Which of the following is a common characteristic of protostars?

Protostars are early stages in the formation of stars, and one of the defining characteristics of this phase is the contraction due to gravity and the accumulation of surrounding material. As a protostar forms from a molecular cloud, gravitational forces pull in gas and dust, causing the material to collapse inward. This contraction results in an increase in pressure and temperature at the core of the protostar.

During this process, the protostar continues to accumulate mass from its surroundings, which further contributes to the gravitational contraction. This phase is marked by instability and variability in luminosity as the core becomes increasingly hot and dense, but it does not yet reach the temperatures necessary for nuclear fusion.

The other options describe phenomena that either occur in different stages of stellar evolution or are not defining characteristics of protostars. For example, high temperatures and brightness are typically associated with main sequence stars rather than protostars, while low temperature and steady light output characterize cooler, evolved stars. The formation of planets occurs in a later stage of stellar evolution when a protostar has formed and a circumstellar disk is present. Therefore, the correct answer highlights the essential process of gravitational contraction that defines the protostellar phase.