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Double vision

April in the northern hemisphere means galaxies -- lots of 'em. But what do you do if you live in the city, can't get to a dark-sky site easily, and it's close to full moon?

How about a few stars, for a change? Recently, I took a look at several bright double stars, for a change of pace from my usual choices of planets and deep-sky objects. More details below.

I observed the four doubles below with my 100mm f/6 refractor. All of them were resolved, even though the seeing wasn't especially stable. I expect all of these to be at least theoretically visible in an 80mm scope, but one (Epsilon Bootis) might be quite challenging.

Castor

This, the second-brightest star in Gemini, is simply dazzling: two brilliant white stellar gems against a dark background. Its components orbit relatively closely, so over several decades their separation changes noticeably. At the moment, it's quite easy to split them even with a 60 mm telescope. Although you can't tell this with a telescope, each of the two brilliant white stars is itself a very close binary, so there are really 4 bright stars here.

Not far from Castor (a little over an arcminute), and visible in a small telescope, is the faint star YY Gem, also known as Castor C. As the name suggests, this is actually another member of the Castor system. In fact, it too is a very close (spectroscopic) binary, so all told there are six stars here, all bound gravitationally as a single system.

Mu Draconis

Like Castor, this is a pair of bright white stars. Unlike Castor, they're almost identical in magnitude, and a little closer together. If the seeing isn't good, they might be a little challenging. Mu Draconis looks a great deal like either one of the 2 pairs in the famous double double in Lyra.

Gamma Leonis

Unlike the first two stars I looked at, this star is bright yellow-orange. I found it to be a little closer than Castor, but still easy to split. There is a noticeable, but not overwhelming, brightness difference between the two stars.

Epsilon Bootis

This is the most challenging of the four, and I expect it might be genuinely difficult in 80mm or smaller telescopes. Its separation is a little greater than Mu Draconis's, but there's a very large (about 2.5 magnitude) brightness difference between the components. I found it to be sensitive to seeing, and there were some times where it wasn't clearly separated. If you can separate it, it's a beauty: the bright primary star is an orange star much like the ones in Gamma Leonis. I had a harder time judging the color of the faint star -- it looked yellow to me, but it's really a white star rather like Castor, and many observers see it as bluish. There might be a clearer color contrast with better seeing or with a larger aperture and hence better resolution. See what it looks like to you!

There are many others to try, of course. High up in the Northern spring are two classics -- Mizar and Cor Caroli -- both much wider than any of these four, and thus very easy with just about any telescope. Alternatively, there are many other stars with separations less than an arcsecond, which make a nice challenge for people with 6" or larger telescopes. If the light pollution or the Moon is bugging you tonight, try a few of these and see what you think.

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