The Music of the Galaxies

August 5, 2014 6:20 PM

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The Music of the Galaxies

There is a common phrase "no two snowflakes are alike." Astronomers in the early part of the twentieth century felt the same about galaxies (or "nebulae," as they were called then). Indeed, at the very detailed level, each galaxy is different. Nevertheless, in 1926, Edwin Hubble (after whom the Hubble Space Telescope is named) published a seminal paper in which he identified a few broad classes of galaxies, based upon their shapes. At the crudest level, he divided galaxies into "Spirals" (S; e.g., Figure 1), "Ellipticals" (E; e.g., Figure 2) and those that did not fit into these two categories, which he called "Irregulars" (Irr; Figure 3). At the next level, he further subdivided the spirals into two subgroups. First, there were those in which the spiral arms appeared to emanate directly from the very core at the center of the galaxy, which he dubbed "normal" spirals (and they were denoted by "S"; e.g., Figure 4). Second, there were the "barred" spirals (denoted by "SB"; Figure 5), in which the arms appeared to be connected to a straight "bar" crossing the galactic center. Within each of the subdivisions E, S, and SB, Hubble introduced a certain progression of shapes. For instance, the ellipticals ranged from E0, denoting a perfectly circular profile, to extremely eccentric (elongated) ellipses, denoted by E7. For the normal and barred spirals, the sequence started with very tightly wound spiral arms (denoted by Sa or SBa) and ended with rather loosely wound or barely discernable arms (Sc or SBc).

About a decade later, Hubble added yet another class of "lenticular" galaxies that he regarded as transitional objects between the spirals and the ellipticals, and these were denoted by S0. While Hubble's classification in itself has been extremely influential, it has gained additional power and app...

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