Galaxy Morphologies

A flattened, rotating disk with spiral arms, a central bulge, and a halo of old stars (and dark matter in the standard picture). Rich in gas and dust; the arms host young, blue stars and active star-forming regions. The Milky Way is a barred spiral.

A subtype of spiral with a central bar of stars, gas and dust crossing the nucleus. According to NASA, about two-thirds of spirals are barred; the bar is often associated with an advanced dynamical evolution of the disk.

Intermediate between spirals and ellipticals: they have a central bulge and a disk but no clear spiral arms. They consist mostly of old stars and show little current star formation.

Round to oval in shape, with little visible structure and very little gas or dust. Their stars are generally old, with little new star formation. Stellar orbits are more random than in spiral disks. They are often linked to galaxy mergers.

Very large ellipticals, often at the centre of galaxy clusters. Dominated by old stars, poor in cold gas, and may contain immense stellar halos. They frequently result from multiple galactic mergers.

No clear symmetric form: they may be distorted, fragmented or chaotic. Often rich in gas and dust, so they can actively form stars. Their shape frequently arises from gravitational interactions or collisions.

Low-mass, low-luminosity galaxies. They may be dwarf elliptical, dwarf spheroidal, dwarf irregular or compact dwarf. They contain far fewer stars than large galaxies; some dwarf irregulars are gas-rich and actively forming stars. NASA gives characteristic masses of about 10⁸ solar masses for certain dwarf irregulars.

An exceptionally high star-formation rate compared with a normal galaxy. Often triggered by an interaction, a merger or gas compression. Rich in molecular gas, warm dust, infrared radiation and recent supernovae.

Galaxies whose centre is far brighter than normal, owing to an accreting supermassive black hole. The accretion disk can emit from infrared to X-rays, and some active nuclei produce relativistic jets. NASA estimates that about 10% of known galaxies are active.

A type of active galaxy, generally relatively nearby and often spiral. The nucleus is very bright, with infrared and sometimes X-ray emission; some produce radio jets. Less luminous than quasars, they allow the same phenomena to be studied at lower energy.

Extremely luminous active galactic nuclei, often observed at very large distance. The nucleus can completely dominate the light of the host galaxy. They emit across a broad part of the electromagnetic spectrum and are linked to strongly accreting supermassive black holes.

A subtype of quasar/AGN in which a relativistic jet points almost directly toward Earth. Because of this alignment they appear very luminous and strongly variable. NASA notes that their brightness can change over timescales of a few months.

Active galaxies, often giant ellipticals, emitting strongly at radio wavelengths via relativistic jets and lobes. The jets can extend well beyond the visible galaxy; NASA mentions jets reaching hundreds of thousands of light-years, and more in some cases.

Two or more galaxies distorted by gravity: matter bridges, tidal tails, rings, multiple nuclei. These interactions can trigger bursts of star formation, feed an active nucleus, and gradually transform spirals into ellipticals or lenticulars.