by apodman » Sun Mar 01, 2009 2:56 pm
APOD Description wrote:Omega Centauri is ... apparent visual magnitude 3.9 ... visible ... with the unaided eye.
Okay, I live well north of the equator, so I have to take someone else's word for it. And I have no trouble comparing magnitudes of stars, planets, moons, and asteroids to guess what will be visible with various sky conditions or in a particular size telescope or binoculars. And the sun and our moon are no problem in any case.
But what of nearby galaxies, nebulae, comets, and clusters? These are distributed objects; that is, their brightness is not concentrated in a point. So here we have cluster Omega Centauri (NGC 5139) about 0.5 degree by 0.5 degree at apparent magnitude 3.9 that I'm told is visible to the naked eye. And I'm familiar with looking at M31 (for example) with the naked eye in various conditions - it is apparent magnitude 4.4 distributed over an area about 1 degree by 3 degrees, several times the size of NGC 5139 (though the size of its nucleus is more comparable in size to NGC 5139), and I would call its nucleus barely visible to the naked eye in average suburban skies.
So I wonder "how visible" NGC 5139 is. Does anyone know of a chart or rule-of-thumb that includes magnitude as well as effective area that I could use to compare apparent magnitudes of distributed objects and get an idea how bright they should look to me? Side-by-side pictures at the same scale and exposure would be great, but that might be asking for too much.
[quote="APOD Description"]Omega Centauri is ... apparent visual magnitude 3.9 ... visible ... with the unaided eye.[/quote]
Okay, I live well north of the equator, so I have to take someone else's word for it. And I have no trouble comparing magnitudes of stars, planets, moons, and asteroids to guess what will be visible with various sky conditions or in a particular size telescope or binoculars. And the sun and our moon are no problem in any case.
But what of nearby galaxies, nebulae, comets, and clusters? These are distributed objects; that is, their brightness is not concentrated in a point. So here we have cluster Omega Centauri (NGC 5139) about 0.5 degree by 0.5 degree at apparent magnitude 3.9 that I'm told is visible to the naked eye. And I'm familiar with looking at M31 (for example) with the naked eye in various conditions - it is apparent magnitude 4.4 distributed over an area about 1 degree by 3 degrees, several times the size of NGC 5139 (though the size of its nucleus is more comparable in size to NGC 5139), and I would call its nucleus barely visible to the naked eye in average suburban skies.
So I wonder "how visible" NGC 5139 is. Does anyone know of a chart or rule-of-thumb that includes magnitude as well as effective area that I could use to compare apparent magnitudes of distributed objects and get an idea how bright they should look to me? Side-by-side pictures at the same scale and exposure would be great, but that might be asking for too much.