What are galaxies? Definition and observation The sun, moon, and the millions of billions of stars that we see in the clear night sky are all part of a vast structure called a galaxy. Galaxies are the basic building blocks of the universe, and they contain stars, gas, dust, and dark matter. Our own galaxy, which we call the Milky Way, is a spiral shape and contains about 200 billion stars. The study of galaxies plays an important role in understanding the mysteries of the universe. By understanding these vast structures, we can gain information about the birth and death of stars, the evolution of the universe, and even the potential existence of extraterrestrial life. Let us now know in more depth what galaxies are and what their structure is like. **Types of galaxies Galaxies can vary greatly in size and structure. They are generally classified into three main types: - Spiral galaxies: These are the most common type of galaxies, with a shape like a flat disc, with a bulge in the middle and spiral arms extending from it. Our galaxy is also a spiral galaxy. Spiral galaxies - Elliptical galaxies: These galaxies are football-shaped or egg-shaped, with no spiral arms. They are generally more massive than spiral galaxies. - Irregular galaxies: These galaxies do not have a regular shape. They are often the result of collisions between galaxies. Irregular galaxies **Structure of galaxies The structure of a galaxy is determined by its type. Spiral galaxies have a central bulge, from which spiral arms extend. The bulge is made up of old, red stars, while the arms are made up of younger, blue stars. The arms are also home to a lot of gas and dust, which is where new stars are born. Elliptical galaxies do not have spiral arms. They are made up of old, red stars, and they are generally more massive than spiral galaxies. Irregular galaxies do not have a regular shape. They are often the result of collisions between galaxies.