Editor's note: A version of this was previously published on the author's website.
Astronomers classify galaxies into three general types: elliptical (oval or ball-like), spiral, and irregular (blotchy), of which the spiral is the most common. This sort has immense arms of stars, dust and gas that stretch out from the center, the nucleus, curving around in a spiral shape. Astronomers have known for many decades that our Milky Way is a spiral galaxy; arms can be seen by the unaided eye.
The sweeping arms of a spiral galaxy give a dramatic aspect, like a spinning ice-skater drawing in her arms as she twirls. Sometimes the galaxy's arms are stretched out of joint by passing galaxies; sometimes they are highly distorted and wound into separate-looking circular patterns. At times they stream across vast distances as the galaxies' gravity tugs at one another. When spiral galaxies merge, these patterns can be demolished and the material reconstituted as a larger elliptical.
A NASA primer notes that spiral galaxies "have a distinctive shape with spiral arms in a relatively flat disk and a central 'bulge.' The bulge has a large concentration of stars. The arms and bulge are surrounded by a faint halo of stars. The bulge and halo consist mainly of older stars, where spiral arms have more gas, dust and younger stars. ...
"Some spiral galaxies are what we call 'barred spirals' because the central bulge looks elongated — like a bar. In barred spirals, the spiral arms of the galaxy appear to spring out of the ends of the bar."
One of these massive bars of stars crossing a spiral galaxy's disk is a sign that the galaxy has reached "full maturity," according to Hubble Space Telescope scientists. One theory is that the interior of the galaxy is rotating slightly faster than the outside, they add.
"(P)assing interactions with other stars can leave stellar orbits unstable. If there is a slight growth of structure in one part of the galaxy, it can have a dominant effect that leads to more instability, creating a situation where stars start wandering off from their smooth, circular orbits and produce a bar-like structure appearing in the overall morphology of the galaxy itself." The scientists promise further studies to improve understanding of the process.
Although mankind has lived here since our beginning, only within the past half-century was the Milky Way found to be a barred spiral. Classifiers have debated whether it should be termed a transitional spiral, a stage between a spiral one without a bar and a fully barred spiral. The latest thinking is to classify it as the latter, with a bar of stars and gas 27,000 light-years long in a galaxy 105,000 light-years across.
Because of the great difficulty in mapping a complex object from the inside out, one that is obscured by clouds of dust and gas, the number of spiral arms in our galaxy has been debated.
In January 2014, the Monthly Notices of the Royal Astronomical Society (Oxford University Press, U.K.) published a 12-year survey of the Milky Way — star ages, luminosity, positions and masses, molecular gas clouds, and other aspects — concluding, "Our results are therefore consistent with a model of the galaxy consisting of four major arms." However, in November 2017, NASA announced that infrared photographs by the agency's Spitzer Space Telescope showed "that the Milky Way's elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Previously, our galaxy was thought to possess four major arms."
Two major arms are shown as attached to a "thick central bar," dominating a pair of lesser structures deemed "now-demoted minor arms (that are) less distinct and located between the major arms." Such arm fragments are observable in many spiral galaxies while a series of four major arms seems less common.
"Our sun lies near a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms," the space agency adds.
The remaining major arms are named the Scutum-Centaurus and the Perseus arms, while the demoted structures are the Norma and Sagittarius minor arms. The study says a radio-telescope survey also identified a previously unknown structure that lies along the bar, identified as the "Far-3 kiloparsec arm" — it’s not a spiral arm.