Photographic investigations of faint nebulae — John Shaqi
Photographic investigations of faint nebulaeHubble, Edwin
Science
Photographic investigations of faint nebulae
Hubble, Edwin
Nebulae
The positions are given for the epoch of the A.G. catalogues, 1875.0.
Two of the N.G.C. nebulae had been measured by Lorenz from photographs
made at Heidelberg. A comparison of his measures with the present
measures shows the same second of arc in declination and the same tenth
of a second of time in right ascension. The agreement is perfect to the
last units used in the present paper, but as the nebulae, N.G.C. 7619
and 7626, are situated near the center of the plate, they cannot serve
as a test of the accuracy of those near the edge. In several cases
comparisons could be made with positions from visual measures, as given
in the _Strassburg Annals_, Vols. III and IV. Here the agreement is not
so good.
In one case, on the same night, positions of four objects were measured
with reference to a certain star, which was, in turn, tied up to an
A.G. star some distance away. The objects are given as N.G.C. 3550,
3552, 3554, and a nova which is designated as K₁₂. The photograph shows
N.G.C. 3550 and 3554 properly placed with reference to the star, both
in distance and in position-angle; there is nothing at all in the place
given for N.G.C. 3552; K₁₂ is properly placed from the star, but is in
the N.G.C. position for No. 3552. The position of the reference star
is given as about 17″ too small in declination. K₁₂ is clearly N.G.C.
3552, and evidently the fourth object does not exist in the published
position. As these objects were all in the same field of view in the
telescope, one is at a loss to account for the discrepancy. A list of
the comparisons is given in Table II.
The descriptions indicate form, brightness, and size, and occasionally
the location of a neighboring star. Wolf’s classification was used.
It is, as he remarks, wholly empirical and probably without physical
significance, yet it offers the best available system of filing away
data, and will later be of great service when a significant order
is established. One class was interpolated between g and h, and was
designated g₀. Brightness was estimated in the order B, pB, pF, F, vF,
eF, eeF, and eeeF. The range is from Herschel’s Class II down to the
limit of the plates. For several of the fields diameters were estimated
to the nearest 5″. Otherwise the size was given in the order L, pL, pS,
cS, S, vS, eS.
The classification given in Table III is illustrated in Plate III,
copied from Wolf’s engraving in Band III, No. 5, of the _Publicationen
des Astrophysikalischen Instituts Königstuhl-Heidelberg_. The most
striking feature is the great predominance of the classes e and f.
These two classes form a continuous sequence from the brightest in the
list to the very limits of the plates, where they are but mere faint
markings on the films. Eleven are clearly spirals, and the spindles are
unexpectedly common. These results are typical.
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