Here's why:

* Main Sequence: This is the longest and most stable phase of a star's life. It's where stars fuse hydrogen into helium in their core, generating energy and emitting light.

* Size: Stars on the main sequence come in a range of sizes, from very small red dwarfs to massive blue giants. Medium-sized stars like our Sun are in the middle of this spectrum.

So, while there isn't a specific name for a medium-sized star, "main sequence star" describes its current stage of life and gives a good indication of its size relative to other stars.