o r i o n a r m

The spiral arms of the Milky Way may resemble those of the barred spiral NGC 6217, which feature
alternating light and dark areas as well as pink star-forming clouds and branching spurs.
Image by Hubble Space Telescope, courtesy NASA/JPL-Caltech

links

Wikipedia
on spirals

spiral structure
of the milky way

gould belt

stellar
nurseries:

orion nebula

trapezium

rho ophiuchi

glossary

references

The Milky Way Galaxy exhibits a barred spiral structure, with two major spiral arms and several minor, irregular arms extending from the central bar (Benjamin 2008). The Solar System is located along a branch or "spur" of one of the lesser arms, about 28,000 light years (8.5 kiloparsecs) from the Galactic Center in Sagittarius (Levine et al. 2006).

Spiral arms are explained as density waves in the clouds of gas and dust that compose our Galaxy, induced by the rotation of the massive Galactic Bulge. Matter gathers in long dusty corridors, encouraging star formation and sustaining large populations of all types of star systems. In the regions between the arms, gas and dust are more tenuous, so stars are fewer and space is darker. See this diagram of the Milky Way's spiral structure

The shining arms of the Galaxy can be understood on the analogy of nighttime traffic patterns in a large city. On popular thoroughfares, vehicles bunch together, creating luminous pathways with their headlights. Side streets remain dark, with relatively little traffic. Although the vehicles themselves are constantly in motion, like stars orbiting the Galactic center, the patterns of light and darkness that they create remain stable over long periods of time. In a city, such periods are measured in hours. In a galaxy, they are measured in hundreds of millions of years.

Spiral arms in other galaxies observable from Earth often take the shape of logarithmic spirals, such that the distance between successive turnings increases geometrically. Other natural phenomena that assume a similar shape include cyclones, spiderwebs, and the shells of molluscs. Although various researchers have suggested such a configuration for the Milky Way, recent mapping of hydrogen abundances suggest a more tightly wrapped, symmetrical structure resembling an Archimedean spiral (Levine et al. 2006).

As repositories of gas and dust, the spiral arms are Galactic nurseries – luminous regions where new stars ignite and begin their orbits around the Galactic center. Among the closest star-forming regions to the Solar System are the Orion Nebula, in the direction of the Galactic rim, and the Rho Ophiuchi Complex, in the direction of the Galactic center. The brightest and most massive stars, of spectral types O and B, have such short life spans that they explode before they can migrate away from their native arms. Nevertheless, their renewed presence in star-forming regions ensures the continued luminosity of the spirals. Dimmer, more temperate stars like Sol live long enough to complete dozens of circuits of the Galaxy.

Given our position within the Galactic disk, we receive an extremely fragmentary view of the Milky Way's spiral structure. A large portion of the Galaxy is invisible to us behind the dense phenomena of the Galactic Bulge. Nevertheless, recent studies indicate the presence of two major arms extending from the bar-shaped central bulge. These have been named the Centaurus and Perseus Arms, after the apparent locations of their most prominent features (Benjamin 2008). Lesser arms include the Sagittarius Arm, the Cygnus (or Norma) Arm, and the Near and Far Three Kiloparsec Arms (Xu et al. 2006, Benjamin 2008).

The Solar System is located on a spur or branch connecting the Perseus and Sagittarius Arms. Our local branch is known as the Orion Arm. A key structural feature of this region is the Gould Belt, an elliptical ring of hydrogen clouds and bright young stars whose longest diameter is about 750 parsecs (Perrot & Grenier 2000). Although our Solar System is currently located well inside the Gould Belt, our proximity is merely a coincidence, since this structure is only about 30-50 million years old (Perrot & Grenier 2000), while the Solar System is orders of magnitude older at 4.6 billion years. Thus the formation of the Gould Belt and the overall history of the Solar System have little in common.

Within the Orion Arm we occupy a relatively depopulated and dust-free region often called the Local Bubble. In dustier regions of the Orion Arm, stars are more numerous and more densely packed, and great star-forming nebulae shimmer and seethe.

All text is copyright Raymond Harris 2006-2008. Image credits appear in the accompanying caption.