What is observed in binary stars?

In binary stars, the defining characteristic is that there are two stars that orbit around a common center of gravity. This relationship occurs due to their gravitational attraction to each other, which leads to mutual orbital motion. The center of mass, or barycenter, lies inside the more massive star if the stars have significantly different masses.

Binary stars can vary greatly in their properties; they may have different sizes, temperatures, and luminosities. The nature of their orbits can also be diverse, ranging from wide separations to very close binaries. Observing the dynamics of these stars can provide valuable insights into stellar masses and evolutionary processes, making them a critical aspect of stellar astronomy.

The other options do not accurately describe binary star systems. A single star without companions indicates a solitary star rather than a binary. While it's possible for two stars to share similar temperatures, not all binary stars will have this characteristic. Finally, while a star with a planetary system features different dynamics, this does not pertain to the defining properties of binary stars. Thus, the accurate observation in binary stars is their mutual orbiting around a common center of gravity.