The term galactic halo is used to denote an extended, roughly spherical component of a galaxy

Galaxy

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:

• the galactic spheroid (star
  Star
  A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
  
  s)
• the galactic corona
  Galactic corona
  The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way...
  
   (hot gas
  Interstellar medium
  In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
  
  , i.e. a plasma
  Plasma (physics)
  In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
  
  )
• the dark matter halo
  Dark matter halo
  A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas...
  
  

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies

Spiral galaxy

A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...

, where the spherical shape contrasts with the flat disc

Disc (galaxy)

A disc is a component of disc galaxies, such as spiral galaxies, or lenticular galaxies.The galactic disc is the plane in which the spirals, bars and discs of disc galaxies exist. Galaxy discs tend to have more gas and dust, and younger stars than galactic bulges, or galactic haloes.The galactic...

. In an elliptical galaxy

Elliptical galaxy

An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...

, there is no sharp transition between the body of the galaxy and the halo.

See also

• Galaxy formation and evolution
  Galaxy formation and evolution
  The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby...
  
  
• Galactic coordinate system
  Galactic coordinate system
  The galactic coordinate system is a celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy. The "equator" is aligned to the galactic plane...
  
  
• Galactic bulge
  Bulge (astronomy)
  In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...
  
  
• Disc galaxy
  Disc galaxy
  Disc galaxies are galaxies which have discs, a flattened circular volume of stars. These galaxies may, or may not include a central non-disc-like region .Disc galaxy types include* spiral galaxies** barless spiral galaxies...
  
  
  • Spiral arm
• Galactic corona
  Galactic corona
  The terms galactic corona and gaseous corona have been used in the first decade of the 21st century to describe a hot, ionised, gaseous component in the Galactic halo of the Milky Way...