The Giant Stellar Systems of the Universe
A galaxy is a vast gravitationally bound system containing stars, planets, gas, dust, dark matter and, in most cases, a supermassive black hole at its centre. Galaxies range in size from small dwarf galaxies containing only a few million stars to giant elliptical galaxies containing more than one hundred trillion stars.
Astronomers estimate that the observable Universe contains hundreds of billions of galaxies, each evolving over billions of years through star formation, mergers and gravitational interactions.
| Property | Typical Values |
|---|---|
| Age | Up to more than 13 billion years |
| Number of Stars | Millions to trillions |
| Diameter | 1,000 to over 1,000,000 light-years |
| Main Components | Stars, gas, dust, dark matter, black holes |
| Dominant Force | Gravity |
| Galaxy Type | Description | Example |
|---|---|---|
| Spiral | Flattened disc with spiral arms and active star formation. | Milky Way, Andromeda |
| Barred Spiral | Spiral galaxy with a central bar of stars. | Milky Way (classified as barred spiral) |
| Elliptical | Smooth, elliptical shape with older stellar populations. | M87 |
| Lenticular | Intermediate between spiral and elliptical galaxies. | NGC 5866 |
| Irregular | No well-defined structure, often distorted by interactions. | Large Magellanic Cloud |
| Dwarf | Small galaxies containing relatively few stars. | Draco Dwarf Galaxy |
Galaxies began forming a few hundred million years after the Big Bang. Small density fluctuations in the early Universe grew under the influence of gravity. Dark matter formed large gravitational halos that attracted ordinary matter, allowing gas to cool and collapse into the first stars and galaxies.
Over billions of years galaxies continued to grow through:
The Milky Way belongs to a collection of more than fifty galaxies known as the Local Group. Its largest members include:
Astronomers study galaxies using telescopes operating across the entire electromagnetic spectrum.
| Wavelength | Information Obtained |
|---|---|
| Radio | Cold hydrogen gas and jets from black holes. |
| Infrared | Dust clouds and star-forming regions. |
| Visible | Stars, nebulae and galaxy morphology. |
| Ultraviolet | Young massive stars. |
| X-ray | Black holes, neutron stars and hot gas. |
| Gamma-ray | Extreme energetic events. |
Galaxies will continue to evolve for trillions of years. Star formation will gradually slow as available gas is consumed. Many galaxies will merge into larger systems, while the accelerated expansion of the Universe will cause distant galaxies to recede beyond our observable horizon.
Ultimately, the Universe may evolve toward a cold, dark state in which most stars have exhausted their nuclear fuel and galaxy evolution has largely ceased.