On a dark night, far from city lights, a faint glowing band arches across the sky. Galileo's telescope showed it is made of countless faint stars. It is our galaxy, the Milky Way, seen from inside. Because light takes time to travel, even its starlight is old: light from the far side of the galaxy set out about 100,000 years ago.
Why it works
Why a band? The Milky Way is a flat disk, and we live inside it. Look along the disk and your line of sight passes through thousands of light-years of stars, blurring into a glow; look out of the disk and you see far fewer. Dust in the disk dims and hides the distant parts, which is why astronomers long thought the Sun was near the center. In 1918 Harlow Shapley mapped the globular clusters, which swarm around the true center, and found it lies far away in the direction of Sagittarius. Infrared and radio telescopes, which see through dust, have since mapped the whole structure.
- Disk: about 100,000 light-years across but only about 1000 thick, with spiral arms of gas, dust and young stars. The Sun lies in it, about 26,000 light-years from the center.
- Bulge and bar: a thick, crowded central region of mostly older stars, crossed by a bar.
- Center: a black hole, Sagittarius A*, of about 4 million solar masses.
- Halo: a faint, roughly round cloud of old stars and globular clusters surrounding the disk.
Key idea
The Milky Way is a barred spiral galaxy of a few hundred billion stars, a disk about 100,000 light-years across. The Sun is in the disk, about 26,000 light-years from the center, orbiting it about once every 220 million years.
Worked examples
Worked example: The galactic year
The Sun orbits the galaxy's center $26{,}000$ light-years away at about $220$ km/s. How long does one orbit take?
- Radius: $26{,}000 \times 9.46 \times 10^{15} = 2.46 \times 10^{20}$ m.
- Path: $2\pi r = 1.55 \times 10^{21}$ m. Time: $\frac{1.55 \times 10^{21}}{2.2 \times 10^{5}} = 7.0 \times 10^{15}$ s.
- In years: $\frac{7.0 \times 10^{15}}{3.156 \times 10^{7}} = 2.2 \times 10^{8}$ years, about 220 million years.
Answer: About 220 million years.
Worked example: How many laps?
The Sun is about $4.6$ billion years old. How many times has it circled the galaxy?
- $\frac{4.6 \times 10^{9}}{2.2 \times 10^{8}} \approx 21$ times.
Answer: About 20 times.
Common mistake
Two old ideas linger: that the Milky Way is the whole universe, and that the Sun sits at its center. In the 1920s Edwin Hubble showed that "spiral nebulae" such as Andromeda are galaxies far outside our own, and the observable universe holds hundreds of billions of galaxies. And the Sun is in the outer disk, about halfway out. Check: the Milky Way's band looks brightest and thickest toward Sagittarius, the direction of the center, not evenly all around.