What characterizes an asterism?

An asterism is characterized as a well-known pattern of stars that is not a constellation. Unlike constellations, which are officially recognized groups of stars that are used for navigation and mapping the night sky, an asterism can be any recognizable shape made by a group of stars, regardless of whether it belongs to an established constellation. For instance, the Big Dipper is an asterism within the larger constellation of Ursa Major. Asterisms are often easier to recognize and remember, making them useful for amateur astronomers and casual star-gazers.

In contrast, the other options describe very different astronomical concepts. A nebula refers to a massive cloud of dust and gas in space, which can form new stars or remnants of exploded stars. Classifying star types involves categorizing stars based on their physical characteristics such as temperature and luminosity, commonly using the spectral classification system. A celestial event involving a meteor shower occurs when Earth passes through the debris left by comets, leading to numerous meteors entering the atmosphere, which is distinct from the concept of an asterism.