US Trends

how are galaxies classified

Galaxies are mainly classified by their shape and visible structure into a few big families: elliptical, spiral (including barred spiral), lenticular, and irregular, with each family broken into finer subtypes based on how detailed features like bulges, spiral arms, and smoothness look.

How Are Galaxies Classified? (Quick Scoop)

1. The Big Picture: Morphological Classification

Astronomers mostly sort galaxies by morphology – what they look like in images, especially how their light and structure are arranged.

The most famous scheme is the Hubble sequence , often drawn as a “tuning fork” diagram showing how ellipticals, lenticulars, and spirals (barred and unbarred) line up in an orderly pattern.

Key ideas behind this kind of classification:

  • Overall shape of the galaxy.
  • Presence or absence of spiral arms.
  • Presence and size of a central bulge.
  • Degree of smoothness vs clumpiness (dust lanes, star-forming regions).

Even though modern astronomy also uses more physical properties (like mass, color, star‑formation rate) and automated algorithms, this shape‑based system is still the standard starting point.

2. The Hubble Sequence: Main Galaxy Types

a) Elliptical galaxies (E0–E7)

Ellipticals look like smooth, featureless blobs of light, ranging from nearly round (E0) to very elongated (E7).

Typical traits:

  • Shape: Oval or football‑like, no spiral arms.
  • Light: Smooth brightness, no obvious structure.
  • Stars: Mostly old, red stars; little current star formation.
  • Gas and dust: Generally poor in cold gas and dust.

b) Spiral galaxies (Sa, Sb, Sc)

Spiral galaxies have flat disks with spiral arms winding out from a central bulge.

They are subdivided by how “tight” those arms are and how big the bulge is:

  • Sa : Large bulge, tightly wound arms, smoother arms with less obvious star‑forming clumps.
  • Sb : Medium bulge, moderately wound arms, noticeable star‑forming regions.
  • Sc : Small bulge, loosely wound, patchy arms rich in gas, dust, and young blue stars.

Example: The Milky Way is classified as a barred spiral (type SB), meaning it has a central bar of stars with arms coming off the ends.

c) Barred spiral galaxies (SBa, SBb, SBc)

Barred spirals are like spirals, but with a bright bar across the center and the arms starting from the ends of the bar.

Subtypes mirror the non‑barred spiral pattern:

  • SBa : Big bulge, tight arms.
  • SBb : Intermediate bulge, moderately wound arms.
  • SBc : Small bulge, loose, clumpy arms.

The bar structure is important dynamically, since it can funnel gas toward the center and help trigger star formation or feed the central black hole.

d) Lenticular galaxies (S0)

Lenticulars are “in‑between” systems with a disk like a spiral, but no visible spiral arms.

Features:

  • Have a prominent central bulge and a disk.
  • Show little or no spiral structure.
  • Often contain older stars and relatively little ongoing star formation.

They sit in the Hubble sequence between ellipticals and spirals and are often thought of as “faded” spirals (though the real story can be more complex).

e) Irregular galaxies (Irr)

Irregular galaxies don’t fit neatly into any of the tidy categories.

Typical traits:

  • No clean overall shape, no clear bulge or arms.
  • Often very rich in gas and dust.
  • Lots of young, bright stars and active star‑forming regions.

They can be small galaxies disturbed by interactions, or systems still assembling into a more ordered structure.

3. How Astronomers Actually Classify a Galaxy

In practice, classifying a galaxy usually starts with its image and then applies a decision process.

Step‑by‑step logic

Astronomers (or algorithms) often go through questions like:

  1. Does it have a well‑defined disk?
    • If no disk, likely elliptical.
    • If a disk is present, move to the next step.
  2. Are there spiral arms?
    • If no arms but a disk and bulge → lenticular (S0).
    • If arms exist → spiral or barred spiral.
  3. Is there a bar across the center?
    • Yes → barred spiral (SB).
    • No → normal spiral (S).
  4. How tight are the arms and how big is the bulge?
    • Tight arms, big bulge → type a (Sa/SBa).
    • Intermediate → type b (Sb/SBb).
    • Loose arms, small bulge → type c (Sc/SBc).
  5. If it doesn’t match any of these patterns, especially if it’s messy, distorted, or strongly interacting, it may be labeled irregular.

Modern projects like Galaxy Zoo use crowdsourced visual classifications plus machine‑learning methods that look for the same kinds of morphological cues in a more automated way.

4. Beyond Hubble: More Detailed Systems

The Hubble sequence is simple and powerful, but more detailed classifications build on it.

De Vaucouleurs system

The de Vaucouleurs system extends Hubble’s ideas to capture more subtle details.

It adds:

  • Extra coding for:
    • Presence of rings or lenses.
    • Degree of bar strength.
    • More finely graded spiral arm structure.
  • A more continuous classification rather than strictly discrete bins.

This helps better describe the variety of spiral and barred spiral galaxies that don’t fit neatly into only three spiral subtypes.

Other schemes and modern trends

More recent approaches can classify galaxies by:

  • Color (e.g., “red and dead” vs star‑forming blue galaxies).
  • Spectral type (which indicates stellar populations and star‑formation rate).
  • Quantitative morphology (e.g., measures of concentration, asymmetry, clumpiness).

Machine‑learning systems trained on large survey images now perform much of this work, particularly for huge datasets from sky surveys.

5. Galaxy Types at a Glance (HTML Table)

Below is an HTML table summarizing common features often used when classifying galaxies.

html

<table>
  <thead>
    <tr>
      <th>Galaxy type</th>
      <th>Basic shape</th>
      <th>Key structures</th>
      <th>Typical stars</th>
      <th>Gas &amp; dust</th>
      <th>Star formation</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Elliptical (E0–E7)</td>
      <td>Round to elongated, smooth light</td>
      <td>No spiral arms, no obvious disk</td>
      <td>Mostly old, red stars</td>
      <td>Little cold gas or dust</td>
      <td>Very low ongoing star formation</td>
    </tr>
    <tr>
      <td>Lenticular (S0)</td>
      <td>Disk plus bulge, no arms</td>
      <td>Central bulge, smooth disk</td>
      <td>Mainly older stars</td>
      <td>Limited gas and dust</td>
      <td>Low to moderate, often faded</td>
    </tr>
    <tr>
      <td>Spiral (Sa–Sc)</td>
      <td>Flat disk with spiral pattern</td>
      <td>Bulge, spiral arms, often dust lanes</td>
      <td>Mix of young and old stars</td>
      <td>Moderate to rich in gas and dust</td>
      <td>Active, especially in arms</td>
    </tr>
    <tr>
      <td>Barred spiral (SBa–SBc)</td>
      <td>Disk with central bar and arms</td>
      <td>Bar across center, arms from bar ends</td>
      <td>Mix of young and old stars</td>
      <td>Moderate to rich in gas and dust</td>
      <td>Active, often enhanced by bar-driven gas flows</td>
    </tr>
    <tr>
      <td>Irregular (Irr)</td>
      <td>No regular overall shape</td>
      <td>Patchy star-forming regions</td>
      <td>Mix, often many young stars</td>
      <td>Often very rich in gas and dust</td>
      <td>Often strong, clumpy star formation</td>
    </tr>
  </tbody>
</table>

6. Mini Story: Classifying a “Mystery Galaxy”

Imagine you’re looking at a new deep‑sky image. You zoom in on a faint smudge and start asking the standard questions.

You notice a bright central region with a flat disk around it and clear spiral arms, so you immediately rule out ellipticals and pure irregulars.

Looking closer, you see a bright straight structure crossing the center from which the arms seem to emerge – that’s a bar, so you tag it as a barred spiral.

The arms are loosely wound and full of bright blue star‑forming knots, and the central bulge is not especially large; that nudges you toward classifying it as SBc instead of SBa or SBb.

With just those visual clues, you’ve placed the galaxy in a widely recognized slot in the Hubble sequence, allowing you to compare it with thousands of similar systems and study how its properties fit into broader patterns in galaxy evolution.

Information gathered from public forums or data available on the internet and portrayed here.