Splitting double stars can still contribute to science, as the image in our banner illustrates.
Other telescopes had shown this particular star as a single – but as a single it broke all the rules. Calculations had shown that it was twice as big as any star could and should be – but no wonder, it really was two stars as the Hubble Telescope proved by splitting it! Here’s how the space agency explained it in a press release in 2006 – oh, and credit for the image should go to NASA, ESA, and J. Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain)
The small open star cluster Pismis 24 lies in the core of the large emission nebula NGC 6357 in Scorpius, about 8,000 light-years away from Earth. Some of the stars in this cluster are extremely massive and emit intense ultraviolet radiation. The brightest object in the picture is designated Pismis 24-1. It was once thought to weigh as much as 200 to 300 solar masses. This would not only have made it by far the most massive known star in the galaxy, but would have put it considerably above the currently believed upper mass limit of about 150 solar masses for individual stars. However, high-resolution Hubble Space Telescope images of the star show that it is really two stars orbiting one another (inset pictures at top right and bottom right). They are estimated to each be 100 solar masses. The Hubble Advanced Camera for Surveys images were taken in April 2006.
Wow! The Hubble Space Telescope – now that’s a star splitter!
John Nanson
Greg Stone
Star-Splitters 
Dear Starsplitter:
I have a 100 mm ED f/9 refractor. What should I be able to split? Is there a table of some kind? I can’t find it. I have split Alnitak with it at 180x once when seeing was very steady. I have never been able to split Alrescha. Should this telescope split Alrescha if seeing is good?
Thank you for a wonderful site. It seems like you do a lot of drawings at the eyepiece. When I do sketch I can almost instantly feel my observing skills sharpening. I learn a lot here. Wish I could afford that Skylight refractor!
Mark Teran
Whittier, CA
Hi Mark,
Yes, you should be able to split Alrescha with a 100mm refractor. As my sketch showed, I split it with a 105mm refractor (which I later learned actually had only 95 mm of unobstructed glass), and if you read the comments at the end of that post, you’ll see I split it with an 85mm refractor. I’ve found Alnitak is actually more difficult to split because there’s a larger difference in magnitude between the primary and secondary (1.07 for Alrescha vs. 1.82 for Alnitak), even though Alnitak is .4″ of a second wider. Alnitak’s 1.88 magnitude primary has some effect on that — the magnitude of Alrescha’s primary is 4.10, so there’s less glare.
The key, though, is good seeing. Without it, both Alrescha and Alnitak are pretty much beyond reach at 100mm. When the seeing is good, they’re both relatively easy.
For a chart, take a look at page five of Sissy Haas’s Double Stars for Small Telescopes. There’s a table there that will give you a rough idea of what you can do with the 100mm refractor under good seeing conditions. Keep in mind that chart is only a rule of thumb. It’s hard to quantify variables such as optical quality, observer experience, and weather conditions.
John