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Mr Q
2014-Mar-10, 04:18 PM
I see so many statements at various astronomical web sites stating that the limits of naked eye star magnitude ranges from 6th to 8th.

Of course this limit depends on many conditions so how about the "average" conditions - say, at highest elevations above sea level of 8,000 feet down to sea level, in a very dark sky site with well dark adapted eyes of people from 20 to 50 years age? This of course assumes excellent sky transparency (no LP, no high altitude dust/ice crystals, etc.). So under the previous conditions, what is the average magnitude limits of white colored stars at the zenith? Is the "6th magnitude" limit a fairly accurate one?

Hornblower
2014-Mar-10, 04:40 PM
I made it to about 6.2 or 6.3 at age 35 at the zenith under slight light pollution but otherwise excellent conditions near sea level. It is my understanding that 7th magnitude is not uncommon in pristine sky conditions, and that a particularly gifted observer reported seeing 8th magnitude stars under pristine conditions in Greece sometime in the 19th century. Unfortunately I do not have a ready reference for that.

Mr Q
2014-Mar-10, 06:49 PM
While living in a high desert area (6700 ft elevation) with no light pollution (nearest streetlight was 1/2 mile away with no other light sources) in a moonless July sky, I estimated my VLM at 6.4 at the zenith. Under these conditions, the star clouds in Cygnus and Scuttum casted vague diffused shadows on the tan colored sandy ground from the bottom half of my legs. The great Omega GC was easy with the naked eye at about 5 degrees above the southern horizon. This observing location had ground horizons at from 20 to 50 miles and the MW was easy with the naked eye from horizon to horizon as well as the N. America Nebula, which was faint but easily seen.

So under these conditions, I find that a realistic VLM of 6.5 at the zenith is proper. Where are people seeing down to 7-8th magnitude???

I find this article to be very realistic and accurate:

http://www.astrometry.org/magnitude.php

glappkaeft
2014-Mar-10, 07:55 PM
A friend of mine who has excellent vision and is an very experienced observer achieved a NELM of 8.2 (IIRC) from the Canaries. I have achieved mag 7.1 from decent but not perfect skies at low altitude in Sweden.

Hornblower
2014-Mar-10, 08:35 PM
While living in a high desert area (6700 ft elevation) with no light pollution (nearest streetlight was 1/2 mile away with no other light sources) in a moonless July sky, I estimated my VLM at 6.4 at the zenith. Under these conditions, the star clouds in Cygnus and Scuttum casted vague diffused shadows on the tan colored sandy ground from the bottom half of my legs. The great Omega GC was easy with the naked eye at about 5 degrees above the southern horizon. This observing location had ground horizons at from 20 to 50 miles and the MW was easy with the naked eye from horizon to horizon as well as the N. America Nebula, which was faint but easily seen.

So under these conditions, I find that a realistic VLM of 6.5 at the zenith is proper. Where are people seeing down to 7-8th magnitude???

I find this article to be very realistic and accurate:

http://www.astrometry.org/magnitude.php
My bold. They must be observing at similarly splendid locations with eyes that are more sensitive than yours. There is considerable person-to-person variation, and we tend to lose some low light sensitivity as we get older. I don't think I can reach 6th magnitude now at age 66.

George
2014-Mar-10, 08:40 PM
... what is the average magnitude limits of white colored stars at the zenith? Is the "6th magnitude" limit a fairly accurate one? The color shouldn't be much of a problem, especially at higher altitudes. Reddish color stars have only a slight advantage over the blue stars since less red light is scattered by our atmosphere. This assumes a zenith view. Stars seen nearer to the horizon will be far more affected, as is obvious with the Sun.

grapes
2014-Mar-10, 09:43 PM
Magnitude six is the historical limit, stars within the 5.5 to 6.5 range. That includes about 9000 stars, so maybe 6.2 or 6.3 would be the 8000 stars often mentioned.

Solfe
2014-Mar-10, 11:04 PM
Lately, in Buffalo, NY, it is hard to find the sun in day time. :)

Less jokingly, for some reason the skies clear up at night and offer a spectacular view of Orion. We are pretty light polluted, if six was the best possible for my eyes, I'd shave off 1.5 for my location. If it clears tonight, I may take a star chart out and see if I can see a mag 6.

Mr Q
2014-Mar-11, 03:36 PM
From the replies so far, it would seem that most observers experience limits from 5.5 to 6.2 mag. near the zenith on their best nights and any beginners reading these posts should also expect some degree of margin to their VLMs since there are many factors involved in any one's eye health, location, etc.

Myself, I usually use the stars of Ursa Minor (Little Dipper) as a rough assessment of sky conditions when it's at least to either side of Polaris or better, above it. My location, being at 42 degrees N, it's high enough most of the time to get a rough idea of seeing transparency. For those harder objects, I consult star charts with mag. listings down to at least 6th mag. in the area of the sky I'm targeting, since I am an avid user of star hopping to locate those tough DSOs.

Then there is the problem of determining the magnitudes of telescopic double stars, especially when contrasting colors are concerned. And variable star observing? I'm guessing there must be a knack to this type of observing that takes years to perfect. Of coarse these subjects warrant separate thread discussions.

George
2014-Mar-11, 04:22 PM
I thought it might be fun to see what the visual star count would be if:
1) all the stars were equal distant from each other using alpha Centauri as the spacing distance.
2) all the stars are equal to the Sun in luminosity.

Assuming my math is not in error,

Mag. 5........ 2,194 stars
Mag. 6........ 8,739 stars
Mag. 7...... 34,863 stars
Mag. 8..... 138,844 stars

[Star count is total no. of stars down to the apparent mag. stated. Mag. 8 takes us to a radius of 137.7 lyrs.]

[Corrected count for 4.367 lyrs to alpha Centauri. I had 4.67 lyrs.]

Hornblower
2014-Mar-11, 05:12 PM
I tested my limit by lying flat on my back and looking at Hercules, using the dew cap from my Celestron 8 to screen off stray light. I got at least fleeting glimpses of every star on the Norton's chart, which has a limiting magnitude of 6.35. Later I tried counting stars in the Great Square of Pegasus and checked my count against a table in Sky and Telescope. The result was similar to that from Hercules. The sky at that location, about 20 miles northwest of Dulles Airport, was still pretty good then but has deteriorated because of the continuing proliferation of sloppy lighting in new developments in the region.

grapes
2014-Mar-11, 05:17 PM
Close packed? or lattice packed?

I guess I could figure it out...

Hornblower
2014-Mar-11, 05:18 PM
I thought it might be fun to see what the visual star count would be if:
1) all the stars were equal distant from each other using alpha Centauri as the spacing distance.
2) all the stars are equal to the Sun in luminosity.

Assuming my math is not in error,

Mag. 5........ 2,194 stars
Mag. 6........ 8,739 stars
Mag. 7...... 34,863 stars
Mag. 8..... 138,844 stars

[Star count is total no. of stars down to the apparent mag. stated. Mag. 8 takes us to a radius of 137.7 lyrs.]

[Corrected count for 4.367 lyrs to alpha Centauri. I had 4.67 lyrs.]
Those numbers are about 3 times the number of stars in the Sky Catalogue 2000.0, which lists about 45,000 stars. The mean luminosity is close to that of the Sun, but the mean spacing of the stars is greater than the distance to Alpha Centauri.

Mr Q
2014-Mar-11, 07:09 PM
Close packed? or lattice packed?

I guess I could figure it out...

What? I'm lost. Did I miss something here :confused:

George
2014-Mar-11, 09:50 PM
Those numbers are about 3 times the number of stars in the Sky Catalogue 2000.0, which lists about 45,000 stars. The mean luminosity is close to that of the Sun, but the mean spacing of the stars is greater than the distance to Alpha Centauri. Yes, it was just a "what if" novelty for comparison. Interestingly, it was not too far from the real star counts at mag. 6. The extra spacing seems to offset the additional star count for the more common lower luminosity stars, which are off the map, of course, after a much shorter distance. That seems to make sense.

grapes
2014-Mar-11, 09:53 PM
What? I'm lost. Did I miss something here :confused:Oops, that question was for George, but Hornblower's post slipped in between there.

George
2014-Mar-11, 10:00 PM
Close packed? or lattice packed?

I guess I could figure it out... For each given magnitude, I calculated what the Sun's distance would be for this magnitude, which became the radius of the sphere volume calculation. Then divided this volume by the volume of the sphere of the 4.367 lyr. distance to alpha centauri to get the counts shown.

grapes
2014-Mar-11, 11:29 PM
For each given magnitude, I calculated what the Sun's distance would be for this magnitude, which became the radius of the sphere volume calculation. Then divided this volume by the volume of the sphere of the 4.367 lyr. distance to alpha centauri to get the counts shown.OK, completely packed then.

One way to look at it, is each star is at the center of a sphere of radius half the distance. None of the spheres intersect, because if two such spheres did, the stars at the centers would be closer than that arbitrary equal distance. Your calculation essentially assumes that we can pack such spheres so tightly that there is no left over space, which would be a density of 1, but that's impossible. The actual best is probably .74048 for close packing (each star is equal distant from 12 other stars). If you "stack" them, one on top another, and left/right, front/back (a cubic lattice pack), so that each one touches six other spheres (each star is exactly that distance from six other stars), the density drops to .52360. (That's how much of a cube that a sphere takes up.) You'd multiply those numbers times your totals.

In both cases, each star is exactly that distance from n other stars, and there is no "wiggle room".

Romanus
2014-Mar-12, 04:28 PM
6.5 is a good, round number; figures higher than that are known, but exceptional. O'Meara--an outstanding observer if ever there was one--says 8th magnitude-plus is at least theoretically possible.

George
2014-Mar-12, 04:30 PM
One way to look at it, is each star is at the center of a sphere of radius half the distance. None of the spheres intersect, because if two such spheres did, the stars at the centers would be closer than that arbitrary equal distance. Your calculation essentially assumes that we can pack such spheres so tightly that there is no left over space, which would be a density of 1, but that's impossible. The actual best is probably .74048 for close packing (each star is equal distant from 12 other stars). If you "stack" them, one on top another, and left/right, front/back (a cubic lattice pack), so that each one touches six other spheres (each star is exactly that distance from six other stars), the density drops to .52360. (That's how much of a cube that a sphere takes up.) You'd multiply those numbers times your totals. I made the assumption that the accuracy improves with larger volumes, which is why I started the star count at mag. 5. :)

A star-centered cubical approach would have meant that there would be 6 stars at the alpha Cen distance and 12 more stars only about 40% further away. I used spherical volumes so the star count for 0.5 mag. is 4.2, an improvement. Yet I am ignoring the fact the system is a binary, or trinary, helping the math.

It would be interesting to know the actual stellar densities for these distances to compare with the result. But to assume a density distribution based only on two stars (Sol and alpha Cen), of course, is a ludicrous extrapolation, but, hey, it is an amateur's privilege to do so. It seems to have worked for Titus & Bode, but with significantly improved efficacy, including George (Uranus) and Fred (Ceres Ferdinand). :)

Mr Q
2014-Mar-13, 02:21 PM
6.5 is a good, round number; figures higher than that are known, but exceptional. O'Meara--an outstanding observer if ever there was one--says 8th magnitude-plus is at least theoretically possible.

Sounds more reasonable to me, that is, for average ranges of seeing conditions. I wonder if this may be the reason for the naked eye "missing" star in Pliedes that was reported in centuries ago or was that star a very long term variable ?

Hornblower
2014-Mar-14, 02:49 AM
Sounds more reasonable to me, that is, for average ranges of seeing conditions. I wonder if this may be the reason for the naked eye "missing" star in Pliedes that was reported in centuries ago or was that star a very long term variable ?

Go to the Wiki article
http://en.wikipedia.org/wiki/Pleiades
and scroll down to Brightest Stars. There you will find a table of names with brightness, and to the lower right is a photo you can click on to identify the stars by name.

Note that there are at least 10 stars brighter than magnitude 6.0. The six brightest ones are 4.29 or brighter, and are relatively easy under conditions that obliterate the others. They form the familiar dipper with its short handle. The seventh brightest, Pleione, is so close to much brighter Atlas that it normally is hard to see, but it is an irregular variable that may flare up occasionally and become easier. The others between magnitudes 5 and 6 are well separated from the brighter ones and should be easier than Pleione. The named stars include parents Atlas and Pleione, the seven sisters and an additional one named Asterope.