Final answer:
Elliptical galaxies are mostly old stars with a spherical or elliptical shape, spiral galaxies feature a mix of old and young stars with spiral arms, and irregular galaxies lack a defined shape with intense star formation.
Step-by-step explanation:
- Elliptical galaxy - B. Forms a perfect sphere or an ellipse and is flattened to some degree
- Spiral galaxy - D. Has a central core from which curved arms spiral outward
- Irregular galaxy - A. Does not have a distinct regular shape
- Galaxy - C. Is a collection of several billion stars and interstellar matter isolated in space
Spiral galaxies, like our Milky Way, are characterized by their central bulge, spiral arms, and the presence of both young and old stars. A significant attribute of these galaxies is their ongoing star formation, evident in the bright nebulae and young, hot stars, especially located within the spiral arms. Spiral galaxies can have varying amounts of interstellar matter and exhibit a range of mass from 109 to 1012 solar masses.
Elliptical galaxies, on the other hand, are dominated by older stars and lack the clear structure seen in spiral galaxies. They have very little interstellar matter and star formation is quite a rare event, having mostly occurred billions of years ago. Their shapes range from nearly spherical to slightly elongated, and their masses vary from dwarf galaxies to those more massive than any spiral galaxy.
Irregular galaxies lack a defined shape and often undergo intense star formation activity. They contain a mix of young and old stars and typically have lower masses and luminosities compared to spiral galaxies.