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Challenge #3 – Izar, Double Star in Boötes

TargetTypeR. A.Dec.ConstellationMagnitudeSeparation
IzarDouble Star14h 45m 53s+26° 59′ 20″Boötes+2.5 & +4.82.9″

Overview

I’ve just published a guide to the constellation of Boötes over on Love the Night Sky, and our third challenge this month is to observe the double star Izar, which is a bright binary near Arcturus.

At magnitude 2.5, this is a naked-eye star to most observers, even under town center light pollution. But, to all of us, it looks like a single star, even though its companion is also relatively bright at just under magnitude 5.

You may be tempted to believe that a nearby reddish star, 34 Bootis, is the companion, but that is not the case. The actual ‘B’ star is less than 3 arcseconds away from the main star and lying on the ‘A’ star’s airy disc in a small scope – making it a decent backyard challenge.

Finder Charts

Izar is simple to find. The SkySafari 6 chart below shows Boötes directly overhead (note the zenith marker) at 10:30pm in the middle of June. Izar is the next brightest star in the constellation after Arcturus.

Izar is the second brightest star after Arcturus in Boötes

If you’re not sure where to find Arcturus, use the finder chart below to get to it from the Big Dipper.

Find Arcturus from the Big Dipper by following the arc of its handle

It’s easy to assume that 34 Bootis, the reddish magnitude 4.8 star nearby, is the double. However, this is just a line-of-sight coincidence. Izar is 200 light-years from us whereas 34 Bootis is 700 light years away!

We need to point our scopes at Izar now to discover the true double. Increase the magnification to begin to split out the two stars. The image below shows them just separating in a 0.2° field of view. Of course, your own field size depends on your telescope/eyepiece combination, but in my scope this equates to roughly 200x magnification with a 50 degree apparent field of view eyepiece.

Izar A and B need decent magnification to separate them out

If your scope and observing conditions can stand it, by all means attempt a higher magnification. You’ll see the companion north of Izar A.

In reality, this is a good test of both the quality of your optics and the seeing conditions.

Good luck!