Red Headed Stepchild
(The Barrett family memoir of Navy Life)
by Sophie Ruth Meranski with photos


Sophie on camel near Sphinx and pyramids p 13-97 ON WEB PAGE THIRTEEN #98 Sophie with Mabel Ganz 1917 Wooster Street #99 Sophie and Jack West Roxbury 1966 color.
Jack Barrett photo Egypt January 1932++ }A{ A plot, in Galactic co-ordinates, of some of the nearby star clusters that encircle the Sun reveals another interesting feature. These star clusters form a band or belt known as GOULD’S BELT – this is inclined relative to the Galactic plane, the reason for this is not clear. It is supposed that the clusters making up Gould’s belt were formed by some common, large scale process. Fig 1.14 Gould’s belt – a band of clusters and associations of bright young stars (OB associations) that encircles the Sun. Note that Gould’s belt is inclined relative to the Galactic plane, which is represented by the horizontal line running across the middle of this figure. Another peculiarity of our part of the disc is that stars of all kinds seem to be more densely packed in our immediate neighbourhood than they are in surrounding regions. Counts of star numbers in different directions indicate that their number density at a distance of 500 – 1 000 pc is only about half the local value. Taking into account the severe obscuring effect of dust on faint stars, it seems the Sun does occupy a region of enhanced star density, known as the LOCAL SYSTEM. Whether the local system is related to the presence of Gould’s belt is unclear. The ability to detect X-rays from all directions seems to indicate that we are not enclosed by a cloud of molecular or atomic hydrogen. Rather, we seem to be contained within a bubble of hot gas, perhaps caused by a supernova that occurred about 10 5 years ago. Studies of absorption features in the UV spectra of stars at various known distances indicate that this so-called LOCAL BUBBLE is about 200 pc across and that it contains a number of warm clouds that may have cold centres. The Sun appears to be located on the edge of one of these warm clouds, though when we look through it in the direction of the constellation Sagittarius – it seems to cause little absorption, so cannot be rich in dust. Fig 1.15 A highly schematic representation of the Local Bubble of the ISM that contains the Sun. The Spiral Arms – their nature, origin and influence. The spiral arms are thought to be important features of the disc, though really firm evidence of the spiral structure is hard to obtain. The results of one attempt at mapping HII regions (glowing ionised hydrogen clouds), thought to be especially correlated with spiral arms is shown in Figure 1.16. It is widely believed that spiral arms are concentrations of bright stars rather than mass. They stand out because they contain bright HII regions and luminous O and B class stars.Fig 1.16 The spiral arms of the galaxy? An attempt at mapping the spiral arms based on optical HII regions, strong radio HII regions and weak radio HII regions. Galactic longitudes are shown. O and B stars are so short lived that, if you see them, they cannot be far from where they were born so it is usual to regard the spiral arms as the main locales of star formation in the Milky Way. Q Why should star formation occur along such reasonably well defined spiral tracks. Q. How do the arms manage to persist over long periods of time. One thing to keep One thing to keep in mind is that if the arms were composed of an unchanging population of stars, the differential rotation of the disc would cause the shape of the arms to alter with time and an initially ‘realistic’ pattern would soon cease to resemble any observed spiral arms. This problem is known as the WINDING DILEMMA. Although the spiral arms may represent a persistent pattern, they cannot always contain the same population of differentially rotating stars if they are as persistent as we think they are. The mechanism described below is the current preferred theory of how the enhanced bright areas within spiral arms are produced and maintained. The conventional theory of spiral arms is based on DENSITY WAVE THEORY developed by the American astronomers C.C.Lin and Frank H. Shu in the 1960s. The theory predicts that areas exist in the galactic disc where the density is slightly increased relative to the disc as a whole. This density increase is only a few percent but is highly significant. Mathematical modelling indicates that these density enhancements can survive the effect of differential rotation (like the red spot on Jupiter). One of these long lived self consistent patterns of density enhancement is shown in Fig 1.17. Such patterns are known as SPIRAL DENSITY WAVES. Fig 1.17 A spiral density wave. This pattern of enhanced density (as shown by the intensity of shading) rotates in the same sense as the orbiting disc material but at a generally slower rate. Only towards the outer edge of the disc, at the so called co-rotation radius, does the rotation speed of the density wave equal that of the matter in the disc. The density wave rotates rigidly despite the fact that the disc material rotates differentially. In fact the density wave moves more slowly than the matter in the disc. As stars and gas approach the density waves from behind and pass through them, they experience compression in the process, and then move on ahead of the wave’s leading edge. A dense cloud entering a density from the rear would encounter a rapid increase in ambient density , as indicated in Fig 1.19. For a giant molecular cloud, just on the verge of forming stars, the encounter with a sudden increase in density (even of only a few percent) will be enough to triger star formation and thus give rise to coherent patters of star formation which we see as spiral arms. Fig 1.19 Average relative gas density encountered during an orbit of the Milky Way. (highly schematic). The above theory is popular but is not well confirmed by observational evidence and has unresolved questions such as what created the density waves in the first place? Open Clusters and OB Associations. OPEN CLUSTERS are another prominent feature of the Galactic disc. Such clusters are localised regions where the (number) density of stars is enhanced relative to that of the immediately surrounding area. A typical open cluster would be 2-3 pc across and might include anything from a few stars to many hundreds. Some open clusters are sufficiently prominent to be visible to the naked eye. The Pleiades (Plate 1.22b) in the constellation of Taurus. In all around 200 open clusters are now known, although this is only about 1% of all the numbers to be found in the disc. Not only are open clusters found in the disc, they are also common in the spiral arms. They are composed of population I stars and include an above average number of bright stars, they are used to map the location of spiral arms. Open clusters must have short lives otherwise differential rotation relative to the arms would carry them out of the arms and spread them across the disc. Detailed studies of open clusters show that all the stars in a particular cluster were formed at the same time and that the cluster as a whole is unlikely to survive for more than 109 years (1 billion years) and normally only last a few million years. Plate 1.22a The open cluster The Pleiades is readily visible to the unaided eye. It is about 50 million years old. The blue light is starlight scattered by dust that the cluster probably encountered after the last vestiges of its own cloud disappeared. The Pleiades is 410 light years away. Plate 1.22b This is one of the oldest open clusters known, about Ten thousand million years old (twice the age of the Sun). It is 2700 light years away. This cluster has a catalogue number M67.
Year: 1932