Final answer:
An active galaxy displays signs of energetic activity in its nucleus, often featuring a supermassive black hole and bright emissions across wavelengths. The color of a galaxy hints at the age of its star population, with blue suggesting recent star formation and red indicating older stars. Jets and mergers are common features of active galaxies.
Step-by-step explanation:
To identify if a galaxy is active or not, we can examine its nucleus for signs of energetic activity that releases a significant amount of energy. For instance, active galaxies or active galactic nuclei (AGN) exhibit characteristics such as jets of high-energy particles, intense electromagnetic radiation, and often feature a supermassive black hole at their center. A telltale sign of an active galaxy is the presence of a central point that is exceptionally bright across a range of wavelengths, especially in the radio and X-ray spectrums.
The color of a galaxy can also provide valuable information about the types of stars it contains. Blue galaxies are typically indicative of hot, massive, young stars and ongoing star formation. In contrast, galaxies that appear more yellowish-red are likely to be composed of older stars that formed billions of years ago. These color observations can roughly estimate the star populations within galaxies, especially when detailed spectral analysis is not possible due to the faint nature of the light received.
Additional evidence that distinguishes an active galaxy includes the observation of galaxy mergers, which often trigger active galactic nuclei, and jets emerging from the galaxy core, aligned perpendicular to the plane of surrounding material, such as seen in photos of NGC 4261.