Galactic Astronomy
Milky Way
Our home galaxy — barred spiral with 200-400 billion stars and Sgr A* at center
The Milky Way is the galaxy containing our Solar System — a barred spiral galaxy with 200-400 billion stars, ~100,000 light-years across, ~1000 ly thick. We orbit at ~25,000 ly from the center, completing one orbit every 225-250 Myr. Sgr A* (4.3 million M_sun BH) anchors the center. Visible from Earth as the "Milky Way" band stretching across the night sky. Future: collision with Andromeda in ~5 Gyr to form Milkomeda. Local Group includes Andromeda, Triangulum, ~50 dwarfs.
- Star count~200-400 billion stars
- Diameter~100,000 light-years
- Mass (incl DM)~1.5 trillion M_sun
- Sun's distance from center~26,000 ly (8.0 kpc)
- Orbital period225-250 Myr (Sun)
- TypeBarred spiral (SBbc)
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why Milky Way matters
- Our galaxy. Direct observations; closest study target.
- Galactic structure. Detailed mapping of stars, gas.
- Star formation. Local star-forming regions.
- Stellar populations. Age, metal content, distribution.
- Galactic dynamics. Rotation curves; dark matter.
- Habitability. Galactic environment of Earth.
- Origin of life. Earth formed in this galaxy.
Common misconceptions
- Milky Way is unusual. Pretty average galaxy.
- Sun is at center. 26,000 ly from center.
- All stars in Milky Way are visible. Most beyond naked eye.
- Milky Way doesn't have bar. Confirmed bar.
- Milky Way is alone. Local Group; many neighbors.
- Galaxy is solid. Sparse — stars vastly separated.
Frequently asked questions
How big is the Milky Way?
Diameter ~100,000 ly; thickness ~1000 ly (disk); halo extends much further (~1 million ly). Mass 1.5 trillion M_sun (most dark matter). Stellar mass ~6×10¹⁰ M_sun. Sun is 26,000 ly from center, in the Orion-Cygnus arm.
What's the structure?
(1) Bulge — central region; mostly old stars; bar present. (2) Disk — flat rotating component; stars + gas + dust; spiral arms. (3) Halo — surrounding region; old stars, globular clusters; mostly dark matter. (4) Sgr A* — supermassive BH at center. (5) Spiral arms — Perseus, Carina-Sagittarius, Scutum-Centaurus, Orion-Cygnus.
How was the bar discovered?
Radio observations (1980s) of gas motions; later confirmed by IR/optical surveys (2MASS, GLIMPSE). Bar length ~16,000 ly. Connects ends of bar to inner ends of spiral arms. Common feature of spirals — ~70% of nearby spirals have bars.
How fast does Sun orbit?
~230 km/s circular velocity. Period: 225-250 Myr. Sun has completed ~20 orbits since formation. Galactic year = 230 Myr (~1 orbit). Sun also moves vertically — oscillates ~80 ly/Myr crossing galactic plane.
Will Milky Way collide with another galaxy?
Yes — Andromeda Galaxy (M31). Approaching at 110 km/s. Closest approach in ~4-5 Gyr. Eventual merger forms "Milkomeda" or "Andromeda-Milky Way." Sun's solar system unlikely to be directly affected (vast space between stars). New galaxy will be elliptical.
What's the Local Group?
Local cluster of galaxies bound by gravity. ~30+ confirmed members. Major: Milky Way, Andromeda (M31), Triangulum (M33). Plus many dwarf galaxies (Sagittarius dSph, Magellanic Clouds, Sculptor, Fornax). Total mass ~1.3 trillion M_sun. ~10 million ly diameter.
What's beyond the Milky Way?
Local Group → nearby galaxy groups → Virgo Cluster → Local Supercluster → Laniakea Supercluster (incl Norma, Vela, Antlia, Hydra-Centaurus, Pavo-Indus, Centaurus). Cosmic web of filaments and voids. Then beyond: Sloan Wall, Hercules-Corona Borealis Wall — biggest known structures.