A galaxy is a huge system of celestial bodies in which hundreds of millions to hundreds of billions of stars are held together by gravity. More than hundreds of billions of galaxies exist in the universe, and their forms and distribution create the large-scale structure of the universe.
Basic Knowledge
Galaxy morphology classification (Hubble classification):
① Spiral Galaxy: A disk-shaped structure consisting of a central bulge and spiral-shaped arms (spiral arms). It contains many young stars and interstellar gas. The Milky Way Galaxy and the Andromeda Galaxy are examples.
② Elliptical Galaxy: Spherical to elliptical in shape. It contains many old, red stars and little interstellar gas. It is common in the central regions of large galaxy clusters.
③ Irregular Galaxy: A galaxy without a regular shape. Examples include the Large Magellanic Cloud and Small Magellanic Cloud.
④ Barred Spiral Galaxy: A spiral galaxy with a bar-shaped structure at its center. The Milky Way Galaxy also belongs to this type.
The Milky Way Galaxy is a barred spiral galaxy about 100,000 light-years in diameter, containing 200 billion to 400 billion stars. The Solar System is located about 26,000 light-years from its center. At the center is a supermassive black hole (Sagittarius A*).
Spiral galaxy (A disk-shaped galaxy with a bulge and arms. The Milky Way Galaxy and Andromeda Galaxy)
Elliptical galaxy (Spherical to elliptical. Contains many old stars. Common in large galaxy clusters)
Irregular galaxy (Irregular in shape. Includes the Magellanic Clouds)
Milky Way Galaxy (100,000 light-years in diameter. A barred spiral galaxy containing the Solar System)
Bulge (The spherical swelling at a galaxy's center. Contains a dense concentration of old stars)
Galaxy cluster/supercluster (Groups of dozens to thousands of galaxies / groups of those groups)
Further Exploration (Background and Significance)
The person who confirmed that galaxies were independent "island universes" was Edwin Hubble (1924). Using Cepheid variable stars in the Andromeda Galaxy, he measured its distance and showed that it lay outside the Milky Way Galaxy.
Galaxies do not exist in isolation but form a hierarchical structure. The Milky Way Galaxy belongs to the Local Group, which consists of about 30 galaxies, and the Local Group is part of the larger Virgo Supercluster. Across the universe, galaxies are distributed in filaments, with voids (spaces containing almost no galaxies) between them, forming a "bubble structure."
The existence of dark matter was suggested by observations of the rotation speeds of galaxies. The reason rotation speeds do not decrease even in the outer parts of galaxies is thought to be that invisible dark matter forms a halo surrounding the entire galaxy.
- Galaxy types: spiral, elliptical, irregular, and barred spiral
- Milky Way Galaxy = barred spiral galaxy, 100,000 light-years in diameter
- Andromeda Galaxy = nearest large galaxy (about 2.5 million light-years away)
- Local Group → Virgo Supercluster hierarchy
- Large-scale structure of the universe = bubble structure of filaments and voids
- Dark matter = suggested by galaxy rotation speeds
- Supermassive black holes = found at the centers of many galaxies
Points of Caution (Easy to Confuse)
① Distinguish the Milky Way Galaxy (our galaxy and a proper name) from a galaxy (a general term). ② The Milky Way is a band seen from inside the Milky Way Galaxy, not its shape as seen from outside. ③ Elliptical galaxies contain many old stars, whereas spiral galaxies contain many young stars and much gas. ④ The Andromeda Galaxy is 2.5 million light-years away and is a large galaxy comparable in size to the Milky Way Galaxy.
Practice
- Give three types in the morphological classification of galaxies.
- What type is the Milky Way Galaxy?
- Describe the characteristic large-scale distribution of galaxies in the universe (the bubble structure).