The distinction between an elliptical galaxy and a "regular" galaxy is primarily based on their different shapes and structural characteristics. Here are some key differences between these two types of galaxies:
Shape: Elliptical galaxies have a rounded, elliptical or spheroidal shape, while "regular" galaxies typically refer to spiral galaxies, which have a more flattened disk shape with spiral arms.
Structure: Elliptical galaxies lack the distinctive spiral arms and flattened disk structure seen in spiral galaxies. Instead, they are characterized by a smooth and featureless distribution of stars. The stars in elliptical galaxies generally follow more random orbits within the galaxy.
Size and Mass: Elliptical galaxies can vary significantly in size, ranging from relatively small to extremely massive. They can contain a vast number of stars and have high mass concentrations at their centers. Spiral galaxies, on the other hand, tend to be less massive and have a moderate size compared to the largest elliptical galaxies.
Stellar Population: Elliptical galaxies are dominated by older stellar populations, consisting mainly of older, redder stars. They have lower rates of star formation and contain a larger proportion of low-mass, low-luminosity stars. Spiral galaxies typically exhibit a mix of stellar populations, with both younger, bluer stars in their spiral arms and older stars in the central bulge.
Dynamics: The dynamics of elliptical and spiral galaxies differ as well. Elliptical galaxies often have a more isotropic velocity distribution, meaning that the motions of stars within them are more randomly oriented. Spiral galaxies, with their flattened disk structure, have more ordered rotational motion, with stars orbiting the galactic center in a coherent manner.
It's worth noting that the universe is incredibly diverse, and there are various types of galaxies beyond the broad categorizations of elliptical and spiral galaxies. There are also irregular galaxies, which lack a well-defined shape and exhibit more chaotic structures. Additionally, there are peculiar or interacting galaxies, which have undergone interactions or mergers with other galaxies, leading to distorted shapes and unique features.