The solar system is part of a large collection of stars called the Milky Way Galaxy. Many more galaxies exist throughout the universe.
Basic Knowledge
The Milky Way Galaxy is a spiral galaxy containing approximately 200 billion〜400 billion stars，with a diameter of approximately 100,000 light-years and a thickness of approximately 1,000 light-years。The solar system is located approximately 26,000 light-years from the center and completes one orbit around the galactic center in approximately 200 million years。
The Milky Way crossing the night sky is our view of the Milky Way Galaxy from within，a phenomenon in which numerous stars in the galaxy's disk appear as a faint band of light。
At the center of the Milky Way Galaxy is a supermassive black hole (Sagittarius A*)，which has approximately 4 million times the mass of the Sun。
There are hundreds of billions or more galaxies in the universe，and galaxies are organized hierarchically into galaxy groups，galaxy clusters，and superclusters。The distribution of galaxies is not uniform，but forms a threadlike large-scale structure (the cosmic web)。
Milky Way Galaxy（A spiral galaxy containing the solar system。Diameter: 100,000 light-years）
Milky Way（A band of stars seen from within the Milky Way Galaxy）
Light-year（The distance light travels in one year。Approximately 9.46 trillion km）
Galaxy（A large group of stars。Classified by shape as spiral，elliptical，or irregular galaxies）
Galaxy clusters and superclusters（Groups of tens〜thousands of galaxies，and even larger groups made from them）
Andromeda Galaxy（Approximately 2.5 million light-years from the Milky Way Galaxy。The nearest large galaxy）
In Depth (Background and Significance)
Until the early 20th century，the Milky Way Galaxy was thought to be the entire universe。In 1924，Edwin Hubble showed that the Andromeda Nebula was another galaxy outside the Milky Way Galaxy (the Andromeda Galaxy)，completely changing our view of the universe。
Galaxies form even larger structures。Our Milky Way Galaxy belongs to the Local Group，which consists of approximately 30 galaxies，and its center is the common center of mass of the Andromeda Galaxy and the Milky Way Galaxy。In approximately 4.5 billion years，the Andromeda Galaxy and the Milky Way Galaxy are expected to collide and merge，forming an elliptical galaxy called “Milkomeda”。
The large-scale structure of the universe is organized by the gravity of dark matter。There is more than five times as much dark matter as observable matter，and it is essential for explaining the rotation speeds of galaxies and the clustering of matter in the universe。
- Milky Way Galaxy=100,000 light-years in diameter・spiral galaxy
- The Sun is 26,000 light-years from the center
- Number of stars in the Milky Way Galaxy=200 billion〜400 billion
- A supermassive black hole lies at the galactic center
- Milky Way=the band seen from within the Milky Way Galaxy
- There are hundreds of billions or more galaxies
- Andromeda Galaxy=the nearest large galaxy (2.5 million light-years)
Important Distinctions (Commonly Confused)
① Do not confuse the Milky Way Galaxy (our galaxy，used as a proper name) with a galaxy (a common noun；many exist in the universe)。② The Milky Way is the Milky Way Galaxy seen from within，not an illustration of the galaxy viewed from outside。③ A light-year is a unit of distance，not time。④ The Andromeda “Nebula” and the Andromeda “Galaxy” are the same celestial object (older name vs modern name)。
Practice
- Approximately how many light-years is the diameter of the Milky Way Galaxy?
- What is the faintly glowing band seen when viewing the Milky Way Galaxy from within called?
- What is the nearest large galaxy to the Milky Way Galaxy?