Our quantitative classification scheme may schematically be depicted in a fork with three prongs, as in Figure 150. An optically late-type Sc galaxy can fall into our alpha class: one example is NGC 5861 (Sc in the optical domain, but class alpha behind the mask). Moreover, an optical early type Sb can be classified as a member of our wide open gamma class, as seen in the galaxy NGC 7083 (Hubble type Sb).
Moreover, the reader will note that it is possible for spirals of a given Hubble type to be distributed within all three dust penetrated classes. For example, NGC 3992, NGC 2543, NGC 7083, NGC 5371 and NGC 1365 illustrated in Figure 150 are all Hubble class b in the optical domain; behind their dust masks, however, NGC 3992 belongs to class alpha, NGC 2543 to type beta, while the three remaining type b spirals NGC 7083, NGC 5371 and NGC 1365 belong to the open gamma class.
At a recent conference held in the ancient city of Rome, we emphasized an important uncertainty principle which has emerged from our investigations. The uncertainty is this: we simply cannot predict what the optical image of a galaxy will look like from its near-infrared counterpart. A human analogy: given an optical photograph of the human mask of the skin, it is impossible to predict what the spinal column will exactly look like. But our flesh always responds to the movement of our backbone - our skeletal structure. In much the same way, masks of gas and dust are highly responsive to the smallest of changes in the backbones of old stars. In a mathematical sense, there is a degree of chaos (uncertainty) in our cosmic masks of swirling gas clouds and dust.
Astronomer Frank Shu gives a weather analogy:
It's like the weather, which is also a chaotic system.. Make all the measurements you can, still no one can predict the weather seven days from now. You can guess the weather tomorrow with some precision, but you really cannot guess well for a week later, no matter how fine are your observations because of the chaos in the system. We need to be prepared for this in our subject.
Dust grains scatter starlight from neighboring stars on the grandest of scales. The famous "Barnard loop" (Figure 151) in the constellation of Orion sweeps out twenty degrees - the equivalent of forty full moons - in our skies. The Barnard Loop, in our Milky Way Galaxy, spans almost six hundred light years across; dust grains within the Loop scatter ultraviolet light from nearby stars in Orion. Such "reflection nebulae" can occur on much larger scales. For example, astronomer Steven Gibson and his collaborators have found that "reflection nebulae" in the Large Magellanic Cloud may occur on scales of the order of three thousand light years across. Very hot, young stars in the north-western sector of that neighboring galaxy illuminate dust grains which then scatter ultraviolet photons of light into our line of sight. A few years ago, an almost straight "strip" of light in the disk of the spiral galaxy NGC 2841 was discussed in the literature by astronomers Bruce Elmegreen, Richard Wainscoat and coauthor David. We had never seen anything like this, before. Could it be a mammoth reflection nebula? Could dust grains in the disk of NGC 2841 be scattering light from stars in the bulge of this galaxy? The answer is yes! The "strip" (arrowed in Figure 152) spans about 6500 light years and is reflected starlight from the luminous bulge of NGC 2841. This "reflection nebula" is over ten times the diameter of the Barnard Loop in our Galaxy and is the largest we have yet studied.
We were much inspired by a pioneering study conducted in 1976 of filaments of dust scattering light from the plane of our Milky Way, at relatively high latitudes of about 50 degrees; these high-latitude reflection nebulosities were identified by Dr Allan Sandage on high-contrast photographic prints. The spatial extent of these wispy clouds in one of Sandage's fields covers about 3 x 4 degrees, which translates to linear sizes of about 35 light years x 45 light years (assuming a distance to these filaments of approximately 650 light years). These structures may be associated with a larger feature known as the North Celestial Pole Loop, whose diameter is some 40 degrees in the sky and which, at a distance of 650 light years, would span about 450 light years across - similar in size to the famous Barnard Loop.
Readers may well enquire as to the smallest reflection nebula studied by astronomers. This is probably a nebula discovered by E.E. Barnard in 1890. Wispy tendrils of interstellar grains Shrouds of the Night of dust in a cloudlet known as the Barnard Merope Nebula reflect starlight from the star
250 Merope in the Pleiades star cluster. The nebula cannot be seen in conventional photographs
of the Pleiades (such as Figure 96 and Figure 101), due to the overwhelming brightness of Merope itself. The cloudlet shows a rich and complex structure, as it passes Merope and in the process, is being shaped by the radiation field of that star. Its size is not measured in light years but rather in astronomical units, where one astronomical unit denotes the average distance of the Earth from the Sun. The Barnard Merope Nebula is a few thousand astronomical units in size.
Penetrating shrouds of cosmic dust or fog we must. In the words of Irish dramatist and novelist Samuel Beckett (1906-1989),
We may reason on to our heart's content, the fog won't lift.
(The Narrator in The Expelled)
Allow British author H.G. Wells (1866-1946) to have the final say:
The science hangs like a gathering fog in a valley, a fog which begins nowhere and goes nowhere, an incidental, unmeaning inconvenience to passers-by.
Shrouds of the Night are ever active - responding as do musical strings to the smallest degree of bowing - and we further explore this theme in the next chapter.
The Grand Bow chapter
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