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Cosmic Challenge: Izar [Epsilon (ε) Boötis]

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[Epsilon (ε) Boötis]



June 2022




Phil Harrington



This month's suggested
aperture range

3- to 5-inch
(76-127mm) telescopes









[Epsilon (ε) Boötis]



04.5 '


2.5, 5.0



Finding this month's challenge object is no challenge at all unless you are trying to starhop to it from the inner city. That can be tough, but for everyone else, Izar (Epsilon [ε] Boötis) is visible easily by eye to the northeast of brilliant Arcturus (Alpha [α] Boötis) as one of six stars that make up the constellation's distinctive kite shape. Swing your telescope its way and it still looks like a single star, as it does to the naked eye. So, what's the attraction?


Above: Early evening star map. Credit: Map adapted from Star Watch by Phil Harrington.

Above: Finder chart for this month's Cosmic Challenge

Credit: Chart adapted from Cosmic Challenge: The Ultimate Observing List for Amateurs by Phil Harrington.
Click on the chart to open a printable PDF version.




Izar is one of the more challenging binary stars in the northern spring sky for small telescopes. Low-power binoculars readily show it to be accompanied by 34 Boötis, a red 5th-magnitude sun 39' to its southwest. They form an attractive pairing, even though they have no relation to each other.


The challenge presented here is to see Izar's actual companion, 5th-magnitude Izar B. Izar B is 2.9" away from the system's primary star, 3rd-magnitude Izar A. Observers usually describe Izar A as looking either yellowish or pale orange, while Izar B ranges from bluish to sea green or emerald.


Two stars separated by 2.9" sounds as though they are wide enough to drive a truck through until you consider the circumstances. The problem is due to the optical properties of light. Because light travels in waves, stars never appear as perfect points through telescopes. Instead, a star will focus to a small central disk -- the Airy disk -- encircled by one or more dim rings called diffraction rings. The distance to the first, most prominent diffraction ring is about six times the area of the Airy disk itself. In the aperture range set for this month's challenge, Izar A's first diffraction ring is approximately 3 arc-seconds away from its Airy disk. As a result, the companion lies nearly superimposed on the primary star's first diffraction ring, lowering image contrast and creating the challenge.


With this in mind, I headed out with my 4-inch (102-mm) f/10 refractor to my suburban backyard a few years ago to see what I could see. Popping in a 7-mm eyepiece (143x), I could readily split the pair, with Izar B appearing as a bright spot on A's diffraction ring. It helped that this particular instrument has very good image contrast and that the seeing was steady. Had I been viewing through a similar size telescope with an obstructed optical design (that is, a reflector or a catadioptric), the resulting lower contrast could have prevented me from seeing the companion.


Above: Izar as portrayed through the author's 4-inch (10.2 cm) telescope.


Like many refractors, mine came with an objective lens cap that has a smaller central opening. In this case, the smaller opening, which is normally covered by a plastic dust cap, measures 1.75 inches (45 mm) across. Could I still see Izar B if I effectively stopped down my refractor to a create 45-mm f/22 instrument? Not expecting much in the way of success, I removed the dust cap, placed the lens cover over the objective, and aimed again at the star. To my surprise, I could see both suns fairly easily, the companion now just inside the first diffraction ring. Incidentally, to make this as "blind" a test as possible, I purposely conducted these observations before looking up the companion's position angle. Sometimes, knowing where something is will cause you to see it even if it is not visible -- witness the plethora of Martian canal observations at the end of the 19th century! Afterward, when I went back inside to check, sure enough it was right where I saw the companion, northwest of Izar A.


To be successful at spotting Izar B, you'll need high quality optics, both in your telescope and in the eyepiece, and steady seeing. Light pollution is of little or no concern once Izar is in view since both stars are so bright.


Now, I know there are going to be readers out there who say that splitting Izar is easy. Sure, if you are viewing it with a telescope larger than the specified aperture range, Izar poses little challenge. So, for those readers, I am offering a bonus challenge, the binary star A570 (ADS 9301). You will see that it is also plotted on the chart above, not quite 3° west of Izar . Here, we find two stars, magnitudes 6.0 and 6.5, separated by only 0.2". That's a challenge for any scope!


Good luck with this month's challenges! And be sure to post your results in this column's discussion forum.


Remember that half of the fun is the thrill of the chase. Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy, including Cosmic Challenge: The Ultimate Observing List for Amateurs. Visit www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2022 by Philip S. Harrington. All rights reserved. No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


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