<p>There is a spectrum of galaxy shapes. The differences between them are like the difference between red and blue.  The three main types are <em>irregular</em>, <em>elliptical</em> and <em>spiral</em> (with or without a bar).</p>
<p>Ellipical galaxies are galaxies that have undergone a major collision or merger in the past, which has randomised the orbits of their stars.  Spiral galaxies have formed more gently allowing the gas in them to settle into a disk. Spiral arms develop naturally due to gravitational interactions in the disk.</p>
<p>But between the two main types, there are galaxies that have a disk-like structure, but no spiral arms.  These are the &quot;<a href="https://en.wikipedia.org/wiki/Lenticular_galaxy" rel="nofollow noreferrer">lenticular galaxies</a>&quot;. By &quot;disk galaxy&quot;, your reference might be referring to all galaxies that have disk including both the classic spiral galaxies and the lenticular. Or it might be referring just the Lenticular galaxies (context probably makes it clear)</p>
<p>IC335 and NGC5866 are lenticular galaxies viewed edge-on.  NGC253 is an intermediate spiral galaxy with little obvious spiral structure in its disk.  So I think your source is using &quot;disk galaxy&quot; for &quot;lenticular galaxy&quot; or a spiral galaxy with indistinct spiral structure.</p>
<p>I would probably avoid the term &quot;disk galaxy&quot;.  It's fine to talk about the disk of a spiral galaxy, or &quot;galaxies with a disk&quot; But I wouldn't choose to use &quot;disk galaxy&quot; as a classification; stick to Spiral, Lenticular,  Elliptical and Irregular</p>

CC BY-SA 4.0
2024-12-21T01:12:20.803Z
“Disk galaxy” = spiral or lenticular, full stop.
Peter Erwin
CC BY-SA 4.0
2024-12-21T01:19:33.637Z
NGC 253 definitely has spiral arms (and a bar). They’re easier to see in the near-IR than the optical, because of the dust. (See, e.g., [Iodice et al. 2014](https://www.aanda.org/articles/aa/full_html/2014/07/aa23480-14/aa23480-14.html).)
Peter Erwin
CC BY-SA 4.0
2024-12-21T08:00:56.387Z
Yes, but why choose those three as examples of "disk"?  They are two lenticular and one spiral galaxy. If you wanted to demonstrate '“Disk galaxy” = spiral or lenticular,' would you choose ngc253 as your example of a spiral galaxy?
James K
CC BY-SA 4.0
2024-12-21T16:06:59.553Z
That's not really a question about "what does 'disk galaxy' mean?", it's a question about "why did the PR/sci-comm. folks at ESA happen to say, 'disk galaxy' when presenting images of those galaxies". It's got nothing to do with what is or isn't a "disk galaxy". (I could hazard a guess that they wanted to point out how vertically thin -- and thus geometrically disklike -- they are, versus the case where you can't directly tell how vertically thin a face-on spiral is; maybe they wanted to be cautious about NGC 5866, which is rather dusty for a lenticular and might be a spiral if seen face-on.)
Peter Erwin
CC BY-SA 4.0
2024-12-21T16:09:58.873Z
(I suppose I could point out that [ UGC 2885](https://esahubble.org/news/heic2002/) is also described as a "disk galaxy"...)
Peter Erwin
CC BY-SA 4.0
2024-12-21T23:16:39.147Z
Now, that's a spiral galaxy!!
James K
CC BY-SA 4.0
2024-12-20T23:18:11.650Z
2024-12-21T08:04:58.353Z
2024-12-21T08:04:58.353Z
James K
<p>&quot;Disk galaxy&quot; means a galaxy with a significant or prominent stellar disk: a basically round and vertically thin structure (&quot;disk&quot; in the geometric sense) made up of stars (and, optionally, gas and dust). There are elliptical galaxies with <em>gas</em> disks, and with compact &quot;nuclear&quot; disks made of stars that only amount to a few percent of the total stellar mass in the galaxy; those are not considered disk galaxies.</p>
<p>It's basically an umbrella term for both lenticular and spiral galaxies. The latter are more numerous; in the local universe, only about 20% of disk galaxies are lenticular, and the fraction gets smaller the farther back in time (higher redshift) you look. The latter fact is consistent with the idea that most if not all lenticular galaxies started life as spiral galaxies and have lost their spiral structure.</p>
<p>By defintion, lenticular galaxies do not have spiral arms (or at least strong, prominent spiral arms) and thus are not &quot;spiral galaxies&quot;; they <em>do</em> share other disk-galaxy characteristics with spirals, such as  stellar bars and rings.</p>

CC BY-SA 4.0
2024-12-22T18:00:53.593Z
Thank you for this answer. It was the most helpful in my goal of explaining galaxies to kids. In particular, the distribution of lenticular and spiral galaxies by redshift really clarifies the evolutionary path in a simple way.
Ray N. Franklin
CC BY-SA 4.0
2024-12-21T16:27:55.637Z
2024-12-30T21:00:04.730Z
2024-12-30T21:00:04.730Z
Peter Erwin
<p>I’m working on a book, <em>Galaxy Astronomy: Field Guide For Kids</em>. During research, I got the impression that a disk galaxy is a type of galaxy. Several descriptions of Hubble images used that term, mostly for galaxies seen edge-on (list below). After reading Wikipedia and other online sources, I suspected disk was more of an attribute that could be applied to spiral and even lenticular galaxies. Now I’m confused and not sure what a disk galaxy really is. </p>
<p>The ESA Hubble website identifies the following as disk galaxies in the source descriptions: IC 335, NGC 5866, NGC 253.</p>
<p>What is the definition of a disk galaxy? Thanks.</p>

CC BY-SA 4.0
2024-12-21T01:13:31.483Z
https://en.wikipedia.org/wiki/Disc_galaxy
Peter Erwin
<galaxy><galactic-evolution>
Disk galaxy definition
Ray N. Franklin
# Disk galaxy definition

(Asked by: username_0 on 2024-12-20)

I’m working on a book, *Galaxy Astronomy: Field Guide For Kids*. During research, I got the impression that a disk galaxy is a type of galaxy. Several descriptions of Hubble images used that term, mostly for galaxies seen edge-on (list below). After reading Wikipedia and other online sources, I suspected disk was more of an attribute that could be applied to spiral and even lenticular galaxies. Now I’m confused and not sure what a disk galaxy really is.

The ESA Hubble website identifies the following as disk galaxies in the source descriptions: IC 335, NGC 5866, NGC 253.

What is the definition of a disk galaxy? Thanks.

**Tags:** galaxy, galactic-evolution

> **Comments:**
> **username_1**: https://en.wikipedia.org/wiki/Disc_galaxy  

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## 2 answers

### Answer by username_1 on 2024-12-21. Score: 5 | Accepted answer

"Disk galaxy" means a galaxy with a significant or prominent stellar disk: a basically round and vertically thin structure ("disk" in the geometric sense) made up of stars (and, optionally, gas and dust). There are elliptical galaxies with *gas* disks, and with compact "nuclear" disks made of stars that only amount to a few percent of the total stellar mass in the galaxy; those are not considered disk galaxies.

It's basically an umbrella term for both lenticular and spiral galaxies. The latter are more numerous; in the local universe, only about 20% of disk galaxies are lenticular, and the fraction gets smaller the farther back in time (higher redshift) you look. The latter fact is consistent with the idea that most if not all lenticular galaxies started life as spiral galaxies and have lost their spiral structure.

By defintion, lenticular galaxies do not have spiral arms (or at least strong, prominent spiral arms) and thus are not "spiral galaxies"; they *do* share other disk-galaxy characteristics with spirals, such as stellar bars and rings.

> **Comments:**
> **username_0**: Thank you for this answer. It was the most helpful in my goal of explaining galaxies to kids. In particular, the distribution of lenticular and spiral galaxies by redshift really clarifies the evolutionary path in a simple way.  

---

### Answer by username_2 on 2024-12-20. Score: 4

There is a spectrum of galaxy shapes. The differences between them are like the difference between red and blue. The three main types are *irregular*, *elliptical* and *spiral* (with or without a bar).

Ellipical galaxies are galaxies that have undergone a major collision or merger in the past, which has randomised the orbits of their stars. Spiral galaxies have formed more gently allowing the gas in them to settle into a disk. Spiral arms develop naturally due to gravitational interactions in the disk.

But between the two main types, there are galaxies that have a disk-like structure, but no spiral arms. These are the "[lenticular galaxies](https://en.wikipedia.org/wiki/Lenticular_galaxy)". By "disk galaxy", your reference might be referring to all galaxies that have disk including both the classic spiral galaxies and the lenticular. Or it might be referring just the Lenticular galaxies (context probably makes it clear)

IC335 and NGC5866 are lenticular galaxies viewed edge-on. NGC253 is an intermediate spiral galaxy with little obvious spiral structure in its disk. So I think your source is using "disk galaxy" for "lenticular galaxy" or a spiral galaxy with indistinct spiral structure.

I would probably avoid the term "disk galaxy". It's fine to talk about the disk of a spiral galaxy, or "galaxies with a disk" But I wouldn't choose to use "disk galaxy" as a classification; stick to Spiral, Lenticular, Elliptical and Irregular

> **Comments:**
> **username_1**: “Disk galaxy” = spiral or lenticular, full stop.  
> **username_1**: NGC 253 definitely has spiral arms (and a bar). They’re easier to see in the near-IR than the optical, because of the dust. (See, e.g., [Iodice et al. 2014](https://www.aanda.org/articles/aa/full_html/2014/07/aa23480-14/aa23480-14.html).)  
> **username_2**: Yes, but why choose those three as examples of "disk"?  They are two lenticular and one spiral galaxy. If you wanted to demonstrate '“Disk galaxy” = spiral or lenticular,' would you choose ngc253 as your example of a spiral galaxy?  
> **username_1**: That's not really a question about "what does 'disk galaxy' mean?", it's a question about "why did the PR/sci-comm. folks at ESA happen to say, 'disk galaxy' when presenting images of those galaxies". It's got nothing to do with what is or isn't a "disk galaxy". (I could hazard a guess that they wanted to point out how vertically thin -- and thus geometrically disklike -- they are, versus the case where you can't directly tell how vertically thin a face-on spiral is; maybe they wanted to be cautious about NGC 5866, which is rather dusty for a lenticular and might be a spiral if seen face-on.)  
> **username_1**: (I suppose I could point out that [ UGC 2885](https://esahubble.org/news/heic2002/) is also described as a "disk galaxy"...)  
> **username_2**: Now, that's a spiral galaxy!!  

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