Step by Step
1
Elliptical galaxies in detail
Elliptical galaxies (E0-E7) contain old red stars, little gas or dust, and show no active star formation — their smooth appearance, and the number in their classification, directly indicates their degree of elongation (E0 nearly round, E7 quite elongated).
2
Lenticular galaxies in detail
Lenticular (S0) galaxies have a disk shape but lack spiral arms entirely, representing an intermediate type between ellipticals and true spirals.
3
Spiral galaxies in detail
Spiral galaxies (Sa-Sc) have a disk with winding arms. Sa-type spirals have tightly wound arms and a large central bulge, while Sc-type spirals have loosely wound arms and a small bulge.
4
Barred spirals and irregulars in detail
Barred spiral galaxies (SBa-SBc) have a bar-shaped structure running through their center instead of a round bulge — our own Milky Way is specifically an SBbc type. Irregular galaxies have no regular shape at all, often as a result of past collisions — the Magellanic Clouds are a well-known example.
Applied Walkthrough
1
Observing a nearly perfectly round, smooth galaxy with old red stars and no ongoing star formation, you would classify it near the E0 end of the elliptical sequence — the least elongated classification.
2
A more visibly stretched-out elliptical galaxy, still smooth but noticeably elongated, would instead be classified closer to E7.
3
A galaxy with tightly wound spiral arms and a prominent central bulge would be classified as Sa, while one with loosely wound arms and only a small bulge would be classified as Sc — with our own Milky Way falling into the barred version of this sequence, specifically as SBbc.
4
A galaxy showing no discernible regular structure at all — like the Magellanic Clouds, likely disrupted by tidal interactions with our own galaxy — would be classified simply as irregular, sitting entirely outside Hubble's tuning fork diagram.
Exam Application
Exams test whether you can place a described galaxy precisely within the Hubble classification system, including specific subtypes (like E0 vs. E7, or Sa vs. Sc), and whether you can correctly identify the Milky Way's specific classification as SBbc.
⚠ Common Trap
The most common trap is forgetting what the numbers and letters within each category actually indicate — for elliptical galaxies, the number indicates elongation (0 = round, 7 = elongated); for spirals, the letter indicates arm tightness and bulge size (a = tight arms/large bulge, c = loose arms/small bulge).
✓ Quick Self-Check
1. What does the number in an elliptical galaxy's classification (like E0 or E7) indicate?
Its degree of elongation — E0 is nearly round, E7 is quite elongated.
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2. What distinguishes an Sa spiral galaxy from an Sc spiral galaxy?
Sa has tightly wound arms and a large bulge; Sc has loosely wound arms and a small bulge.
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3. What distinguishes a barred spiral from a regular spiral?
A barred spiral has a bar-shaped structure through its center instead of a round bulge.
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4. What is the Milky Way's specific Hubble classification?
SBbc.
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5. What galaxies are a well-known example of the irregular classification?
The Magellanic Clouds.
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