avoiding 'imagination' taking over compared with the other way round.

Here's what I found:

Castor Alpha Gem. - this is a great double star system and one that I return to a lot. It's easy to split the two main bright stars which are approximately equal in brightness and very bright white. Separation is about 4". Don't forget to look for the tiny red star which is also a part of the system at about 71" separation.

Pollux Beta Gem. - I was actually surprised to read that Pollux is a double star system. I could see a few stars in the same field at reasonable power and I think this was my problem - the stars in the atlas are stated as being 201, 234 and 260" separation. On this basis I am not counting having seen this system.

Kappa Gem. - A bright yellowish primary with a tiny speck of a secondary at 7" sep. Reminiscent of Rigel in Orion but obviously different colour. Could not detect colour of the secondary.

Wasat Delta Gem. - very similar to Kappa but with a whiter primary. The separation was also quite small at 6".

Mekbuda Zeta Gem. - I marked this as a wide pair at 160x with a whitish yellow primary and a dirty white fainter secondary. However, I am now doubting that I actually saw the double as it appears to be much closer than I recorded it. I had the separation as pretty much similar to Mebsuta (see below) but the only information I can find is that this Cepheid variable star has a secondary 0.1" separation. Again, not counting this one. Anyone got further info?

Mebsuta Epsilon Gem. - The description of this is pretty much as described for Mekbuda above with a separation of 111" and with a whitish yellow primary and a dirty white fainter secondary.

Nu Gem. - pretty much identical to Epsilon but with a whiter primary star. Separation 112"

15 Gem. - This was remarkably like the previous three doubles other than the stars appeared much closer together (separation 25") and the stars were smaller in the field also.

Propus Eta Gem. - this is also a double star as well as being a variable star. The primary was very yellow and I recorded what I took to be the secondary, spaced as per others this night at around 100". Unfortunately, the separation is actually much closer at 1.8" so another one I missed and I'll try again another night.

To round things off I had a look at M35 and also the Eskimo Nebula.

A really enjoyable night and I like the atlas as it gives great info.One thing that could be improved I feel is more finely spaced lines across the maps. The bulk of the listed doubles require you to establish the RA and Dec co-ordinates to locate them in the index and this is tricky in the dark. I may need to use a ruler when using this book in the future.

[Crimson Sunset]

Gemini has great doubles, doesn't it? Wasat is fantastic, I must admit that I take a peek at it almost every session as the secondary has a really nice red hue to it!

[cotterless45]

Kappa is ace, it has a lovely story:

Due south of Gemini's Pollux, separated from it by about the angular distance between Pollux and Castor, lies easily-found almost-but-not- quite-third-magnitude (3.57, so it's barely fourth) Kappa Geminorum. Not really part of the classical outline, it at first comes across as just one more helium-fusing class K or G (this one G8) giant. But we should not dismiss such stars so lightly, as along with class A and B hydrogen-fusing dwarfs, they form the hearts and souls of the patterns we have come to know and love. Though in outward appearance these stars all seem rather alike, no two are quite the same, this one especially harboring a delightful secret (as noted below). A large number of temperature measures (in part as a result of the star being used as a statistical standard) yield a well-determined average of 4990 Kelvin. From a VERY well determined distance of 141 light years (accurate to just 1 l-y), the star shines at us with a luminosity (that includes the invisible infrared radiation estimated from temperature) of 74 times that of the Sun, from which we find a radius of 11.5 solar and (from theory) a rather hefty mass of around (with low accuracy) 2.7 Suns with an age of some 500 million years. Kappa Gem is near and large enough for direct measure of angular diameter by interferometry. An observation in yellow light gives a radius of 11.6 times that of the Sun, right on the mark, while another at a redder color makes the star seem much larger at 14 solar radii. While distended giants and supergiants commonly appear to have different sizes when seen in different colors, this discrepancy is disconcertingly large. A projected equatorial rotation speed of 5 kilometers per second (with a very large range of measures from 2 to 9 km/s, and using the smaller radius) gives a rotation period that could be as long as 115 days.

What makes Kappa Gem special, however, are not these statistics, but that it is linked to an intriguing solar-type companion. Set off at a current separation of 7.5 seconds of arc lies eighth magnitude (8.2) Kappa Gem B. Smythe and Chambers in the 19th century called the pair "a very delicate double star" with colors "orange" (certainly appropriate for bright Kappa A) and "pale blue" for Kappa B. Hardly. Over nearly 200 years, the separation has changed by only 2.5 seconds of arc, so they are probably a real orbiting binary. (If they were a line-of-sight coincidence, the rate of separation should have been much greater.) Given that they do belong to each other, Kappa-B's brightness however shows it to be a class G4 dwarf that is rather much like our Sun, which is yellow-white. The "pale blue" is the result of a visual contrast effect. Placement of stars right next to each other enhances, or even changes, the stars' apparent colorings. The effect is so strong, and the apparent colors so vivid, that old-time astronomers once thought that the components to such "wide" (non-interacting) double stars were physically different from single stars. Given a true separation between the stars and that "B" has about a solar mass, Kepler's Laws give an orbital period greater than 3000 years. X-rays coming from the system are probably due to magnetic activity associated with the solar-type companion. If it (Kappa has any planets (which is unknown), Kappa A would present a fantastic sight, shining in their skies with the light of over 1000 full Moons, which when up would REALLY blot out the stars. Thanks to Paolo Colona, who suggested this star, and to Bill Hartkopf for expert opinion.

From "Luminos" app,

Nick.

[PorkyB]

I like the Cambridge Double Star Atlas, I use it for all my observing because of its large scale and clear maps. But I do wish the index was sorted by constallation and name, rather than by RA, as it takes me ages to find anything, especially with frozen fingers.

[lunator]

Shane

Nice report

The CDSA is a great resource for us Double Star fans

I have found that the easiest way to use the index is the Struve numbers. If you know what they are then it is easy to track them down in the index.

Cheers

Ian

[Moonshane]

Cheers Ian - top tip. I'll try that next time out.

[michael.h.f.wilkinson]

Nice report. Really makes me think I should get into this double-star more. Slight haze or LP do not matter as much as for most other DSOs

[lunator]

Michael

Having lived in SW London all my life I learnt that doubles can be observed on any night that can be considered clear-ish.

Faint fuzzies are just not visible in my current location.

Cheers

Ian

[John]

Interesting report Shane

Some good targets there for those nights when the seeing is OK'ish.

[Special K]

Good stuff Shane and thanks for sharing: I've split Castor recently but could spend more time in Gemini for sure. Favorites are still the double doubles Eta Lyra. I've only recently realized that doubles are good fare on average nights and thankful for em!

The Cambrige Star Atlas is very nice and can imagine the CDSA is cool, and from what I can tell does down to mag 7.5 whereas CSA stops at 6.5. I can see why you use it for all your obs, PorkyB!

[chiltonstar]

At last - a clear night (not enough for the comet though....!

Tried delta and kappa-gem; both easy'ish. although the seeing wasn't wonderful. Colours? Not sure, I thought the secondary of kappa was definitely purple, but colours through my scope are never too accurate.

Chris