Types of galaxies

Galaxies as stellar systems

A galaxy is a gravitationally bound system of stars, gas, dust, dark matter, and compact objects. Galaxies range from tiny dwarf galaxies with millions of stars to giant ellipticals with trillions.

Galaxy classification began with visible appearance, but modern astronomy connects morphology to dynamics, gas content, star formation, environment, and merger history.

Spiral galaxies

Spiral galaxies have flattened rotating disks, spiral arms, gas, dust, and ongoing star formation. The Milky Way is a barred spiral. Spiral arms contain bright young stars and glowing gas regions, making them visually prominent.

Spiral galaxies often have older stars in a central bulge and younger stars in the disk. Their rotation is ordered, with stars and gas orbiting around the galactic center.

Barred spirals

Many spirals have central bars. A bar is an elongated structure of stars crossing the central region. Bars can redistribute angular momentum, drive gas inward, trigger star formation, and influence spiral patterns.

Barred spirals are common, so bars are not unusual features.

Elliptical galaxies

Elliptical galaxies range from nearly spherical to elongated. They contain mostly older stars, relatively little cold gas and dust, and low current star formation. Their stellar orbits are more random than the circular disk orbits of spirals.

Ellipticals are common in dense environments such as galaxy clusters. Giant ellipticals can form through mergers of smaller galaxies.

Lenticular galaxies

Lenticular galaxies, or S0 galaxies, are intermediate between spirals and ellipticals. They have disks and bulges but little spiral structure and less gas for star formation. They may be faded spirals that lost or used up their gas.

Environment can transform galaxies by stripping gas and shutting down star formation.

Irregular galaxies

Irregular galaxies lack a regular spiral or elliptical form. They are often rich in gas and may show active star formation. Some are distorted by gravitational interactions with nearby galaxies.

Dwarf irregular galaxies are important because they can be chemically primitive and provide clues about galaxy formation.

Hubble sequence and limitations

The traditional Hubble sequence arranges galaxies as ellipticals, lenticulars, spirals, barred spirals, and irregulars. It is sometimes called the tuning fork diagram.

Despite historical names like early-type and late-type, the sequence is not a simple evolutionary path from one type to another. Galaxy evolution is more complex and depends on mass, gas supply, mergers, and environment.

Color and star formation

Galaxies are often described as blue or red. Blue galaxies usually have young massive stars and active star formation. Red galaxies often have older stellar populations and little ongoing star formation.

A useful diagnostic is specific star formation rate: star formation per unit stellar mass.

The big idea

Galaxy types reflect structure, motion, gas content, and history. Spirals are rotating, gas-rich, and star-forming; ellipticals are older, gas-poor, and dynamically hotter; irregulars are often gas-rich and disturbed. Classification is a starting point for understanding how galaxies form and evolve.