Arcturus

Arcturus (α Boo, α Boötis, Alpha Boötis) is the brightest star in the constellation Boötes. To the naked eye, orangey-yellow Arcturus has a visual magnitude of −0.04, making it the brightest star in the northern hemisphere and the fourth brightest star in the night sky, after -1.46 magnitude Sirius, -0.86 magnitude Canopus and -0.27 magnitude Alpha Centauri. However, Alpha Centauri is a bright binary star, whose unresolved components to the naked eye are both fainter than Arcturus. This makes Arcturus the third brightest individual star, just ahead of Alpha Centauri A (α Cen A), whose visual magnitude is −0.01.

Arcturus is visible from both hemispheres in the sky, as it is located less than 20 degrees north of the celestial equator. The star culminates at midnight on about the 30th April, being visible during the northern spring or the southern autumn. From the northern hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find Spica, "Arc to Arcturus, then speed on to Spica." The star is also a member of the Local Interstellar Cloud.

Arcturus is a type K1.5 IIIpe orange giant star—the letters "pe" stand for "peculiar emission," which indicates that the spectrum of light given off by the star is unusual and full of emission lines. This is not too uncommon in red giants, but Arcturus has a particularly strong case of the phenomenon. It is at least 110 times visually more luminous than the Sun, but this underestimates its strength as much of the "light" it gives off is in the infrared; total power output is about 180 times that of the Sun. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun.