OBSERVING THE SIRIUS COMPANION STAR Last updated: 9 March 2001

OBSERVING THE COMPANION TO THE BRIGHT STAR "SIRIUS"

In our telescopes large and small the bright star Sirius is a breathtaking sight, filling the eyepiece with its brilliant bluish light, overshadowing all else which surrounds it. Indeed, the star field encircling the brightest star of the heavens is one of the prettiest that can be found in low power telescopic viewing. This star field comprises just a small part of the beautifully delicate winter Milky Way, with brilliant Sirius, at magnitude -1.58, serving as a beacon toward the southern winter sky.

Sirius is so bright in a medium power telescopic field of view - indeed, NINE TIMES brighter than a "regular" first magnitude star - that it is briefly a rather painful sight to behold. It is one of the few stars (or even objects, for that matter!) that can be easily seen in daylight, visible in anY ETX at low power. Any computerized telescope can GO TO this object by merely "dummy-aligning" from near north (pressing enter at each alignment star instead of centering, of course) and then doing a "GO TO" under "Star" to Sirius.

Sirius is our fifth closest star, only 8.7 light years from our sun and slightly more luminous; in fact as recently as only 2000 years ago, there are historic Chinese and Greek references to Babylonian cuneiform text telling of Sirius as "the ruddy [reddish] star". Even as recently as 140 AD, the Greek philosopher Ptolemy grouped Sirius among all of the "fiery red" stars of the northern hemisphere.

Today we know it as brilliant blue-white, and we also have known since 1862 by the discovery of famous telescope-maker Alvan Clark, that it has a very, very close companion, about 5 arc seconds distant (during recent times) from the brilliant primary star. At magnitude 8.6, and a full 5" arc distant.....WHY have more skilled observers NOT seen this interesting "double star?"

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A TOUGH DOUBLE TO SPLIT

I have known of - and seen many times - the duplicity of Sirius since 1966. Recently, it's one of those astronomical statistics/facts that I had just stored away and forgotten until one of our keen ETX 125 users posted an inquiry on the Weasner Mighty ETX Site perplexed because he could not see the companion.

And perplexed for all the right reasons...he was RIGHT, after all! He had a 5" aperture telescope with the finest of optics (an ETX 90 should be able to see it, according to "resolution" and "light gathering" and all those empirical values that we hold so dear) and he seemingly was doing "everything right."

So WHY could he not see it? A star of magnitude 8.6 so far from its "mother star" should be seen in a 2" target spotting scope (you would think).

In 1993 to 1995 the star was only about 2.5" arc from Sirius and almost invisible to all telescopes (even though, on a good night at medium power the ETX 90 could easily separate any OTHER star of that separation!). At that time I would have advised observers to "forget trying to split Sirius" and worry about some other celestial challenge. But now, things are changing; for the next three decades, beginning after about 1999, the faint companion will be moving (as seen from Earth) farther and farther away from the bright primary in its 49.9-year orbit around Sirius.

So the time is right....we have telescopes that SHOULD be able to resolve this star.....let's go look at "Sirius B", the orbiting companion star to its more famous big brother, "Sirius A".

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SPECIAL OBSERVING TECHNIQUES

You do not need a special, or even a larger, telescope to view Sirius B for the coming years....what you need is "technique." The problem in observing this double is the overwhelming brightness (and glare in the optics you use) of Sirius A. It literally FILLS the eyepiece field of view with brightness, some of it scattered through the Earth's air through which you are looking, and a lot of it scattered within the optical elements of your telescope and its eyepiece.

Most of you are familiar with my "star test" techniques in which I emphasize that the "Airy Rings" - faint concentric rings of light which seem to encircle a bright star - indicate excellence in both optical quality and alignment.

However, when attempting to separate bright, and close, double stars, the Airy Rings can also "get in the way" and that is exactly what happens when attempting to view the elusive Sirius B. Note in Figure 1 (TOP) the naked-eye chart indicating Sirius and a little triplet of stars to its west (lower right); now move to the chart on the right which is "a closer view" and REVERSED, like images appear in your catadioptic telescope with the right angle prism for the eyepiece. You will see the same three (3) reference stars in both Figure 1 diagrams, but those in the reversed field will be a mirror image of what the naked eye sees. This has been included to show RELATIVE to Sirius A where the faint companion is located.

Don't get any ideas that it is this "easy" to see. Not by a long shot.

Now, look at Figure 2, showing a "telescopic" appearance that you might see of Sirius IF there were no diffraction rings, scattered light nor brightness from bright Sirius. Notice the current position of Sirius B (denoted by the YEARS of each position in its orbit) relative to Sirius. This field would be a VERY high (i.e., 80x to 100x per inch aperture) magnification and only usable on the steadiest of all nights.

Indeed, it is important to note here that ONLY THE PERFECTLY STEADY NIGHTS when you can "defocus" Sirius and observe its broad bright Airy "disk" to have NO perceptible motion whatsoever, can you expect to see the fainter companion star. It takes, quite simply, the most steady nights you have ever witnessed; hazy nights with very high clouds in late winter are many times very steady.

Figure 3, is like Figure 2, except all of the dates, titles and N,S,E,W cardinal axes have been removed as it might look under very high magnification. Have you learned WHERE the star is now? Chances are, you STILL cannot find it....there are likely diffraction rings covering it up, or there is so much scattered light from Sirius A that you might be STARING RIGHT AT IT, and never see it! It happens all the time.

It's there....you just can't see it.

Now....Figure 4 is the same field of view, but notice what I have done to eliminate all the glare...the diffraction rings...the light scatter: SIRIUS HAS BEEN MOVED OUT of the field of view on the edge OPPOSITE the direction of the tiny companion star! (and you're saying, "....gosh, why didn't I think of that!?").

It's a good and helpful idea, but it is very tricky and does not always work. There are two keys to using this "trick" to spot Sirius B:

1) the air must be absolutely perfectly steady (no star twinkling even on the horizons) and Sirius must be nearly on the Meridian (as much overhead as it's going to get);

2) you must use as MUCH magnification as your telescope can "hold;" I know this goes against everything I have preached on every other subject, but not on this one. It takes POWER to see Sirius' elusive companion - and lots of it. Use Sirius as a guide on your magnification limits. Turn the star just slightly out-of-focus until the bright star becomes an extended disk with a few rings within it; if the overall image is moving ("wiggling") then forget it, you're not going to see anything but bright blue Sirius A. If, on the other hand, the unfocused disk is steady with no motion....well, you have a "shot."

Using your highest practical magnification (where your star images are still faint points of light and not scattered - if they are "fuzzy", back down on magnification until they are points again - slowly move Sirius up and to the slight left (North-West) until it suddenly touches the NW edge of your eyepiece field.

Gently, either manually or with your electronic controller, tap the telescope until bright Sirius A is SUDDENLY just right over the edge and not visible (you will still see its bright light very vividly however); just to the lower right (SE) of where Sirius is "amost there", and very, very close to the edge of your field of view next to Sirius you should barely see Sirius B in the position marked on my accompanying chart (Figures 2 and 3).

You have "occulted" the bright star by the edge of the eyepiece, eliminating most of its glaring light that prevents Sirius B from being seen. You will probably have to repeat this technique time and time again within a short period of time until you FINALLY can actually spot the companion star.

But it CAN be done, and YOU can do it. You have the telescope, you know where to look, and now you know HOW to look for the star. If you don't see it tonight, just remember.....the conditions just keep getting more and more favorable for the next 25 years! By then, like me in 1966, you'll be a master and conquer this giant of all our celestial splendors!

P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
Arkansas

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