1.0 Physical

An overview of the Milky Way's physical statistics, spiral-arm structure, satellite galaxies, the Local Group, and the two stellar populations.

Title spread for the Milky Way Galaxy chapter, showing a top view of the spiral galaxy with the Sagittarius and Scutum arms labeled, an inset side view across 100,000 light-years, and an inset top view, between the Gamma and Alpha Quadrants.

Top view of the Milky Way with its spiral arms labeled from the core outward: 3 KPC Arm, Norma Arm, Crux Arm, Carina Arm, Orion Arm, Perseus Arm, and Outer Arm, between the Delta and Beta Quadrants, with the galactic statistics panel.

Side and top views of the Milky Way showing Local Space (a sphere 1,500 LY in diameter), globular star clusters, the dark-matter halo, the Large Magellanic Cloud (distance 179,000 LY), the Small Magellanic Cloud (distance 210,000 LY), and an arrow toward the Andromeda Galaxy (distance 2.2 million LY); marked with the routes of the S.S. Valiant (2065) and U.S.S. Enterprise.

Our Local Group includes two very large spiral galaxies—the Milky Way and the Andromeda (M33) galaxies—and several dozen smaller, mostly irregular galaxies that are clustered around these two giants. The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way, and SagDEG is a dwarf galaxy currently in the process of colliding with our own.

Globular star clusters populate the galaxy's halo, and are concentrated near the galactic core. There are also “tramp” globular clusters located between the galaxies in intergalactic space.

The Population I stars in the galaxy's spiral arms tend to be young and blue, with an age range of one million to several billion years. The Population II stars in the galactic core and globular star clusters are redder and much older, 10 billion years or more.

Galactic Statistics

Type: Sbc
Age: 13 billion years
Diameter: 100,000 light-years
Thickness at center: 15,000 light-years
Stellar population: 500,000 million stars
Orbital period of UFP: 220 million years
Mass of Milky Way: 875 billion solar masses
Mass of Sagittarius A: 2.6 million solar masses
Diameter of Sagittarius A: 15 million km
Companions of Milky Way: Large Magellanic Cloud, Small Magellanic Cloud, 200 globular star clusters, Galaxy SagDEG