Occasionally observers report a feeling of seeing DSO's, particularly globular clusters, in three dimensions. This feeling, however, can not have anything with reality to do. The image with any instrument at such distances will be flat. The 3-D feeling is probably a result of differences in brightness of the stars. When a globular like M13 is viewed through a big scope at a magnification of 200 - 300x, the cluster fills nearly the whole field of view, and the observer gets the feeling of being located not far from the edge of the cluster.
If you were located just outside a globular cluster where all the stars had the same absolute magnitude (hypothetical) the nearer stars would appear much brighter than the stars at the opposite side of the cluster. Assuming the centre of the cluster to be 100 light years away, the nearest stars 40 and the most distant ones 160 light years away, the nearest stars would be 2 magnitudes brighter than the stars near the centre and 3 magnitudes brighter than the most distant stars of the cluster.
If this hypothetical globular was observed with a telescope from a distance of 40.000 light years all members of the cluster would appear equally bright (far all practical purposes) regardless of aperture and magnification. I assume that such a globular would appear flat in the telescope.
Any real globular cluster contains a number (several hundreds or even thousands) giant stars in addition to myriads of fainter main sequence stars. The brightest giants will appear brightest through the telescope regardless of their position in the cluster, many of them being located at the far side. Observing the real cluster through the telescope at high magnification may give the illusion of a 3-D view with the brighter stars being the nearest. The truth is that the members of a globular cluster appearing brightest in the telescope are the ones with the highest true luminosity. (This facilitates the process of constructing the HR-diagram of a globular cluster.)
Something to think about...
Inge S
If you were located just outside a globular cluster where all the stars had the same absolute magnitude (hypothetical) the nearer stars would appear much brighter than the stars at the opposite side of the cluster. Assuming the centre of the cluster to be 100 light years away, the nearest stars 40 and the most distant ones 160 light years away, the nearest stars would be 2 magnitudes brighter than the stars near the centre and 3 magnitudes brighter than the most distant stars of the cluster.
If this hypothetical globular was observed with a telescope from a distance of 40.000 light years all members of the cluster would appear equally bright (far all practical purposes) regardless of aperture and magnification. I assume that such a globular would appear flat in the telescope.
Any real globular cluster contains a number (several hundreds or even thousands) giant stars in addition to myriads of fainter main sequence stars. The brightest giants will appear brightest through the telescope regardless of their position in the cluster, many of them being located at the far side. Observing the real cluster through the telescope at high magnification may give the illusion of a 3-D view with the brighter stars being the nearest. The truth is that the members of a globular cluster appearing brightest in the telescope are the ones with the highest true luminosity. (This facilitates the process of constructing the HR-diagram of a globular cluster.)
Something to think about...
Inge S
