We assume the stars to be gathered together into galaxies and that these form the basic building blocks of the visible matter in the Universe. Many of the galaxies are similar to our own Milky Way, but many are rather different.
The adjacent image shows the Hubble Deep Field: the Hubble Space Telescope was pointed at the same region of the sky (in the constellation Ursa Major) for 10 consecutive days and images were combined to give the most distant optical view of the Universe yet obtained. In this image, almost every smudge is a galaxy, and objects down to visual magnitude +30 can be seen.
Because looking out to such large distances implies looking back in time because of the finite speed of light, this image is actually giving us a view of the Universe early in the history of galaxy formation. This image may represent what the Universe looked like only a billion years or so after the Big Bang.
The Hubble Classification
Galaxies may be viewed as the basic building blocks for the large-scale visible stucture of the Universe. There are may galaxy types, having rather diverse features. Therefore, it is useful to have a way to classify galaxies into different types.
The Tuning Fork Diagram
Hubble introduced the classification scheme illustrated in the following figure, which separates most galaxies into elliptical, normal spiral, and barred spiral categories, and then sub-classifies these categories with respect to properties such as the amount of flattening for elliptical galaxies and the nature of the arms for spiral galaxies. The galaxies that do not fit into these categories are classified separately as irregular galaxies.
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| Hubble Classification of Galaxies |
This diagram is termed the Hubble classification scheme, or (because of its shape) the "tuning fork diagram".
Examples of Hubble Galaxy Types
Here are some examples of specific galaxies that illustrate some of the Hubble classification types.
- M81: Type Sb spiral
- NGC2997: Type Sc spiral
- M95: Type SBa barred spiral
- NGC1365: Type SBb barred spiral
- Leo I: Type E3 (dwarf) elliptical
- M110: Type E6 elliptical
- Small Magellanic Cloud: Irregular type
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