Galaxy classification helps astronomers organize the huge variety of galaxies seen across the universe. This cheat sheet covers the main galaxy types, how they are labeled, and what their shapes reveal about stars, gas, dust, and formation history. Students need these patterns to interpret telescope images and connect galaxy appearance to physical properties.
Key Facts
- The three main galaxy types are spiral, elliptical, and irregular, with lenticular galaxies often listed as a transition type between spiral and elliptical.
- Spiral galaxies are classified as S if they have no central bar and SB if they have a central bar through the bulge.
- Spiral subtypes Sa, Sb, and Sc describe increasing openness of spiral arms and increasing gas and dust from Sa to Sc.
- Elliptical galaxies are classified from E0 to E7, where the number estimates how stretched the galaxy appears: n = 10(a - b)/a.
- An E0 galaxy appears nearly round, while an E7 galaxy appears very elongated.
- Lenticular galaxies are labeled S0 because they have a central bulge and disk but little gas, dust, or clear spiral arm structure.
- Irregular galaxies are labeled Irr and usually have no clear spiral or elliptical shape, often because of gravitational interactions or recent star formation.
- Galaxies with more gas and dust usually have more active star formation, so spirals and irregulars often contain many young blue stars.
Vocabulary
- Galaxy
- A large system of stars, gas, dust, dark matter, and planets held together by gravity.
- Hubble sequence
- A classification system that groups galaxies by visible shape, including elliptical, spiral, barred spiral, lenticular, and irregular forms.
- Spiral galaxy
- A disk-shaped galaxy with a central bulge and spiral arms that often contain gas, dust, and young stars.
- Elliptical galaxy
- A smooth, rounded or stretched galaxy with little gas and dust and mostly older stars.
- Barred spiral galaxy
- A spiral galaxy with a straight bar-shaped structure of stars crossing the central bulge.
- Lenticular galaxy
- A disk galaxy labeled S0 that has a bulge and disk but little gas and no obvious spiral arms.
Common Mistakes to Avoid
- Calling the Hubble sequence an evolution timeline is wrong because Sa galaxies do not automatically turn into Sb, Sc, or elliptical galaxies.
- Classifying every flat galaxy as a spiral is wrong because lenticular galaxies can be flat disks without visible spiral arms.
- Ignoring the central bar in a spiral galaxy is wrong because barred spirals use SB labels instead of S labels.
- Assuming elliptical galaxies have no stars forming anywhere is too absolute because most have little star formation, but small amounts can occur in special cases.
- Using color alone to classify a galaxy is unreliable because dust, distance, and star formation can affect color without changing the galaxy's main shape.
Practice Questions
- 1 A spiral galaxy has a clear central bar and loosely wound arms. Which classification is most likely: Sa, SBc, E3, or Irr?
- 2 An elliptical galaxy has a longest visible axis a = 100 units and a shortest visible axis b = 70 units. Use n = 10(a - b)/a to find its E classification.
- 3 A galaxy has a smooth disk and central bulge but almost no gas, dust, or spiral arms. Which type is it most likely to be?
- 4 Explain why a galaxy's shape can give clues about its gas content, star formation, and past gravitational interactions.