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Counting the stars in the Pleiades

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#26 BrettG


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Posted 30 September 2013 - 09:52 AM

I need to really give the Pleiades a count once again. I have not really done this since my youth when I could count anywhere from 7 to 9 depending on the seeing. That always caused me to wonder why it was called the seven sisters but everything in the 60's said that seeing 7 was the norm for the average person. That's when I could see the milky way from in town, today on the best of nights I can see to mag. 4.5, but normaly to 4.2 is more typical.

I really have no idea what the mag from my backyard is, but on a moonless night, I can see the milky way, very obviously. It is more difficult to do the "count the stars in the little dipper" trick too, as the large tree in my yard blocks the north! :)

#27 Tony Flanders

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Posted 30 September 2013 - 10:53 AM

Granted that no documents show that anybody has described seeing moons of Jupiter before Galileo, it's not entirely beyond the realm of possibility that someone with keen eyes saw them but didn't report it ...

Sure, that's quite likely. However, it's impossible to confirm and not particularly relevant.

Likewise, surely many people saw the Andromeda Galaxy before it was recorded by al Sufi in the 10th century. But an unrecorded observation doesn't count for anything.

It's partly a matter of what you think is worth recording. The sky is full of fuzzy spots, some of them unresolvable star clusters, a few nebulae, and probably some tight, unresolved double stars. Very few of them were recorded before the telescopic age.

Likewise, Jupiter is fairly frequently close to Ganymede-brightness stars, so a single observation of Ganymede would be altogether unremarkable. Why bother recording it?

What really made Galileo's observations special, of course, was his ability to see the moons repeatedly and consistently, which allowed him to confirm that they do in fact orbit Jupiter.

#28 Starman1


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Posted 30 September 2013 - 03:20 PM

5-7 is normal (and think of the name).
I used a technique I used when looking for stars at the limit in my scope:
I take a chart out to the field with me with stars down to a magnitude much lower than I can possibly go (say 9.0). Then, I circle each star I can see, using averted vision (and, in the case of the Pleiades, my glasses to correct my open-pupil vision).
I am 62 and have only average vision, yet I can typically make out 12 of the stars.
Part of that is knowing where they are and using averted vision until the star winks in and out (as you would do with any limit observation).
Here is a chart to try your luck:
Without my glasses? Well, let's just say fewer :grin:

By the way, if you draw an outline to the glow around the Pleaides and compare it to the outline of the nebulosity on a photograph, you'll see that the naked eye picks up the nebulosity around the stars. For comparison, look at the Hyades nearby (no nebulosity or glow at all) or use averted vision on the Orion Nebula. The naked eye can see a lot more than people think it can. The Pleiades nebula is brighter than the Gegenschein.

If you have dark-enough skies, mount an H-Beta filter on an extension tube and look at Orion. You can see Barnard's loop extending around almost to Rigel with the [filtered] naked eye.

#29 Astrodj


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Posted 30 September 2013 - 03:37 PM

I took a good long look at the Pleiades last night from my backyard (I was able to see 8, using averted vision on 16 Tau and 28 Tau).

You make a good point, eight may be as likely as seven, although I found 16 Tau to be easier to pick out than 28 Tau so I can see where some may say 6 or 7 visible as opposed to 6 or 8.

It is the first time since I broke down and acquired glasses that correct for distance vision that I have viewed the Pleiades (I had been noticing a decline in my distant visual acuity for a few years but stubbornly stuck with just readers for near vision correction). It is also the first time I have seen 8 from my backyard in many years. Go figure..

I do however think light pollution is an important factor for seeing 12 or 13 stars (or more), if not 6 or 8. There are at least 16 stars brighter than magnitude 7 that are close enough to the asterism to be considered as a part of the cluster, at least in my opinion, when attempting a visual count. 21 and 22 Tau, as you stated, are so close together that if they are seen I think it is most likely they are seen as one star (my past experience), although I am sure some will claim to have seen both individually, who knows?

24 Tau is practically on top of 25 which is 3 magnitudes brighter so it is probably not claimed by many, I have never seen both individually.

Likewise, 26 Tau is at nearly magnitude 6.5 and close enough to 27 Tau to be very difficult for most. It was never one of my 12-13.

HD 23568, near 21/22 Tau and as bright as mag 6.7 in some sources could possibly be seen by some, even though it is relatively near the brighter stars of the the asterism. I have not seen it naked eye.

Getting back to the light pollution issue...

HD 23923, the toughest of the "12" I could see, at mag 6 and HD 23753 at mag 5.5 are far enough from 27 Tau to pick out and are two of the 12 I used to see from mag 6.5 or better skies. The light pollution in my backyard would prevent that observation, even in my youth, as 5.5 (without something much brighter very nearby) is the best I ever do at zenith and that is rare.

18 Tau at mag 5.6 is another of the "12" I could see from mag 6.5 skies that the light pollution of my backyard would kill.

HD 23712 is a variable that is brightest at mag 6.5 or so and lies above the dipper asterism kind of off by itself but still close enough that I would include it in a count attempt. From a very dark or pristine site, this is the one I felt like I could just barely detect as number 13, a few times. It is not near a much brighter star as is similar magnitude 26 Tau which is what made it possible for me I think.

As you say, if you want to stray further and further out there are more and brighter stars one could add. I always felt the ones I mentioned were near enough to use for a count.

So here is a list of my 12-13:

28, 27, HD23923, HD23753, 25, 23, 17, 16, 19, 20, 21/22 as one, 18, and very occasionaly under only the best conditions, HD23712.

Of these 13 stars, for those with the visual acuity to see them near the brighter cluster members at all, the detectability of the HD stars listed along with 18 Tau (and for that matter 26 Tau, or possibly HD 23568, which I guess could be #14/15 for some very keen eyed souls) can all be impacted enough by light pollution so as to render them invisible IMO.

#30 blb



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Posted 01 October 2013 - 09:40 AM

Well, I counted them early this morning, and at 3 o-clock I could only count 5 stars but when I went back out at 6 o-clock and tryed again, when the Pleiades were just past the meridian, I counted 7 stars. At 3 o-clock the cluster was only about 45-50 degrees in altitude and at 6 o-clock the cluster was within about 10 degrees of the zenith, where light pollution has it's least effect. So even with my old eye's I can still see seven stars in town, from my white zone, when the cluster is positioned best for observation.

#31 schang


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Posted 01 October 2013 - 02:08 PM

I counted 6 stars there at about 5:30AM today without scope while a quarter moon hanging between the Mars and Jupiter. I could not tell Tau 28 apart from Tau 27 which is brighter nearby. Tau 16 are the two that evaded my eye due to their close vicinity to other brighter stars. Tau 21 and 16 also evaded my eyes.

#32 Deb and Todd

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Posted 02 October 2013 - 01:11 AM

After counting with naked eye, try it with binoculars, and then with a scope.

I've tried to count with my 8", but have given up every time. Once I've crossed a couple of hundred I start losing track.

#33 AE1AW


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Posted 02 October 2013 - 03:52 AM

Twelve to fourteen Pleiades is NOT uncommon with younger (or very well preserved) eyes with adapted vision at a moderately dark site... YMWV :ubetcha:

Do you have statistics to back that up?

All four Galilean moons are well above the limiting magnitude for a decent suburban sky

Of course they are -- as I said. But that doesn't mean that people can actually see them. The obstacle is Jupiter, not light pollution.

As I consistently proved in college through my dorm room window (sans screen). ... Two are easy, the third is more difficult, but all four together are a visual challenge most youth and many adults can achieve ...

Are you claiming to have seen all four Galilean moons simultaneously without optical aid? I have never heard anybody claim to do that before. It is certainly far beyond the ability of any normal person.

It is, of course, a matter of record that nobody described seeing any moons of Jupiter before the invention of the telescope.

I have seen 3 moons on a few occasions by squinting really hard, or poking a hole in a piece of paper/making a tiny hole in my hand(I guess both of the latter would be considered optical aid though).
As far as the Plieades, with LP around my house I got to 4-1/2 and gave up quickly last I tried. I may be taking a walk to the beach tonight after work to give it an honest go while looking out over the ocean...

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