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.