Unbarred spiral galaxy
Type of spiral galaxy without a central bar
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Unbarred spiral galaxies are one of three general types of spiral galaxies under the de Vaucouleurs classification system, the other two being intermediate spiral galaxy and barred spiral galaxy. Under the Hubble tuning fork, it is one of two general types of spiral galaxy, the other being barred spirals.
Overview
Roughly one third of known spiral galaxies are unbarred. Spiral galaxies range in diameter from about 20,000 to over 100,000 light-years; smaller spiral galaxies are less likely to have a stable bar than larger ones. Unbarred spiral galaxies tend to have a higher redshift and are older than barred spirals, suggesting they represent an earlier stage in galactic evolution. The ratio of unbarred to barred spiral galaxies has fallen over the history of the universe.
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Classification
In 1926, Edwin Hubble introduced the Hubble sequence for morphological classification of galaxies. Spiral galaxies, defined by their disc shape and presence of star-forming spiral arms, were divided into unbarred (SA) and barred (SB) types. Both types were further subdivided along a continuum of how tightly wound their spiral arms were, from tightly to loosely (a–c).
Gérard de Vaucouleurs further expanded on this system in 1959. He described an intermediate class of spiral galaxy (SAB), between barred and unbarred, that shared features of both. He noted the continuity between loosely wound spiral galaxies and Magellanic spirals (galaxies with only one spiral arm), which he assigned the classes Sd and Sm. He also introduced classes for a galaxy's inner structure, sorted by whether the arms originated from a central ringlike structure or were purely spiral, allowing for mixed types: (r), (rs), (sr), and (s).
Unbarred lenticular galaxy
An unbarred lenticular galaxy is a lenticular version of an unbarred spiral galaxy. They have the Hubble type of SA0.
An example of this is the galaxy AM 0644-741. For other examples, see Category:Unbarred lenticular galaxies.