<<Edwin Hubble classified these types of spiral galaxies as "SB" (spiral, barred) in his Hubble sequence, and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between. A fourth type, SBm
, was subsequently created to describe somewhat irregular barred spirals
, such as the Magellanic Cloud galaxies, which were once classified as irregular galaxies
, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are: spiral galaxy, elliptical galaxy and irregular galaxy.
In 2005, observations by the Spitzer Space Telescope backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. Observations by radio telescopes had for years suggested the Milky Way is barred, but Spitzer's vision in the infrared region of the spectrum has provided a more definite calculation.
Barred spiral galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy.
The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure.
Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time, transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars. Since so many spiral galaxies have a bar structure, it is likely that it is a recurring phenomenon in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years.
Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts