Step by Step
1
Size and structure
The Milky Way is about 100,000 light-years in diameter, with a disk thickness of roughly 1,000 light-years, surrounded by a much larger dark matter halo extending to about 600,000 light-years. It's home to roughly 300 billion stars.
2
Barred spiral structure and our location
Classified as a barred spiral (SBbc), with a central bar roughly 27,000 light-years long. Our solar system sits in the Orion Arm, about 26,000 light-years from the galactic center.
3
Sagittarius A* — the galactic center
At the Milky Way's center sits Sagittarius A* (Sgr A*), a supermassive black hole with a mass of about 4 million solar masses. Its existence and mass were confirmed by tracking the orbits of stars near the galactic center — work that earned Andrea Ghez and Reinhard Genzel the Nobel Prize in 2020.
4
Rotation and satellites
At the Sun's location, the galaxy rotates at about 220 km/s. The Milky Way has satellite galaxies, most notably the Large and Small Magellanic Clouds.
Applied Walkthrough
1
Looking up at a dark night sky, you're seeing our own galaxy from the inside — a barred spiral roughly 100,000 light-years across, containing an estimated 300 billion stars, of which our Sun is just one.
2
Our solar system sits in the Orion Arm, roughly 26,000 light-years from the galactic center, orbiting the galaxy's center at approximately 220 km/s.
3
At that distant galactic center sits Sagittarius A*, a supermassive black hole with about 4 million times the Sun's mass — its existence was confirmed by tracking the precise orbits of stars swinging closely around it, a discovery that earned Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in Physics.
4
Beyond the visible disk, the Milky Way's dark matter halo extends much farther still — out to roughly 600,000 light-years — vastly larger than the visible galaxy itself, and orbited by satellite galaxies like the Large and Small Magellanic Clouds.
Exam Application
Exams test whether you know the Milky Way's basic size and structure figures (diameter, star count, classification), whether you know Sagittarius A*'s mass and how its existence was confirmed, and whether you know our solar system's location within the galaxy.
⚠ Common Trap
The most common trap is confusing the size of the visible galactic disk (about 100,000 light-years) with the much larger dark matter halo (extending to about 600,000 light-years) — these are two very different scales describing different components of the galaxy.
✓ Quick Self-Check
1. What is the Milky Way's approximate diameter, and how many stars does it contain?
About 100,000 light-years in diameter, with roughly 300 billion stars.
Tap to reveal / hide
2. What is the Milky Way's Hubble classification?
SBbc — a barred spiral.
Tap to reveal / hide
3. What is Sagittarius A*, and what is its approximate mass?
The supermassive black hole at the Milky Way's center, about 4 million solar masses.
Tap to reveal / hide
4. How was Sgr A*'s mass confirmed, and who won a Nobel Prize for this work?
By tracking the orbits of stars near the galactic center; Andrea Ghez and Reinhard Genzel (2020 Nobel Prize).
Tap to reveal / hide
5. Where is our solar system located within the Milky Way?
The Orion Arm, about 26,000 light-years from the galactic center.
Tap to reveal / hide