About Spiral Galaxies

Types

• Edwin Hubble, best known in relation to the Hubble telescope, was a galaxy researcher. It was Hubble who invented the main classification system for spiral galaxies. Not all spiral galaxies are the same. Some have loose, somewhat indistinct spirals, while others are tightly wound and have a predominant spiral structure. Different researchers distinguish between varying types of spiral galaxies in different ways. Some refer to tight spirals as Type A galaxies and loose spirals as Type B. Type-C galaxies are the loosest spiral galaxies under this definition.

  Hubble's method of classifying spirals is based on the Hubble Sequence. This method splits spirals into two families: barred and ordinary. Ordinary spiral galaxies are designated with the letters S or SA. Barred spirals, which appear to have a bar running through their central bulge, are known as SB. A third type exists that mixes traits of both families. Ordinary and barred spiral galaxies are further defined by the tightness of their spiral pattern, and are termed Sa,Sb,Sc, Sd and Sm (the most loosely based).

  Finally, some researchers prefer to name spiral galaxies by their appearance aside from the spiral structures. Those whose arms emerge from a discernible central core are known as "grand design" spirals. Those that look lumpy in the middle, with shorter spirals, are called flocculent.

Size

• Spiral galaxies come in a wide variety of sizes and brightnesses. The best-known spiral galaxies average more than 1,000 times the mass of the sun. From Earth, many spiral galaxies appear with an angular size of just over 4 arc-minutes. This corresponds to 200,000 light years in diameter.

  Some spirals are more luminous than others, but none can be seen in full by the naked eye; magnitudes range from -16 to -23. However, telescopes bring out many of the distinguishable features.

  While they are uncommon, dwarf varieties of the spiral galaxy exist. These low-luminosity disks have a diameter of less than 5 kpc and contain little hydrogen. It is more common to find "imperfect" variations of galaxies, resembling elliptical or irregular galaxies, than true spiral structures. Dwarf spirals can be found outside larger galaxy clusters.

Identification

• Because there are so many of them, spiral galaxies are easy to recognize. With a central bulge or bar of luminosity, the galaxies have a spiral shape, with "arms" that wrap around the center--sometimes in a dramatic fashion. They appear blue because of the number of bright, new stars that line the spirals, which can be an indication of star formation in these regions. However, there is actually more of a reddish glow to the central bulge, meaning that many older stars occupy the center of spiral galaxies.

  From the side, spiral galaxies look like circles, so there may be many more such galaxies out there than have been identified. Two very prominent examples of spiral galaxies are Messier 101 (also known as the Pinwheel Galaxy) and M31, or Andromeda, which is the closest spiral galaxy to our own.

Geography

• Although there are quite a few spiral galaxies, we on Earth don't often see these galaxies head-on. On the contrary, we see them from the edge or angled, so that we can't fully appreciate the total spiraled structure. Nor can we appreciate the Milky Way galaxy in its full splendor, for there is too much dust obscuring the central bulge.

Theories/Speculation

• It is thought that spiral galaxies evolved from lower-mass galaxies, some 10 times smaller than the Milky Way. These small galaxies have very few stars and very little mass in comparison to spirals as we know them. In fact, they are often initially defined as individual stars, but the ultraviolet light they emit betrays them. This intense light is a sign of stellar nurseries, in which temperatures are so high that hydrogen atoms are stripped of their electrons. Using the knowledge gleaned from research into the way galaxies cluster, scientists speculate that several small galaxies such as these merge over a few billion years to create a standard-sized spiral galaxy.

Misconceptions

• It was originally believed that the arms of spiral galaxies were composed of solid material. Scientists knew, however, that if that were the case, the arms would become so tightly wound (due to the effects of gravity) that they would become indistinguishable from the rest of the galaxy in a short amount of time. This "winding problem" is the reason two Chinese scientists predicted that the spiral arms were a result of density, rather than the presence of material. These scientists, Lin and Shu, suggested that the spirals had resulted because dense areas of the galaxy compressed nearby dust and gas into discernible shapes; what scientists saw as solid spirals were areas of increased density.