Weltevreden SA
11-09-2013, 06:44 AM
Hi fellas, I've been observing & reading about the low-density, rather faint globular NGC 7492 Aquarius. It's a beautiful object and not as difficult as its reputation has it, provided you know ahead of time that it is a very loose Class XII cluster with a great deal of visual darkness between its Mag <15.5 core stars. IOW, don't expect a white haze, look instead for an overdensity of faint stars in a round shape. Attached is a hi-rez detail shot from C. Seligman's website (http://cseligman.com/text/atlas/ngc74a.htm#7492). In our eyepieces 7492 will never look like this. (Still we can dream, right?) I've observed it half a dozen times in as many nights in my 180mm and 200mm scopes under mag 6.5 and darker skies, spending at least half an hour on it each time. The visual impression is always the same: 8 to 12 tiny specks twinkling briefly in & out with no white haze in the middle. At 100x and up it presents as very faint shimmer of similar-brightness stars at the limit of vision—and is all the more pleasing because of its now-you-see-it, now-you-don't effect. The sparseness of the surrounding field is a great help—the cluster would be impossible if closer to the MW band. The only other cluster that gives the same impression is NGC 6256 in Sco. (Though if you breathe on the eyepiece to fog it up a little like I did unwittingly last night, M55 gives a great imitation.)
For a long time I've frequently noticed something in photos like the hi-rez Seligman image that I hardly ever see visually: star chains. Across the bottom centre and up the right side of the image are numerous demi-loops of stars of approx similar magnitude and clearly delineated direction. I rarely see any visual sign of chains in objects which show chains very clearly in images. In my finder at 50x I often see what appear to be chains in star clusters, but raising the magnification above 150 dissolves them into random patterns. Blue bandwidth images seem to show the effect more often than yellow-red, and IR images seem to have quite few. That said, there are a few regions where I see multiple chains visually, the most distinct being the outer NE reaches of the M24 Sagg star cloud and the OC NGC 5480 in Centaurus, these are the exceptions.
With the 7492 image here, simple physics denies the reality of organized bound chains of the length and numbers involved in the images. NGC 7492 is >100 lyr across and 82,000 lyr away, which makes the stellar densities as seen in the image at least 3 or 4 stars per 10 cubic lyr—a figure supported by the mass-luminosity relation studied in a recent professional paper (http://iopscience.iop.org/1538-3881/128/6/2838/fulltext/). Moreover, 7492 has a high ellipticity of 77% of spherical and is well mass-segregated with the heavy stars in the middle and the lightweights in the suburbs—which is where most of these 'chains' occur. There's simply no way gravitational or magnetic field effects could support a linear bond in a cluster whose stars orbit in such eccentric wobbles as 7492's. Also roughly 40% to 50% of the stars you see in the inner region are binaries in random orbits, which disrupts binding energy beyond 1.4 times the distance of a pair's own orbit—that's why smaller stars that blunder too close to a globular binary are ejected from the cluster altogether.
Are these 'chains' an effect of astro-imaging that produces illusion in my eyes accustomed to eyepiece views? Any suggestions?
For a long time I've frequently noticed something in photos like the hi-rez Seligman image that I hardly ever see visually: star chains. Across the bottom centre and up the right side of the image are numerous demi-loops of stars of approx similar magnitude and clearly delineated direction. I rarely see any visual sign of chains in objects which show chains very clearly in images. In my finder at 50x I often see what appear to be chains in star clusters, but raising the magnification above 150 dissolves them into random patterns. Blue bandwidth images seem to show the effect more often than yellow-red, and IR images seem to have quite few. That said, there are a few regions where I see multiple chains visually, the most distinct being the outer NE reaches of the M24 Sagg star cloud and the OC NGC 5480 in Centaurus, these are the exceptions.
With the 7492 image here, simple physics denies the reality of organized bound chains of the length and numbers involved in the images. NGC 7492 is >100 lyr across and 82,000 lyr away, which makes the stellar densities as seen in the image at least 3 or 4 stars per 10 cubic lyr—a figure supported by the mass-luminosity relation studied in a recent professional paper (http://iopscience.iop.org/1538-3881/128/6/2838/fulltext/). Moreover, 7492 has a high ellipticity of 77% of spherical and is well mass-segregated with the heavy stars in the middle and the lightweights in the suburbs—which is where most of these 'chains' occur. There's simply no way gravitational or magnetic field effects could support a linear bond in a cluster whose stars orbit in such eccentric wobbles as 7492's. Also roughly 40% to 50% of the stars you see in the inner region are binaries in random orbits, which disrupts binding energy beyond 1.4 times the distance of a pair's own orbit—that's why smaller stars that blunder too close to a globular binary are ejected from the cluster altogether.
Are these 'chains' an effect of astro-imaging that produces illusion in my eyes accustomed to eyepiece views? Any suggestions?