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DSC-60: Beta Orionis – Rigel, the brighter giant

This is a DSC-60 Project observation for project details go here.
Beta Orionis 05h 14m.5 -08° 12′ 0.1, 6.8 9.5″ 202

Maybe I should call my new little scope Jack – as in Jack, the Giant Tamer. No bean stalks here, but on it’s first night out I turned it on the difficult double Rigel on a whim and to my surprise, it brought the blue giant to heel immediately. There, shining steadily while stellar flames darted all around it, was the 7th magnitude companion!

Wow! I was impressed. OK, it’s a Televue 60, so it no doubt has good genes and it had just done a fine job on some other doubles, but Rigel is no piece of cake for any telescope. The 9.5 second separation between primary and secondary certainly is generous enough for any scope, but the nearly seven magnitudes in difference between primary and secondary makes that separation seem much smaller than it is. In fact, I remember reading a few years back that Rigel was a great warm-up star for those wishing to tackle Sirius whose “pup” has been notoriously difficult to see in recent years, but the separation is widening making it easier.

Then I reviewed the  table in the introduction to the Sissy Haas book. Oops. The “pop” you heard was my bubble bursting.  By the numbers, Rigel shouldn’t be all that difficult.

In the table I’m talking about  she shows that a 60mm should be able to handle a 2″ separation – if the stars are the same magnitude.   She then builds that table up to include a four magnitude difference where she says the split, if it is to be detected by a 60mm scope, should be at least 4.5″ – and there she stops.  I haven’t been able to deconstruct a formula from the table, but the trend is pretty obvious – and it says I shouldn’t be so shocked because with 9.5″ separation even a 60mm should find it reasonably easy to split two stars despite a seven magnitude difference between them.

Nevertheless, I still think I’ll call it Jack in honor of a no hassle split of Rigel, because more than one night I have found this star difficult with much larger scopes, though, of course, seeing plays a major role.  The seeing on this night was good – probably a four out of five where five is the best.  I should add it was good only for about an hour, then everything deteriorated quickly – transparency and seeing. Got colder, too – though it was already darned cold in what has been one of the most wintry winters in memory.

But the heck with the cold, let’s get to the hot stuff – the real hot stuff – Rigel. Do I need to tell you how to find it? It’s the blue/white star int he foot of Orion – his western-most foot


Orion and friends as Stellarium represents them with, I think, a little whimsy. I mean, should Orion be more concerned about Taurus charging down on him? Or Monoceros - the Unicorn - who has a gleam in his eye and a mean look on his lips and a single sharp horn aimed at the hunter's backside! (Oh- note Rigel - and yes, if you click this image you'll get a larger version. And check out our "resources" link at the top of the page for more information on Stellarium.)



Like nearby Pollux, it looks like Rigel was short-changed having been designated the “Beta” star of the constellation Orion while dimmer Betelgeuse is the Alpha. Of course, Betelgeuse, being variable, may have been brighter when Johann Bayer made his designations in 1603. Bayer’s “system” is inconsistent, however, to say the least, so there’s no sense getting too worried about this.

Like Betelgeuse, Rigel is a supergiant. It’s huge and it’s brilliant too – and since it is more distant (860 light years), it is intrinsically more brilliant than Betelgeuse. Jim Kaler writes: “Only about 10 million years old, Rigel should eventually expand to become a red supergiant very much like Betelgeuse is today, by which time it will be fusing helium into carbon and beyond in preparation for its eventual explosion as a supernova.”

Rigel’s radius is 74 times that of the Sun, 0.34 Astronomical Units, nearly as big as the orbit of Mercury. Words don’t do that justice. I came up with this little graphic to show how large Rigel is in comparison to nearby Adebaran.

Get the picture? Our Sun, large enough to hold one million Earth’s, deserves the appelation “dwarf” when you compare it to Rigel and Aldebran,  Before you look through your telescope at Rigel, pause. Take a look with the naked eye. Imagine Rigel expanding to cover all of the stick figure of Orion – and that’s only half of it! Of course we’ll leave Betlegeuse out of it. In fact, I had to leave Betelgeuse out of the graphic because it’s just too darned big to fit into it. Where Rigel would reach past the planetMercry were it taking the place of our Sun, we would be inside Betelgeuse  and it would cover everything else almost out as far as Saturn!

I guess in that sense it deserves to be called Alpha. But Rigel, with its brilliant blue light, will always hold my imagination in part because i can imagine it,  And now it also stands as one of Jack’s first conquest and one the first of the Double Star Club 60 mm  observations. Betelgeuse, after all, is only a single, right? And sure, it might go super nova in 2012 and that has the internet rumor mill going crazy already – but that sort of ignores the fact that while that prediction has some scientific validity, it would also be scientifically accurate to say it might go super nova a million years from now 😉 Rigel, thankfully, is caught up in no such rumor mill yet.

Vital stats:

  • Brilliance: Magnitude 0.12, the 7th brightest star in our sky. Shines with the luminosity of about 90,000 Suns.
  • Distance: 860 light years
  • Spectral Type: B
  • Position: 05h:55m:10s, +7°:24′:25″

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