Which star is known as the old North Star?

The correct answer is Arcturus. This star has been referred to as the "old North Star" due to its prominence in the Northern Hemisphere's night sky and its historical significance in navigation before Polaris became the primary North Star.

Arcturus, a red giant located in the constellation Boötes, is one of the brightest stars visible from Earth. It is often described as having a reddish hue, distinguishing it from other stars and making it easily recognizable. Its position in the sky and brightness meant that it was used by ancient navigators familiar with the constellations to find their way.

The other choices include Vega, Deneb, and Sirius, which are all bright stars but do not hold the same historical significance as navigational aids in relation to the North Star. Vega, located in the constellation Lyra, is not associated with the same historical context regarding navigation. Deneb, a blue supergiant in the constellation Cygnus, is also not considered an "old North Star." Lastly, Sirius is the brightest star in the night sky but is located in the Canis Major constellation, far from the North Celestial Pole.