<p><strong>Capitalization is critical here</strong>. (As is knowing which classification system is being used.) The traditional spiral stages (going back to Hubble) are Sa - Sb - Sc; this was a sequence from more centrally concentrated (more dominant central "bulge") + smoother disk + more tightly wrapped spiral arms (Sa) to less centrally concentrated (smaller or even nonexistent bulge) + less smooth disk + more open spiral arms (Sc). Later, S0 (for lenticular galaxies, which are not spirals but seem to have significant or dominant disks) was added at the beginning and Sd (and sometimes Sm) were added as stages beyond Sc.</p>

<p>Hubble also specified that clearly/strongly barred galaxies should be labeled SB0 - SBa - SBb - SBc (with a later extension to SBd), creating a parallel sequence to the nominally unbarred sequence S0 - Sa - Sb - Sc - Sd.</p>

<p>De Vaucouleurs emphasized the existence of <em>intermediate</em> stages: thus, Sab is in between Sa and Sb, Scd is in between Sc and Sd, and so on. (There is also a stage intermediate between S0/lenticular galaxies, which are not spirals, and the Sa spirals: "S0/a".) He also applied this to bar classifications, arguing that there were weakly barred galaxies in between unbarred and strongly barred galaxies. To do so, he changed the Hubble classifications so that "SA" mean unbarred, "SAB" mean weakly barred", and "SB" meant strongly barred. These precede the lower-case Hubble-sequence labels, so you end up with SA0, SAB0, SB0 - SA0/a, SAB0/a, SB0/A - SAa, SABa, SBa - SAab, SABab, etc.</p>

<p>To add to the confusion, de Vaucouleurs <em>also</em> recognized that some spiral galaxies had an unclear bar status (especially true for edge-on galaxies). His famous and widely used RC3 catalog uses "S" for those cases, so "Sa" in RC3 actually means "Hubble stage a, unknown bar status". This, of course, looks exactly like the traditional Hubble classification (continued by other classifiers such as Allan Sandage) meaning "Hubble stage a, <em>unbarred</em>), which has confused people over the years.</p>

<p>To directly address your question: the "a" and "ab" mean what you said in your comment: "Sa galaxies tend to have significant bulges, and tightly-wrapped and relatively smooth spiral arms. Sab galaxies are similar to Sa galaxies, but show more obvious resolution of the arms." Loosely, people will often use "Sa", etc., to refer to spirals regardless of their bar status, as in your quoted text (or in something like, e.g., "Sa galaxies tend to have lower star-formation rates than Sc galaxies."). If it's a specific classification of an individual galaxy, then it might mean "also unbarred" or even "bar status unclear".</p>

CC BY-SA 4.0
2020-02-03T12:52:21.343Z
2020-02-03T13:04:53.920Z
2020-02-03T13:04:53.920Z
Peter Erwin
<p>I have learned that the notation to classify spiral galaxies in the <a href="https://en.wikipedia.org/wiki/Galaxy_morphological_classification#De_Vaucouleurs_system" rel="nofollow noreferrer">De Vaucouleurs system</a> goes like "SA for spirals without bar, SB for spirals with bar and SAB for intermediate spirals". But in some places I've seen galaxies classified as Sa or Sab for example. What it means and what is the difference between a Sa and a Sab spiral?</p>

CC BY-SA 4.0
2020-01-31T23:50:10.783Z
Ok, reading [this](https://ned.ipac.caltech.edu/level5/Sept11/Buta/Buta5.html), it seems that Sa and Sab are just abbreviations for the usual SA, SB and SAB classification of the De Vaucouleurs system + "a" and "ab" at the end, denoting intermediate stages. Also, it says that "Sa galaxies tend to have significant bulges, and tightly-wrapped and relatively smooth spiral arms. Sab galaxies are similar to Sa galaxies, but show more obvious resolution of the arms."
Elismar Lösch
CC BY-SA 4.0
2020-02-01T01:24:03.300Z
de Vaucouleurs SA = [Hubble](https://en.wikipedia.org/wiki/Hubble_sequence) S.
Mike G
CC BY-SA 4.0
2020-02-03T12:56:13.843Z
@ElismarLösch Sa and Sab are *not* abbreviations for SA and SAB (see my answer for details); the second part of your comment is correct.
Peter Erwin
<galaxy><classification><notation>
What does Sa and Sab mean for spiral galaxies?
Elismar Lösch
# What does Sa and Sab mean for spiral galaxies?

(Asked by: username_0 on 2020-01-31)

I have learned that the notation to classify spiral galaxies in the [De Vaucouleurs system](https://en.wikipedia.org/wiki/Galaxy_morphological_classification#De_Vaucouleurs_system) goes like "SA for spirals without bar, SB for spirals with bar and SAB for intermediate spirals". But in some places I've seen galaxies classified as Sa or Sab for example. What it means and what is the difference between a Sa and a Sab spiral?

**Tags:** galaxy, classification, notation

> **Comments:**
> **username_0**: Ok, reading [this](https://ned.ipac.caltech.edu/level5/Sept11/Buta/Buta5.html), it seems that Sa and Sab are just abbreviations for the usual SA, SB and SAB classification of the De Vaucouleurs system + "a" and "ab" at the end, denoting intermediate stages. Also, it says that "Sa galaxies tend to have significant bulges, and tightly-wrapped and relatively smooth spiral arms. Sab galaxies are similar to Sa galaxies, but show more obvious resolution of the arms."  
> **username_1**: de Vaucouleurs SA = [Hubble](https://en.wikipedia.org/wiki/Hubble_sequence) S.  
> **username_2**: @ElismarLösch Sa and Sab are *not* abbreviations for SA and SAB (see my answer for details); the second part of your comment is correct.  

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## 1 answers

### Answer by username_2 on 2020-02-03. Score: 4 | Accepted answer

**Capitalization is critical here**. (As is knowing which classification system is being used.) The traditional spiral stages (going back to Hubble) are Sa - Sb - Sc; this was a sequence from more centrally concentrated (more dominant central "bulge") + smoother disk + more tightly wrapped spiral arms (Sa) to less centrally concentrated (smaller or even nonexistent bulge) + less smooth disk + more open spiral arms (Sc). Later, S0 (for lenticular galaxies, which are not spirals but seem to have significant or dominant disks) was added at the beginning and Sd (and sometimes Sm) were added as stages beyond Sc.

Hubble also specified that clearly/strongly barred galaxies should be labeled SB0 - SBa - SBb - SBc (with a later extension to SBd), creating a parallel sequence to the nominally unbarred sequence S0 - Sa - Sb - Sc - Sd.

De Vaucouleurs emphasized the existence of *intermediate* stages: thus, Sab is in between Sa and Sb, Scd is in between Sc and Sd, and so on. (There is also a stage intermediate between S0/lenticular galaxies, which are not spirals, and the Sa spirals: "S0/a".) He also applied this to bar classifications, arguing that there were weakly barred galaxies in between unbarred and strongly barred galaxies. To do so, he changed the Hubble classifications so that "SA" mean unbarred, "SAB" mean weakly barred", and "SB" meant strongly barred. These precede the lower-case Hubble-sequence labels, so you end up with SA0, SAB0, SB0 - SA0/a, SAB0/a, SB0/A - SAa, SABa, SBa - SAab, SABab, etc.

To add to the confusion, de Vaucouleurs *also* recognized that some spiral galaxies had an unclear bar status (especially true for edge-on galaxies). His famous and widely used RC3 catalog uses "S" for those cases, so "Sa" in RC3 actually means "Hubble stage a, unknown bar status". This, of course, looks exactly like the traditional Hubble classification (continued by other classifiers such as Allan Sandage) meaning "Hubble stage a, *unbarred*), which has confused people over the years.

To directly address your question: the "a" and "ab" mean what you said in your comment: "Sa galaxies tend to have significant bulges, and tightly-wrapped and relatively smooth spiral arms. Sab galaxies are similar to Sa galaxies, but show more obvious resolution of the arms." Loosely, people will often use "Sa", etc., to refer to spirals regardless of their bar status, as in your quoted text (or in something like, e.g., "Sa galaxies tend to have lower star-formation rates than Sc galaxies."). If it's a specific classification of an individual galaxy, then it might mean "also unbarred" or even "bar status unclear".


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