The focus for those learning the stars this month is the beautiful star cluster, the Pleiades – known in many cultures as the “Seven Sisters.” But in December 2010 there’s also a great lunar eclipse; the Geminids should put on a terrific meteor show; and the planets promise several “cool” appearances.
For details on the eclipse, Geminids, and planet show, be sure to see the December 2010 Events post. Here we’ll focus on the sky spectacular that happens every December when you look east starting about 45 minutes after sunset. Here’s what you should see.
Capella, which we met last month, dominates the northeast and now it’s easy to pick out the familiar kite figure which, lead by Capella, covers the heart of the constellation Auriga. About parallel with Capella, but south of it will be the Pleiades – but don’t expect to see them well until it gets darker. You may pick them up with binoculars an hour after sunset, but to really appreciate them, wait an hour and a half after sunset.
East of the Pleiades – below it as you look at the eastern sky – is the bright guidepost star, Aldebaran. It highlights a “V” asterism that marks the head of Taurus the Bull. Dominant as it is, imagine just for a moment what it would like if Aldebaran were our Sun. James Kaler points out that it would span 20 degrees in our sky – our Sun spans half a degree! So rising in the east, it would nearly fill the space between the Pleiades and the horizon. Get the following vision of Aldebaran in your head as you gaze to the east on a December evening.
Aldebaran is what is classified as a “giant,” and it is indeed huge when compared to our Sun, but there are many stars much larger. It’s the 14th brightest star in our sky – compare it to Capella and you will notice that Capella is brighter. Aldebaran is 67 light years away – reasonably close – and in the ecliptic – the path the Sun, Moon, and planets take in our sky. This means it frequently flirts with Mars and it’s fun to compare the color of these two reddish objects. It also gets occulted, from time to time, by our Moon – meaning the Moon passes in front of it. Its surface temperature is a bit lower than our Sun’s, thus the orange tint. It radiates quite a lot of its energy in infrared and is about 425 times as luminous as our Sun.
Vital stats for Aldebaran (al-DEB-ah-ran)
• Brilliance: Magnitude .85; its luminosity is the equal of 425 Suns.
• Distance: 67 light years
• Spectral Types: K5
• Position: 04:36, +16:32
Aldebaran appears to be the brightest star in another star cluster, the Hyades. In reality, it is not part of that cluster, for it’s much closer to us. Its name – Aldebaran – means “follower” – for it appears to follow the Pleiades up the sky. (Actually, skywatchers sometimes use the terms “precedes” and “follows” to indicate sky direction. A star that “follows” is to the east of the object it is following – and one that precedes, is to the west.)
In classical depictions of the constellations, Aldebaran is the “bull’s eye,” and the “V” of stars near it is the bull’s head. But that V is, as mentioned , another open star cluster, the Hyades.
Hyades and Pleiades
Now what’s fun here is to pause a moment and go back and forth between the Hyades and the Pleiades. Both are open star clusters, and in reality they cover roughly the same area of space – about a dozen light years – but, you will notice immediately that the Hyades appear much larger. There’s a simple reason for that – the Hyades are just 151 light years away, while the Pleiades are more like 400 light years from us.
A careful observer will also notice that the Hyades tend to be yellowish stars, while the Pleiades are icy, blue diamonds. That’s because the Hyades at 660 million years are about ten times as old as the Pleiades. Of course, in astronomical terms both contain young stars, our Sun being about 5 billion years old and our galaxy something like 12 billion years. But the older Hyades does contain more yellow stars.
One more thing you might notice about the Pleiades – they look like a tiny dipper – in fact, I’ve had more than one visitor ask me if this is the “Little Dipper.” I guess you could call it The Littlest Dipper! You also could call it “Subaru.” That’s the Japanese name for this little purse of celestial gemstones, and the car maker does include them in its logo. And here are a couple of Pleiades challenges for you:
1. How many Pleiads can you see with the naked eye?
2. And can you see – with naked eye, binoculars, or telescope – the faint nebulosity that surrounds these stars?
It was that nebulosity that apparently inspired Alfred Lord Tennyson as he penned this famous tribute in “Lockesley Hall”:
Many a night I saw the Pleiades,
Rising thro’ the mellow shade,
Glitter like a swarm of fireflies
Tangled in a silver braid.
Beautiful, but no words or image can do justice to the live, real-time experience of standing outside on a crisp December evening, raising binoculars to your eyes, and seeing these icy diamonds! (Oh they can be seen with the naked eye, but binoculars give a much better view.)
Even without binoculars, the Pleiades can be quite dazzling for those with good eyes and dark skies. Not me. With my aging eyes they tend to blend together, and even when I put my glasses on I can only with care see four or five separate stars. Younger eyes do much better.
So how many stars do you see? Take your time. Patience is the key. I suggest you get a comfortable beach chair, lean back, relax, and look for at least a solid minute at a time. How many should you see? I suspect most people who take the time to observe carefully get as many as six to 10. Walter Scott Houston, who wrote a Sky and Telescope magazine column when astronomy was new to me in the 1950s, counted 18 with the naked eye! And the visual observer I most admire today, Stephen James O’Meara, says in his book “The Messier Objects:”
Although largely symbolic, the age-old association of the Pleiades with the number seven remains fixed to this day – to the point that some observers swear they cannot see more than seven members, even though the Pleiades contains 10 stars brighter than 6th magnitude. Some observers question how it is possible to see 10 Pleiads in The Seven Sisters (a demonstration of the power of words . . . ) The fact is that almost three times that magic number of stars can be seen without magnification by an astute observer under dark skies.
O’Meara says he logged 17 while observing in Cambridge, MA – which hardly has dark skies. “The trick,” he says, “is to spend a lot of time looking and plotting.” This business of “time on target” is something I find hard to convey to new observers. But it is the key. Another key is simply experience. I frequently see things that those with younger eyes don’t see, simply because I’ve seen them before and know exactly what to expect. Crossen and Tirion in their book “Binocular Astronomy” have this general piece of advice, which certainly applies here:
When I first began observing with binoculars I could not see the Rosette Nebula at all, but now it is not difficult for me even under poor sky conditions.
The most important thing in observing is to really look – a mere glance at an object or a field is simply not enough. You must keep your eye at the oculars for at least a full minute at a time.
That said, don’t let the numbers and reports by others discourage you – the Pleiades are yours to enjoy no matter how many you count. Another noted popular astronomy author, Terrence Dickinson, writes in his book “Nightwatch,” that he has “a tough time seeing more than six stars with the unaided eye, even under excellent conditions,” but he also notes that some of his “astronomy students have reported seeing as many as 11.”
And turn binoculars on them and you should be able to easily count between 25 and 50.
The second challenge is more subtle. It involves the nebulosity that shows up in nearly every photograph of this cluster. No, don’t go looking for such a photograph. It will only prejudice you as to both the nebulosity and the fainter stars – and besides, you’ll never match a long exposure photograph with your eyes because film, or the modern CCD accumulate much more light than our eyes.
The Pleiades, as I mentioned, are “young” stars – about 100 million years old, and in astronomical terms that means they’re mere babes. (Our star – the Sun – is about 5 billion years old. ) The Pleiades are not far removed from the cosmic womb of gas and dust in which they were formed. Until fairly recently it was assumed that this nebulosity we see was the last wispy remains of the nebulae in which the Pleiades were formed. Today it is more generally thought that this nebulosity is just a happy accident – an entirely different gossamer cloud of gas and dust that is reflecting the brilliant light of the Pleiades as they pass through it.
In any event, Tennyson seems to reference it when he refers to his “swarm of fireflies” being in a “tangled braid.“ When I look with the naked eye I certainly don’t see it. But be careful. A couple of these stars are quite bright, and because they’re close together, their light tends to blend and perhaps give the impression of being surrounded by nebulosity. Perhaps that’s all Tennyson saw, especially as the stars were near the horizon – or at least that’s where he puts them in his poem.
So while I assume Tennyson was talking about a naked eye view and perhaps glimpsed the nebulosity in pristine Victorian skies free of modern light pollution, I feel this second challenge is best pursued with binoculars and small telescopes. While there is nebulosity near several stars, the brightest part is southeast of Merope. (Merope is identified in the downloadable charts at the end of this section.) So I would look for this first. What you need to do is look for a difference in the darkness of the background sky in this region. Using binoculars move away from the cluster a tad to avoid the glare – see how dark the sky is? Now move closer to it – do you detect any change in the background brightness? Again, be careful you don’t confuse the glow around a bright star with nebulosity.
When you think you have spotted the nebulosity, it would be helpful to quickly sketch its location on the provided chart – then compare it with a picture of the Pleiades, such as this one, to see if your impression of the location and size of the nebulosity matches what the camera reveals.
When to look
To take the challenge you want the Pleiades high in a dark – moonless – sky. In December of 2010 both the first and last weeks give good, Moonless skies at a reasonable hour. At the start of the month the Moon is in the early morning sky. It’s new on December 5 and won’t start to be a problem until around the 10th. By Christmas the Moon isn’t rising until five hours after sunset, and it rises later each night through the end of the month. The Pleiades will be well up about two and a half hours after sunset.
This is a good lesson, however, for looking at any faint astronomical object. When we do that we are constantly balancing these different factors of how high the object is above the horizon – the higher the better because the higher it is the less atmosphere you need to look through to see it – and where the Moon is, because it is constantly changing position and brightness, and it tends to wash out the sky anywhere near it. But as you can see, there’s at least a two-week window when you can take the Pleiades’ challenge – assuming the weather cooperates! And, of course, the Pleiades will still be with us through the winter.
Some helpful charts
There are three printer-friendly charts listed here, but for starters I suggest you download only the first two. They both show the brightest Pleiads but the second one has no names on it and is meant for you to use – and add to – when taking either challenge. Put it on a clipboard and take it, a pencil, and a soft red light to your observing location. Then when you spot something you can mark its location in relation to the brightest stars. Once you’ve done this, take a look at the third chart which shows the Pleiades as seen through a typical pair of binoculars. This chart will tell you whether fainter stars you identified and noted on your chart are in the sky or just in your imagination 😉
Chart 1 – Download this chart as a starting point for your observations – and to get to know the names of the Pleiads. (Atlas and Pleione are the parents of the seven sisters.)
Finally, compare your observation of the nebulosity with a picture of the Pleiades, such as this one.