spiral galaxy
When people picture a galaxy, they usually picture a spiral: a glowing pinwheel of stars, a bright bulge at the middle with luminous arms curling outward through a flat disk, threaded with dark dust lanes and sparkling blue star clusters. Our own Milky Way is one. A spiral galaxy is a flattened, rotating disk of stars, gas, and dust wound into a graceful pattern of spiral arms.
A spiral has a few distinct parts: a central bulge of older stars; a thin disk where almost all the action happens; spiral arms within that disk; and, surrounding it all, a faint stellar halo and a vast invisible dark-matter halo. The disk is full of cold gas, so it is actively making new stars — and the spiral arms are where that gas piles up and lights up with hot, short-lived blue stars, which is why arms look bluer and brighter than the rest of the disk. The arms are best understood not as fixed structures of fixed stars but as a traffic-jam-like density wave that stars and gas drift through. The whole disk rotates, but oddly: the outer parts move just as fast as the inner parts, a flat rotation curve that betrays the dark-matter halo.
Spirals matter because they are the universe's great star factories: most new stars today are born in spiral disks. Their orderly rotation lets astronomers weigh them and discover dark matter, and their brightness ties to their rotation speed through the Tully-Fisher relation, making them useful distance indicators. A spiral is a galaxy still vigorously alive, in contrast to the red, quiescent ellipticals — until a merger or gas starvation eventually quenches it.
The Whirlpool Galaxy (M51) is a textbook 'grand-design' spiral: two clean, sweeping arms studded with pink star-forming regions and blue young clusters, its shape sharpened by the gravity of a small companion galaxy tugging on its disk.
A rotating disk of gas and stars, arms lit blue by newborn stars — a galaxy still making suns.
Spiral arms are not solid pinwheels of the same stars; they are density waves that stars orbit through, like cars bunching through a slow patch of highway. Otherwise the arms would wind up tight in a few rotations — the 'winding problem'.