Izell was a blue giant star with an associated system located in Federation space within the Milky Way Galaxy. It had a diameter of approximately ten million kilometers, and was 51,500 times more powerful than Sol. It was a fifth magnitude star, and one of the brightest stars known to the Federation.
In the year 2268, during testing of a new deflector shield system linked to the warp drive, the USS Enterprise inadvertantly created an accretion disk that allowed a cosmic string to form and consume nearly a quarter of Izell's mass. (TOS novel: First Frontier)
Izell is described as a blue giant in First Frontier (before it lost a quarter of its mass) but also larger than Rigel which is a supergiant star. The fact that it is a fifth magnitude star also creates uncertainty as to being one of the brightest known stars.