Sirius

Sirius (α Canis Majoris) is the brightest star visible from Earth other than the Sun (Sol). Sirius is sometimes called the "Dog Star" because it is located in the southern constellation Canis Major, the Bigger Dog (RA 6h 45m 8.7s, declination -16°43’03”). As is true of almost all stars Sirius is not visible during all seasons. In the Northern Hemisphere it is most easily viewed in the winter in early evening.

Sirius is the seventh nearest star to Earth at a distance of 8.61Ly, according to the latest measurements of the Hipparcos satellite. Sirius is a binary star system consisting of Sirius A (spectral class A1 V, absolute visual magnitude +1.47) and Sirius B (spectral class B0wd, absolute visual magnitude +8.44). This means Sirius has a much brighter absolute visual magnitude than the Sun (absolute visual magnitude +4.85). Sirius is approximately 23.2 times more luminous than the Sun. Luminosity is the actual amount of light put out by an object. Both Sirius's luminosity and its nearness to Earth contribute to it being the brightest star in the night sky.

Other than its brightness and location, Sirius is noteworthy as the first discovered instance of a “white dwarf” star (Sirius B), an extremely hot, small and dense stellar remnant. Although an orbiting companion of Sirius was suspected as early as 1844 by German astronomer Friedrich Bessel, due to a slight, gravitationally induced and very regular “wobble” in Sirius’ proper motion, the presence of Sirius B was not confirmed until 1862. While conducting star-tests of the 18.5-inch Dearborn refracting telescope on January 31, 1862, American telescope maker Alvan Graham Clark noted a very small star close to Sirius that had not been previously noted. Further observations confirmed Sirius B’s binary nature in an orbit with a period of about 50 years and at a distance from Sirius A of about 20AU (3,000,000,000km).