### aggregation | galaxy:

Active galaxy
* Active galaxies are amongst the most exciting and challenging phenomena in modern astrophysics
  - brighter than normal galaxies but fainter than quasars
  - usually many, many times brighter than their normal galaxy counterparts
* Active galaxies contain an accretion disk around a central black hole, with two perpendicular jets
  - tightly beamed jets, along which matter becomes ultra relativistic
  - emit abnormally large amounts of energy from their central nuclei
  - have more luminosity than normal galaxies
  - produce significant emission at their centers
  - show especially violent activity, usually near the centers of the galaxies
* Some active galaxies eject long jets of gas from their nuclei
  - emit jets and blobs of highly energetic, electrically charged particles

Distant galaxy
* Distant galaxies are also sometimes relatively strong sources of radio waves
  - anomalously blue
  - smoother than close ones
* Many distant galaxies look significantly different from their nearby counterparts.
* More distant galaxies are fainter
  - have larger shifts, which means they are moving away even faster
  - recede faster from Earth than nearby galaxies

### aggregation | galaxy:

Elliptical galaxy
* Elliptical galaxies appear as reddish blobs.
* Elliptical galaxies are actually the most common type of galaxy
  - almost void of interstelar gas and dust
  - galaxies that are shaped like ellipses
  - just big, ovoid blobs of stars
  - more common in large clusters of galaxies than spirals are
  - oval and more compact, lacking the arms
  - probably a mix of all three shapes
  - rather like the halo component of spiral galaxies
  - round or egg-shaped, often with a bright, starlike center
  - sort of the galactic equivalent of a heap of tangled rubble
  - spheroidal in shape, and range from perfectly round to quite flattened
* Elliptical galaxies are the most common in the universe
  - numerous type of galaxy
  - vast ellipsoids of stars
  - very unphotogenic
  - consist of old stars and have little or no gas and dust
* Elliptical galaxies contain older stars and very little dust and gas
  - very little gas and dust
  - only old stars, and so they appear in a uniform color as seen above
  - form when two spiral galaxies collide
  - have a very large range of sizes
  - make up both the largest and smallest of all galaxies
  - range in shape from almost perfect spheres to flattened globes
* Elliptical galaxies show only small evidence for young stars, dust or gas
  - very little structure and are flat to spherical in shape
  - tend to have a reddish color - being relatively devoid of young stars
* Some elliptical galaxies appear to have more hot horizontal-branch stars than others
  - are nearly circular in shape
  - resemble nebula, or large clouds of gas, because they are so old
* is galaxy.

### aggregation | galaxy:

Irregular galaxy
* Irregular galaxies are ot disk-like or spheroidal and have no nucleus.
* Irregular galaxies are usually filamentary or very clumpy in shape
  - small and have no particular shape
  - very gas-rich systems as a class
  - are, well, irregular
  - can come in many different shapes
  - exhibit no uniform structure overall and have no discernable arms
  - get their odd shapes in many ways
  - have an undefined shape, and are full of young stars, gas, and dust
* Irregular galaxies have no apparent structure or shape
  - obvious spiral or elliptical structure
  - organized shape, no symmetry, contain young stars and much gas
  - specific shape
  - odd differing shapes and are hard to classify

Large galaxy
* All large galaxies appear to contain super-massive black holes.
* Large galaxies can have more than a trillion stars
  - swallow their smaller neighbors in slow acts of cosmic cannibalism
* Large galaxies have more than a trillion stars
* Many large galaxies are the result of mergers of smaller galaxies.