a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction

How do we know the Milky Way is a spiral galaxy?

The clues we have to the shape of the Milky Way are:
1) When you look toward the Galactic Center with your eye, you see a long, thin strip. This suggests a disk seen edge-on, rather than a ellipsoid or another shape. We can also detect the bulge at the center. Since we see spiral galaxies which are disks with central bulges, this is a bit of a tipoff.
2) When we measure velocities of stars and gas in our galaxy, we see an overall rotational motion that differs from random motions. This is another characteristic of a spiral galaxy.
3) The gas fraction, color, and dust content of our Galaxy are like other spiral galaxies.