Unbarred spiral galaxy
Topic: Astronomy
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An unbarred spiral galaxy is a type of spiral galaxy without a central bar, or one that is not a barred spiral galaxy. It is designated with an SA in the galaxy morphological classification scheme. A nearby example of an unbarred spiral is the Triangulum Galaxy.
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.[1][2]
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.[3] 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.[4]
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).[3][5]
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).[6]
| Example | Type | Image | Information | Notes |
|---|---|---|---|---|
| SA0- | SA0- is a type of lenticular galaxy | |||
| SA0 | SA0 is a type of lenticular galaxy | |||
| SA0+ | SA0+ is a type of lenticular galaxy | |||
| NGC 3593 | SA0/a | 100px | SA0/a can also be considered a type of unbarred lenticular galaxy | NGC 3593 is an "SA(s)0/a" |
| NGC 3169 | SAa | 100px | NGC 3169 is an "SA(s)a pec" | |
| Messier 81 | SAab | 100px | M81 is an "SA(s)ab" | |
| Messier 88 | SAb | 100px | M88 is an "SA(rs)b" | |
| NGC 3949 | SAbc | 100px | NGC 3949 is an "SA(s)bc" | |
| NGC 4414 | SAc | 100px | NGC 4414 is an "SA(rs)c" | |
| Triangulum Galaxy | SAcd | 100px | Triangulum is an "SA(s)cd" | |
| NGC 300 | SAd | 100px | NGC 300 is an "SA(s)d" | |
| NGC 45 | SAdm | 100px | SAdm can also be considered a type of unbarred Magellanic spiral | NGC 45 is an "SA(s)dm" |
| NGC 4395 | SAm | 100px | SAm is a type of Magellanic spiral (Sm) | NGC 4395 is an "SA(s)m" |
Unbarred lenticular galaxy
An unbarred lenticular galaxy is a lenticular version of an unbarred spiral galaxy. They have the Hubble type of SA0.
References
- ↑ Astronomical Journal, "Near-infrared surface photometry and morphology in virgo cluster spiral galaxy nuclear regions", Bernard J. Rauscher, April 1995, Bibcode: 1995AJ....109.1608R, doi:10.1086/117389
- ↑ Astronomy Pictures, "M99" (accessed 18 April 2010)
- ↑ 3.0 3.1 Melvin, Thomas; Masters, Karen; Lintott, Chris et al. (11 March 2014). "Galaxy Zoo: an independent look at the evolution of the bar fraction over the last eight billion years from HST-COSMOS★" (in en). Monthly Notices of the Royal Astronomical Society 438 (4): 2882–2897. doi:10.1093/mnras/stt2397. ISSN 1365-2966. http://academic.oup.com/mnras/article/438/4/2882/1088340/Galaxy-Zoo-an-independent-look-at-the-evolution-of.
- ↑ "20.3: Types of Galaxies" (in en). 21 July 2025. https://phys.libretexts.org/Courses/Prince_Georges_Community_College/Introduction_to_Astronomy_(2025)/03%3A_An_Introduction_to_the_Universe/3.06%3A_Galaxies/3.6.03%3A_Types_of_Galaxies.
- ↑ "Types of Galaxies" (in en-US). 21 September 2020. https://science.nasa.gov/universe/galaxies/types/.
- ↑ "Galaxies and the Universe - Galaxy Classification". https://pages.astronomy.ua.edu/keel/galaxies/classify.html.
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