Look East In January 2011 – a trio of twins; Orion, man of the world; Betelgeuse, giant among giants! |

Look East In January 2011 – a trio of twins; Orion, man of the world; Betelgeuse, giant among giants!

Posted on December 24, 2010 by Greg Stone

January brings a host of bright and wondrous winter stars. There are four new guidepost stars to meet this month and one new guidepost asterism, Orion. Orion is probably the best known figure in the heavens because it actually looks like a person and can be seen from most locations in the world since it’s centered on the celestial equator. That’s a lot for one month, but fun to think about on a dreary winter day and more fun to observe on a brilliant, winter evening.

Here’s the chart for the eastern sky one hour after sunset for mid-northern latitudes. Remember, going out about 45 minutes after sunset and looking east, you’ll see only the brightest stars as they come out. This makes it easier to identify and learn our guidepost stars. Our guidepost asterisms may not be as readily seen until a little later as the sky gets darker and more of the fainter stars come out.

: The eastern sky as seen on a January evening about one hour after sunset. Click image for larger version. Use link below to download a printer-friendly, black and white version of this chart. (Chart is based on a screen shot, modified by me, of Starry Nights Pro software.)

Click here to download a black-on-white (printer-friendly) version of this chart.

The January eastern sky – what to remember

Castor – A trio of twins
Pollux – The bigger, brighter twin
Orion – A man for all to see
Betelgeuse – a giant among giants
Rigel – Blue and brilliant

Castor – A trio of twins

When you see Castor, think “twins” – a trio of twins. Well, in a sense there are really four pairs!

: Click image for larger view.

But the fourth pair is just mythological – Castor is one of the “heavenly twins” of the constellation Gemini – the other twin being Pollux. This is nothing but a fanciful relationship, though, based on how the stars appear to us. But there is more, much more, to Castor. And, it’s what we don’t see that makes this bright star so fascinating.

Were you to look at Castor in a backyard telescope, you would see it has a twin – another bright star that appears quite close – so the two are Castor A and B. These two are related, orbiting one another about every 400 years. But there’s more. Each of these two are twins! However, you can’t see this in a telescope because in both cases the pairs of stars are extremely close to one another, orbiting one another in periods of less than 10 days. And as noted, each pair orbits the other pair in about 400 years. But there’s more.

Returning to that backyard telescope you may notice a third star, Castor C, quite a distance from the first two and significantly dimmer. This star is also part of the Castor family and it too has a twin that also is so close we can’t detect it without special instruments. In fact, Castor C consists of the closest pair of all, orbiting one another in less than a day! This pair, in turn, orbits the other four stars in the system once every 10,000 years or so.

So when you look at Castor, remember that in classic mythology it has a twin, Pollux – and remember that what looks to you like a single bright star is really the combined light from six stars, all held together in one of the most complex star systems we know. (I wrote much more about the Castor system on the double-star blog. That post includes a scale model that puts Castor and company into perspective with the Earth and Sun. You’ll find it here. )

Vital stats (for just the brightest star in the Castor system):

Brilliance: Magnitude 1.58, the 23rd brightest star in our sky and the brightest second magnitude star. Absolute magnitude is 0.9.
Distance: 50 light years (not among the 200 nearest stars)
Spectral Type: A
Position: 07h:34m:36s, +31°:53′:18″
Compared to the Sun: Castor radiates 14 times as much energy as our Sun.

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Getting to know Pollux – the bigger, brighter twin

Pollux should feel a little cheated because it’s the brightest star in the constellation of Gemini and usually the brightest star was given the designation “alpha.” Not Pollux. It is designated “Beta Geminorum” and follows its slightly dimmer twin brother around the sky. But Pollux has its own way of standing out: It has a slight edge in brilliance in our sky; it is a tad closer to us; and it is an orange giant. What’s more, in 2008 it was confirmed to have a planet orbiting it.

As an orange giant, it has moved off the “main sequence,” and instead of fusing hydrogen into helium, as our Sun does, it is fusing helium into carbon and oxygen. It will eventually blow off a lot of its substance becoming a planetary nebula. It is currently about eight times the diameter of our Sun – that’s huge, but nowhere near as large as our next star, Betelgeuse. The planet circling Pollux is also large – “Jupiter class” – and was first detected in 1993, but not confirmed until 2008.

Vital stats:

Brilliance: Magnitude 1.14, the 17th brightest star in our sky. Absolute magnitude is 0.7 .
Distance: 34 light years (not among the 200 nearest stars)
Spectral Type: K
Position: 07h:45m:19s, +28°:01′:35″

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Orion – A man for all to see

If you’re in the same general latitude as I am in Westport, MA, then you see Orion like this as it rises in the east on a January evening.

: Orion – as seen when rising in mid-northern latitudes. (Click for larger image.)

What always sticks with me about Orion is how Robert Frost described him in his wonderful poem, “The Star Splitter.”

‘You know Orion always comes up sideways.

Throwing a leg up over our fence of mountains,

And rising on his hands, he looks in on me . . .

But if I lived in Sydney, Australia, I wouldn’t see it this way. What I would see is a man standing on his head!

: Orion, as seen when rising in the east from Sydney, Australia. (Click image for larger version.)

The real point here is that these stars do look like a man, and they can be seen from deep into both the southern and northern hemispheres. What’s more, the three distinctive stars that form Orion’s belt also mark the approximate position of the celestial equator in your sky, a handy thing to know. Of course, if you’re in the southern hemisphere, the celestial equator appears to make an arc in your sky to your north. In the northern hemisphere it appears to make an arc in the sky to the south.

But in either case the belt stars of Orion will rise just about due east and set due west. How high they get in your sky is calculated simply by subtracting your latitude from 90. That is, if your latitude is 42 degrees, as mine is, then Orion’s belt will be, at its highest, about 48 degrees above the horizon when it passes due south. From Sydney, Australia, the stars in the belt will cross about 56 degrees above the horizon as they pass due north. And yes, if you live on the equator these stars will cross directly over head. Anyway you look at it, Orion is a man for all latitudes – well, almost. At the north pole you would only see his top half, and at the south pole, only his feet! Return to Menu

Betelgeuse – giant among giants

When you look at the eastern sky early on a January evening, get this picture in your head!

Here's what our eastern sky would look like on a January evening if Arcturus and Rigel, two genuine giants, were as near to us as our Sun. The Sun, to scale, is also shown. What isn't shown to scale is Betelgeuse. That's because we couldn't see it as a star if it were the same distance from us as the Sun - for we would be buried deep inside it, and Betelgeuse would be everywhere.

If the Sun looks smaller than you think it should in the above image. Classified as a red supergiant, Betelgeuse is one of the largest stars you can see – and certainly up there with the biggest of all stars – yet it won’t look any bigger in our sky than other stars because all stars, except the Sun, are so far away they appear only as a point source of light to our eyes. Last month we showed what Aldebaran would look like if it were in our sky and the same distance from us as the Sun, and this month we’ve added Rigel to the picture. We can’t do a similar thing with Betelgeuse – it wouldn’t be in our sky – we would be in it!

What’s more, it’s hard to put a number to the size of Betelgeuse, not because it can’t be measured, but because it’s hard to decide exactly what you want to measure when you’re dealing with a ball of gas – especially one like Betelgeuse. Our Sun is a little easier case. While it does not have a surface, it does appear to us to have an edge that’s fairly easy to define – it’s the place where its gases are dense enough to be opaque to our vision.

Exactly how we define the size of Betelgeuse is a bit more difficult. I rely on James B. Kaler as my stellar authority. I love his books, and in one, “The Hundred Greatest Stars,” he describes the size of Betelgeuse variably as:

650 times that of the Sun, or 2.8 AU (Astronomical Units – an Astronomical Unit is the distance between the Earth and the Sun – roughly 93 million miles)
800 times the diameter of the Sun, or about 4 AU
1600 times the Sun – about 8 AU when measured by modern observation in ultraviolet light

And on his Web site, after opting for a figure of around 8-9 AU, he writes:

However, the star is surrounded by a huge complex pattern of nested dust and gas shells, the result of aeons of mass loss, that extends nearly 20,000 AU away (Betelgeuse so far having lost over a solar mass). That, an extended atmosphere, and the pulsations make it difficult to locate an actual “surface” to tell just how large the star actually is. Moreover, because of changes in gaseous transparency, the “size” of the star depends on the color of observation.

Betelgeuse has other problems. The pulsations he refers to are a sort of puffing up that occurs from time to time and changes both size and brightness significantly. Betelgeuse is usually thought of as about magnitude 0.55, but it can be as bright as 0.3, or as dim as 1.1. All this huffing and puffing will soon lead to an explosion, and Kaler says it will then be as bright as a crescent moon! But don’t hold your breath. “Soon” in astronomical terms means sometime in the next million years or so! Its distance, too, is uncertain, but 500 light years is a good ballpark figure.

Let’s focus on that 8 AU size for a moment. When we build a scale model of our solar system and reduce the Sun to something about the size of a volleyball, the tiny speck of the Earth orbits at around 75 feet away. But at 8 AU Betelgeuse would be more like 600 feet in diameter. So pause for a moment as you look at Betelgeuse on a winter evening. Imagine yourself holding an 8-inch volleyball in one hand – our Sun – while you stand next to a red, raging, unstable monster ball that is 600 feet in diameter!