Milky Way vs Andromeda (M31)
| Milky Way | Andromeda (M31) | |
|---|---|---|
| Diameter | ~100,000 light-years | ~152,000 light-years |
| Stars (estimate) | 100-400 billion | ~1 trillion |
| Mass (incl. dark matter) | ~1.5 × 10¹² M☉ | ~1.5 × 10¹² M☉ (recent estimates converging) |
| Distance from us | We are inside it | 2.537 million light-years |
| Type | Barred spiral (SBbc) | Spiral (SA(s)b) |
| Naked-eye visibility | Galactic core visible as a band | Faint smudge from dark sites |
| Apparent size in sky | 360° (we are inside) | 3° × 1° (6× the full Moon's diameter) |
| Approach speed | N/A | ~110 km/s toward us |
| Collision estimated in | ~4.5 billion years | ~4.5 billion years |
Verdict
Andromeda is somewhat bigger and richer in stars, but in mass the two are surprisingly close. They will merge in 4-5 billion years to form an elliptical galaxy sometimes nicknamed "Milkomeda".