Milky Way
The barred spiral galaxy containing the Solar System — our home galaxy. The galactic environment within which the local-bubble, interstellar-medium, oort-cloud, and all other aphelion notes exist. One of two dominant members of the Local Group alongside the Andromeda Galaxy.
Key Dimensions
| Property | Value |
|---|---|
| Disk diameter | ~80,000–100,000 light-years (~26 kpc) |
| Disk thickness | ~1,350 light-years (~0.4 kpc) |
| Dark matter halo diameter | ~1.9 million light-years (584 ± 122 kpc) |
| Total mass | ~0.88 trillion M☉ (8.8×10¹¹ M☉) |
| Dark matter mass | 1–1.5×10¹² M☉ (most of total) |
| Stellar mass | 4.6–6.4×10¹⁰ M☉ |
| Number of stars | 100–400 billion |
| Age | ~13.61 billion years (formed shortly after Big Bang) |
Structure
Central bar and bulge — A central barred structure surrounded by a spheroidal bulge of older stars. At the very center: Sagittarius A\* (Sgr A*), a supermassive black hole of ~4 million M☉, confirmed by stellar orbital measurements and the Event Horizon Telescope image (2022).
Disk — The flat rotating plane containing most young stars, gas, and dust. Star formation is concentrated in the disk. Warped into an S-curve shape. The Sun is located ~26,000 light-years from the Galactic Center, in the Orion-Cygnus spur — a minor arm or spur between the Perseus and Scutum-Centaurus major arms.
Spiral arms — Four arms traced by gas and young stars: Norma, Scutum-Centaurus, Carina-Sagittarius, Perseus. However, infrared surveys reveal only *two* major stellar overdensities (Perseus and Scutum-Centaurus) — the Milky Way appears to have 2 major stellar arms but 4 gas arms.
Stellar halo — A spheroidal distribution of old stars and ~150 globular clusters surrounding the disk; 90% within 100,000 light-years of the center. ~40% of clusters orbit retrograde. No active star formation.
Gaseous halo — A massive hot gas halo (1–2.5 million K) extending hundreds of thousands of light-years — nearly as massive as the entire galaxy. May account for "missing baryons" in the universe.
Solar System Location
The Solar System orbits the Galactic Center at ~220 km/s with a period of ~225–250 million years (one "galactic year"). Its current position within the local-bubble places it in a region of unusually sparse interstellar gas carved by supernovae over the past 10–20 million years.
Formation and History
- ~13.61 billion years ago: Milky Way began as small overdensities after the Big Bang
- Halo formation first: Earliest stars formed in globular clusters now comprising the stellar halo; nearly half of Milky Way matter may have come from other galaxies
- ~11 billion years ago: Major merger with the "Kraken" galaxy
- ~10–8 billion years ago: Disk formed as the galaxy spun up; star formation dropped by an order of magnitude during the hot-gas epoch when interstellar gas was too hot to form stars efficiently
- Ongoing: Currently accreting material from the Large and Small Magellanic Clouds via the Magellanic Stream
Fate
The Milky Way sits in the "green valley" on the galaxy color-magnitude diagram — transitioning from active star formation (blue cloud) toward quiescence (red sequence). Star formation will likely extinguish within ~5 billion years. In ~4 billion years, the Milky Way will collide with the Andromeda Galaxy; the merged system will likely become an elliptical galaxy, completing the transition to the red sequence.
Intergalactic Context
- Local Group: 50 closely bound galaxies dominated by the Milky Way and Andromeda
- Virgo Supercluster: The Local Group's parent supercluster
- Laniakea: The broader supercluster complex drawn toward the Great Attractor; ~500 million light-years across