There are hundreds of billions of galaxies in the universe. The Hubble classification (tuning-fork diagram), which classifies galaxies by shape, is fundamental to galaxy morphology.
Basic Knowledge
A galaxy is a celestial system in which hundreds of millions to hundreds of billions of stars, gas, dust, and dark matter are gravitationally bound. In 1926, Hubble classified galaxies by morphology and represented them as the tuning-fork diagram (Hubble tuning fork).
Main morphological classifications:
・Elliptical galaxies (E type): spherical to elliptical. E0 (spherical) to E7 (elongated ellipse). They contain little gas and few young stars and appear reddish.
・Spiral galaxies (S type): disk-shaped with spiral arms. Sa (tightly wound arms and large nucleus) to Sd (open arms and small nucleus). Young stars are abundant and star formation is active.
・Barred spiral galaxies (SB type): spiral galaxies with a bar-shaped structure at the center. The Milky Way is type SBbc.
・Irregular galaxies (Irr type): irregular in shape. The Magellanic Clouds are examples.
・Lenticular galaxies (S0 type): an intermediate form between elliptical and spiral galaxies.
Elliptical galaxy(E type) (spherical to elliptical; an old galaxy in which star formation has almost ended; reddish)
Spiral galaxy(S type) (disk + spiral arms; rich in young blue stars and interstellar gas)
Barred spiral galaxy(SB type) (a bar-shaped central structure; the Milky Way is type SBbc)
Irregular galaxy(Irr type) (no clear shape; the Large and Small Magellanic Clouds are representative examples)
Tuning-fork diagram (Hubble’s galaxy morphology classification diagram; changes in shape are represented in a tuning-fork form)
Active galactic nucleus(AGN) (the nucleus of a galaxy that contains a supermassive BH at its center and emits intense radiation)
Further Study (Background and Significance)
Hubble classification was initially considered to suggest an evolutionary direction from “elliptical → spiral,” but modern understanding holds that morphology changes through mergers and environment, not a fixed evolutionary sequence. Galaxies repeatedly collide and merge (galaxy mergers), and small galaxies can combine to form large elliptical galaxies.
A supermassive black hole (SMBH, with a mass of 10⁶〜10¹⁰M☉) lies at the center of a large elliptical galaxy. There is a strong correlation between the mass of a galaxy and that of its SMBH, and they are thought to have coevolved.
In 2019, the Event Horizon Telescope (EHT) took the first image of the black hole shadow at the center of the M87 galaxy (BH mass about 6.5 billion M☉).
- Elliptical galaxies(E): old stars, reddish, little gas
- Spiral galaxies(S): young stars, blue, rich in gas
- Barred spiral galaxies(SB): the Milky Way is one of these (type SBbc)
- Irregular galaxies(Irr): the Magellanic Clouds are representative
- Galaxy mergers often produce large elliptical galaxies
- Supermassive BHs lie at the centers of large galaxies
- In 2019, the EHT took the first image of M87’s BH shadow
Points of Caution (Easily Confused)
① Hubble classification is a morphological classification, not an evolutionary sequence (according to modern interpretation). ② A barred spiral galaxy is a type of spiral galaxy (SB is S + Bar). ③ The Milky Way is not simply a spiral galaxy but a barred spiral galaxy. ④ The Magellanic Clouds are satellite galaxies of the Milky Way and are classified as irregular galaxies.
Practice
- State the morphology of the Milky Way according to Hubble classification.
- Describe the characteristics of elliptical galaxies in terms of stellar age and color.
- What type of galaxies are the Large and Small Magellanic Clouds according to Hubble classification?