The structure, composition and rotation of our Galaxy
A galaxy is a system of hundreds of billions of stars, gas, dust and dark matter bound by gravity. Our Galaxy, the Milky Way, is a barred spiral galaxy: a central bulge of stars crossed by a “bar”, a flat disc (diameter ≈ 100,000 light-years, thickness ≈ 1000 light-years) with spiral arms, and a sparse halo of gas and stars around it. It contains ≈ 100–400 billion stars. The Sun lies ≈ 26,000 light-years (≈ 8 kpc) from the centre, in the Orion Arm, so we see the disc from inside as the band of the Milky Way. The Sun revolves about the Galactic centre at ≈ 220–230 km/s, one full turn taking ≈ 230 million years (a galactic year). The rotation speed hardly falls farther from the centre – evidence of much more unseen, dark matter than visible matter: the Galaxy’s total mass is close to 10¹² M☉. At the centre lies the black hole Sgr A* of ≈ 4 million M☉.
On a dark night away from city lights (with an adult) find the band of the Milky Way in summer. It crosses Cygnus and Aquila; sketch its direction. No instruments are needed.