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Galaxies and Large-Scale Structure
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaktikos, meaning "milky," a reference to the Milky Way galaxy.
The current understanding of galaxy types is often categorized using the Hubble sequence:
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Type |
Description |
Image Placeholder |
|---|---|---|
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Elliptical |
Smooth, featureless, elliptical light distribution. |
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Spiral |
Flat, rotating disk containing stars, gas, and dust, with a central concentration of stars known as the bulge. |
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Barred Spiral |
A spiral galaxy with a bar-shaped structure of stars extending from the center. |
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Irregular |
Galaxies that do not have a distinct regular shape. |
For further details on specific galaxy classifications, refer to File.
The Milky Way Galaxy
Our own galaxy, the Milky Way, is a barred spiral galaxy with a diameter between 100,000 and 200,000 light-years. It contains 100 to 400 billion stars.
Key components of the Milky Way:
- Galactic Center: Contains a supermassive black hole, Sagittarius A*.
- Disk: Where most of the young stars, gas, and dust reside.
- Halo: A large, spherical component containing older stars and globular clusters.
Large-Scale Structure of the Universe
Galaxies are not distributed randomly throughout space; they cluster together to form a hierarchy of structures.
Hierarchy of Cosmic Structures
- Groups: Collections of a few to a few dozen galaxies (e.g., The Local Group).
- Clusters: Gravitationally bound structures containing hundreds to thousands of galaxies.
- Superclusters: Massive groups of galaxy clusters, forming the largest known structures in the universe (e.g., The Laniakea Supercluster).
- Filaments and Voids: Superclusters are organized into long, thread-like structures called filaments, which surround vast, empty regions called voids, creating a large-scale "cosmic web."
The Local Group, which includes the Milky Way and Andromeda, is located near the edge of the Laniakea Supercluster, approximately .
Galaxy Evolution and Mergers
Galaxies grow and change over cosmic time through mergers and accretion.
When two galaxies interact or collide, the gravitational forces can trigger intense bursts of star formation, known as starbursts, and dramatically alter the shape of the galaxies.
Current research suggests the Andromeda galaxy will collide with the Milky Way in approximately 4.5 billion years, forming a new, larger elliptical galaxy dubbed "Milkomeda."
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