National Solar Observatory/Sacramento Peak
Galaxy

       Bigger  Smaller

                                       +-- Star Clusters
                                       |
Types: Galaxy Cluster ---- Galaxy    --+-- Clouds
                             :         |
                           Spiral      +-- Interstellar Medium
                           Elliptical
                           Barred Spiral
                           Lenticular
                           Irregular
                           Quasar
                           BL Lac
                           Radio
                           Seyfert
                           N Galaxies

Size:    5 Mpc        ---- 50 kpc    ----- 5 pc

Earth: Local Group    ---- Milky Way ----- (none)

A galaxy is a large collection of stars, gaseous clouds, and interstellar medium that are all bound together by gravity and that is separated from other galaxies by vast expanses of virtually starless space.

Galaxies come in different kinds. The classification scheme used most often today is due to Edwin Hubble (of Hubble-constant fame). In Hubble's scheme, there are four major galaxy classes which may each be subdivided:

• S = spiral galaxies.
• E = elliptical galaxies.
• SB = barred spiral galaxies.
• Irr = irregular galaxies.

The picture shows three galaxies in the constellation Leo: M 105 (an elliptical galaxy), NGC 3384 (a lenticular galaxy), and NGC 3389 (a spiral galaxy). (Picture from the STScI Digitized Sky Survey).

If a galaxy does not fit one of these classes because it is strange qin some way, then it may be classed as P (= peculiar). In addition to this classification scheme that is based on visual appearance, galaxies are also classified based on how they behave at other wavelengths:

• Quasars
• Radio galaxies
• Seyfert galaxies
• BL Lac objects
• N galaxies

Our own galaxy is a spiral galaxy, probably of type Sb or Sc.

Hubble Deep Field

The Hubble Space Telescope looked at a small patch in the sky for many days in December 1995 to get the Hubble Deep Field. Because the telescope looked in this direction for so long, galaxies could be detected in the pictures that were very much fainter (with greater magnitudes) than had ever been seen before. Every smudge in this picture is a galaxy, and many point-like objects are galaxies, too. For more information, see the Astronomy Picture of the Day for 24 January 1996.

[LS 25 March 1998]