Step by Step
E
Elliptical galaxies (E0-E7)
Smooth, ranging from round (E0) to elongated (E7). Composed mostly of old, red stars, with very little gas, dust, or ongoing star formation.
S0
Lenticular galaxies
Have a disk shape but lack spiral arms — an intermediate type between elliptical and spiral galaxies.
Sa-Sc
Spiral and barred spiral galaxies
Spiral galaxies (Sa-Sc) have arms that go from tighter with a larger bulge (Sa) to looser with a smaller bulge (Sc), along with more gas and younger stars. Barred spiral galaxies (SBa-SBc) have a central bar structure instead of a round bulge — our Milky Way is classified as SBbc.
Irr
Irregular galaxies, and an important caveat
Irregular galaxies have no clear organized structure, like the Magellanic Clouds. Crucially, Hubble's tuning fork diagram was never intended as an evolutionary sequence — galaxies don't actually transform from one type into another by moving along this diagram; it's purely a classification scheme based on appearance.
Applied Walkthrough
1
A smooth, round galaxy with old red stars and no visible ongoing star formation would be classified as an elliptical galaxy, likely near the E0 end of the range if it appears nearly circular.
2
A galaxy with a clear disk and winding spiral arms, but a central bar structure rather than a round central bulge, would be classified as a barred spiral — exactly the category our own Milky Way falls into, specifically as an SBbc type.
3
A galaxy with no organized structure at all, like the Magellanic Clouds visible from the southern hemisphere, would be classified as irregular.
4
It would be a mistake to assume a galaxy is somehow "evolving" from elliptical toward spiral (or vice versa) simply by moving along Hubble's original tuning fork diagram — the diagram was designed purely as a classification tool based on appearance, not as a depiction of how galaxies actually change over time.
Exam Application
Exams test whether you can correctly classify a described galaxy into the appropriate Hubble category (elliptical, lenticular, spiral, barred spiral, or irregular), and specifically whether you know that the tuning fork diagram does not represent an evolutionary sequence.
⚠ Common Trap
The most common trap is treating Hubble's tuning fork as an evolutionary sequence, with galaxies transforming from one end to the other over time — this was never Hubble's intention; it's purely a classification system based on observed shape.
✓ Quick Self-Check
1. What characterizes elliptical galaxies?
Smooth appearance (round to elongated), old red stars, little gas/dust, little ongoing star formation.
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2. What is a lenticular galaxy?
A galaxy with a disk shape but no spiral arms — an intermediate type.
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3. What distinguishes a barred spiral from a regular spiral galaxy?
A barred spiral has a central bar structure instead of a round bulge.
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4. What Hubble classification is the Milky Way?
SBbc — a barred spiral.
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5. Does Hubble's tuning fork diagram represent an evolutionary sequence?
No — it's purely a classification system based on shape, not a depiction of how galaxies transform over time.
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