What shapes do galaxies have?
Galaxies can be categorised in terms of their apparent shape. The two main types are spiral and elliptical. Those which do not fit easily into either of these categories are referred to as “irregular” and “peculiar” galaxies.
In the early 20th century, Edwin Hubble classified galaxies on the basis of their shape. Hubble recognised four classes: (1) spirals and barred spirals (loosely or tightly wound); (2) ellipticals (more or less flattened or elongated); (3) lenticulars (composed of an elliptical core together with a disc containing no spiral arms); and (4) irregulars. We now know that galaxies do not slowly evolve from one type into another, as Hubble thought. Instead, collisions and mergers of spirals can form giant elliptical galaxies, while gravitational interactions can produce irregularshapes. The central bars in some spiral galaxies may come and go over billions of years.
Our Milky Way galaxy is a nice spiral. Many other galaxies are spirals, too. But the Universe also contains elliptical and irregular galaxies. Can you spot them around here?