Understanding the structure of the Milky Way Galaxy and the position of the Solar System means understanding our place in the universe.
Basic Knowledge
The Milky Way is a barred spiral galaxy (type SBbc). It consists of a disk-shaped galactic disk about 100,000 light-years in diameter and about 1000 light-years thick, a central bulge (a spherical, swollen nucleus), and a halo that surrounds the disk spherically.
The Solar System is about 26,000 light-years from the galactic center and belongs to the Orion Arm, one of the galaxy’s spiral arms. It completes one orbit around the galactic center in about 220〜250 million years (a galactic year).
At the galactic center is the supermassive black hole Sagittarius A* (Sgr A*), whose mass is about four million times that of the Sun. In 2022, the EHT successfully obtained its first direct image.
The Milky Way belongs to the Local Group, which consists of about 50 galaxies. The largest galaxy is the Andromeda Galaxy (M31, about 2.5 million light-years away).
Galactic disk (the flat disk component of the Milky Way; 100,000 light-years in diameter, rich in young stars and gas)
Bulge (the spherical swelling at the galactic center; rich in old red stars)
Halo (the region surrounding the galaxy spherically; globular clusters and the dark matter halo are distributed there)
Sagittarius A* (Sgr A*) (the supermassive BH at the galactic center; mass about 4 million M☉)
Local Group (a group of about 50 galaxies including the Milky Way; about 10 million light-years in diameter)
Galactic year (the time required for the Solar System to orbit the galactic center once; about 220〜250 million years)
Further Study (Background and Significance)
A large amount of dark matter is distributed throughout the galactic halo and is estimated to account for about 85% of the Milky Way’s total mass. Dark matter does not emit light and therefore cannot be observed directly, but its existence is strongly suggested by the rotation curve of the galaxy’s outer regions (the rotation speed does not decrease in the outer regions, indicating invisible mass).
The Milky Way and the Andromeda Galaxy (M31) are predicted to collide and merge in about 4 billion years. The Solar System may be thrown into a different galaxy, but collisions between stars will be rare because the distances between stars are so great.
Globular clusters (old groups of stars distributed in the halo, about 10〜13 billion years old) provide evidence of early star formation in the Milky Way and an independent clue that constrains the lower limit of the age of the universe.
- Milky Way: 100,000 light-years in diameter; barred spiral galaxy
- Solar System: about 26,000 light-years from the galactic center; Orion Arm
- Galactic year: about 220〜250 million years
- Galactic center: Sgr A* (supermassive BH, about 4 million M☉)
- Local Group: about 50 galaxies including the Milky Way and Andromeda
- Dark matter: about 85% of the galaxy’s mass
- About 4 billion years from now: predicted collision and merger with Andromeda
Points of Caution (Easily Confused)
① The Milky Way is a band of light seen from inside the galactic plane, not the appearance of the entire galaxy from outside. ② Distinguish between the Milky Way Galaxy and a galaxy (the former is the name of our galaxy). ③ Sgr A* is the BH at the center of the Milky Way (smaller than M87*, about 4 million M☉). ④ The Local Group and the Virgo Cluster (a large cluster including the Local Group) are different scales.
Practice
- What is the approximate diameter of the Milky Way in light-years?
- About how many light-years from the galactic center is the Solar System?
- State the name of the supermassive object at the center of the Milky Way.