9.1 Galaxy classification and properties

Galaxies come in various shapes and sizes, each with unique characteristics. The Hubble classification scheme organizes them into elliptical, spiral, lenticular, and irregular types based on their appearance. This system helps astronomers understand galaxy evolution and structure.

Each galaxy type has distinct properties. Spirals have young and old stars, gas, and dust, allowing ongoing star formation. Ellipticals are mostly old stars with little gas. Irregulars lack symmetry but can have active star formation. Most galaxies harbor supermassive black holes at their centers.

Galaxy Classification and Properties

Hubble galaxy classification scheme

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• Organizes galaxies based on their morphology and appearance (Hubble tuning fork diagram)
• Main categories include:
  • Elliptical galaxies (E0 to E7) have smooth, elliptical shapes and are numbered based on their ellipticity, with E0 being nearly spherical and E7 being highly elongated
  • Spiral galaxies feature a disk-like structure with spiral arms and are further divided into normal spirals (Sa, Sb, Sc) and barred spirals (SBa, SBb, SBc), with the former having decreasing bulge size and spiral arm tightness from Sa to Sc, and the latter featuring a bar-like structure across the center
  • Lenticular galaxies (S0) are a transitional type between elliptical and spiral galaxies, with a disk-like structure but no spiral arms
  • Irregular galaxies (Irr) lack symmetry and regular structure and do not fit into the other categories

Properties of galaxy types

• Spiral galaxies:
  • Contain a mix of young and old stars and are rich in gas and dust, allowing for ongoing star formation
  • Feature a disk-like structure with spiral arms and a supermassive black hole at the center (Milky Way, Andromeda galaxy)
• Elliptical galaxies:
  • Primarily composed of old, red stars with low amounts of gas and dust, resulting in little to no ongoing star formation
  • Have a smooth, elliptical shape without spiral arms and a supermassive black hole at the center (Messier 87, Centaurus A)
• Irregular galaxies:
  • Lack symmetry and regular structure and are often smaller than spiral and elliptical galaxies
  • Contain a mix of young and old stars and may have high amounts of gas and dust, allowing for ongoing star formation in some regions (Large Magellanic Cloud, Small Magellanic Cloud)

Galaxy morphology and composition

• Spiral galaxies contain a mix of young, blue stars and old, red stars, are rich in gas and dust, and have ongoing star formation in the spiral arms
• Elliptical galaxies are primarily composed of old, red stars, have low amounts of gas and dust, and feature little to no ongoing star formation
• Irregular galaxies contain a mix of young and old stars and may have high amounts of gas and dust, allowing for ongoing star formation in some regions

Supermassive black holes in galaxies

• Supermassive black holes (SMBHs) are found at the centers of most massive galaxies and have masses ranging from millions to billions of times the mass of the Sun
• The mass of an SMBH correlates with the mass of its host galaxy's bulge (M-sigma relation), suggesting a co-evolution of SMBHs and their host galaxies
• Active galactic nuclei (AGN) are powered by the accretion of matter onto the SMBH and can outshine the entire host galaxy, with types including quasars, Seyfert galaxies, and blazars
• Feedback from SMBHs can affect galaxy evolution, with outflows and jets heating and expelling gas, suppressing star formation, which may explain the lack of gas and star formation in massive elliptical galaxies