The characteristics that are considered most typical of an old galaxy in decline are:
- Low gas content: As a galaxy ages, it tends to deplete its supply of interstellar gas, which is crucial for the formation of new stars. Therefore, older galaxies generally have lower gas content compared to younger galaxies.
- Little star formation: A declining galaxy typically experiences a decrease in the rate of star formation. This is due to the depletion of gas and a decrease in the availability of the materials necessary for new star formation.
- Shrinkage in size: Over time, gravitational interactions with other galaxies and the loss of gas can lead to the contraction or shrinking of an old galaxy. This can result in a smaller overall size compared to its earlier stages.
- High dust content: Older galaxies often have a higher dust content. Dust particles can accumulate over time through various processes, such as the remnants of dead stars and supernova explosions. The presence of high dust content can make the galaxy appear redder in color.
- Relatively small brightness: While older galaxies can still emit light, their overall brightness tends to be lower compared to younger, more active galaxies. This decrease in brightness is typically a result of the reduced star formation rate and depletion of gas.
It is important to note that the presence of spiral structure and abundant planets are not necessarily characteristic of an old galaxy in decline. Spiral structure can be observed in both young and old galaxies, and the presence of abundant planets is unrelated to the age or decline of a galaxy.