What defines the structure of a galaxy?

The structure of a galaxy is defined by its mass, shape, size, and the distribution of its stars, gas, and dark matter.

A galaxy's structure is primarily determined by its mass, which includes the combined mass of all its stars, gas, and dark matter. The mass of a galaxy influences its gravitational pull, which in turn affects the movement and distribution of its components. For instance, a galaxy with a higher mass will have a stronger gravitational pull, causing its stars and gas to cluster more densely.

The shape of a galaxy is another defining factor. Galaxies can be spiral, elliptical, or irregular in shape. Spiral galaxies, like our own Milky Way, have a flat, rotating disc containing stars, gas, and dust, with a central bulge composed of older stars. Elliptical galaxies are round or elliptical in shape and contain mostly older stars with little gas or dust. Irregular galaxies, as the name suggests, do not have a regular shape and are often chaotic in appearance, with large amounts of gas and dust for new star formation.

The size of a galaxy also plays a role in its structure. Galaxies can range in size from dwarf galaxies, with just a few billion stars, to giant galaxies with over a trillion stars. The size of a galaxy can influence its structure by determining the density and distribution of its stars and other components.

Lastly, the distribution of a galaxy's stars, gas, and dark matter can define its structure. Stars in a galaxy are not evenly distributed but are often found in clusters or along certain paths, known as arms in spiral galaxies. Gas and dust in a galaxy are typically found in the interstellar medium, the space between stars, and can be concentrated in certain areas, leading to star formation. Dark matter, although invisible, makes up a significant portion of a galaxy's mass and can influence its structure through its gravitational effects.