The Local Group
Our gravitational neighborhood — the ~80+ galaxies bound together by gravity, spanning ~10 million light-years. The arena for any conceivable intergalactic travel.
Key Facts
- Size: ~10 million ly across
- Members: 80+ known galaxies (3 large spirals + dozens of dwarfs)
- Total mass: ~2-3 trillion solar masses (including dark matter)
- Center of mass: Between Milky Way and Andromeda
The Major Members
| Galaxy | Type | Distance (ly) | Diameter (ly) | Stars | Notes |
|---|---|---|---|---|---|
| Milky Way | SBbc barred spiral | 0 (home) | ~100,000 | 200-400 billion | Our galaxy |
| dest-andromeda (M31) | SA(s)b spiral | 2,537,000 | ~220,000 | ~1 trillion | Largest in group, approaching us |
| Triangulum (M33) | SA(s)cd spiral | 2,730,000 | ~60,000 | ~40 billion | 3rd largest, may orbit Andromeda |
Satellite Galaxies of the Milky Way
| Galaxy | Distance (ly) | Type | Notes |
|---|---|---|---|
| Sagittarius Dwarf | 70,000 | dSph | Being consumed by Milky Way |
| dest-large-magellanic-cloud | 160,000 | SBm irregular | Largest satellite, visible naked eye |
| Small Magellanic Cloud | 200,000 | SBm irregular | Visible naked eye from Southern Hemisphere |
| Ursa Minor Dwarf | 200,000 | dSph | |
| Draco Dwarf | 260,000 | dSph | |
| ~50+ ultra-faint dwarfs | Various | dSph/UFD | Many discovered in 2010s-2020s |
Intergalactic Travel Prospects
Even within the Local Group, distances are staggering:
| Destination | Distance | At 0.9c (Earth time) | At 0.9c (ship time) |
|---|---|---|---|
| dest-large-magellanic-cloud | 160,000 ly | 178,000 years | 77,500 years |
| dest-andromeda | 2.54M ly | 2.82M years | 1.23M years |
| Triangulum | 2.73M ly | 3.03M years | 1.32M years |
| Edge of Local Group | ~10M ly | 11.1M years | 4.84M years |
Bottom line: Intergalactic travel within the Local Group is beyond any plausible biological civilization. Only relevant for:
- Self-replicating von Neumann probes (million-year timescales)
- Civilizations that have transcended biological form
- tech-alcubierre-drive or equivalent spacetime manipulation
The Milky Way-Andromeda Merger
In ~4.5 billion years, the Milky Way and dest-andromeda will merge. This is the one intergalactic “trip” that requires no propulsion — the galaxies come to us. The merger will:
- Take ~1-2 billion years to complete
- Likely form an elliptical galaxy (“Milkdromeda”)
- Not destroy individual star systems (space between stars is vast)
- Dramatically reshape the night sky over millions of years
Beyond the Local Group
The Virgo Cluster (54 million ly) and beyond are receding due to cosmic expansion. Eventually (tens of billions of years), everything outside the Local Group will be unreachable even at light speed — the universe’s expansion will carry them beyond our cosmic horizon.
The Local Group is our permanent neighborhood. Everything we will ever reach, across all of cosmic time, is within these ~80 galaxies.