The brightest star in the night sky is Sirius. Only 8.6 million light years from Earth, Sirius A is 25 times more luminous than the sun. Because of its brightness, the star was well known in ancient times—it was named Sopdet in Ancient Egypt and it was the basis of the Egyptian calendar. After a 70-day absence from the skies Sirius (or Sopdet) first became visible just before sunrise near the Summer solstice—just prior to the annual Nile floods. Greek and Roman astronomers philosophized and speculated about Sirius, (which they called “the dog star” because of its closeness to the constellation Canis Majoris). Arabs knew the star as Aschere “the leader”. Polynesians used it as a principle focus of their astonishing oceanic navigation. Over countless millennia, Sirius has worked its way deep into human consciousness as one of the immutable landmarks of the night sky.
So imagine the shock when it was discovered that Sirius is not alone. The bright star we know is actually Sirius A, a star with twice the mass of the sun. In 1844 the German astronomer Friedrich Bessel hypothesized a tiny companion for Sirius based on the irregular proper motion of Sirius. Then, in 1862, as the American Civil war was being fought an American astronomer in Chicago first observed the tiny companion, Sirius B (thereafter affectionately known as “the Pup”). Sirius B has nearly the same mass as the sun (.98 solar mass) but it is only 12,000 kilometers (7,500 mi) in diameter—nearly the same distance around as Earth.
Today Sirius B is the closest white dwarf star to planet Earth. However it has not always been so, Sirius B began its life as a luminous blue B-type main sequence star with a mass five times that of the sun. About 124 million years ago—as the dinosaurs grazed on the first magnolias—Sirius B fused its way through the hydrogen and helium in its mass. As Sirius B began to fuse together larger elements like oxygen and carbon it expanded into a red giant star with a diameter 10 to 100 times that of the sun. Then Sirius B ran out of nuclear fuel. Without the heat generated by nuclear fusion to support it, the star underwent gravitational collapse and shrank into a hyper dense white dwarf star. These tiny stars are extremely dense and hot when they are formed, but since they generate no new energy their heat and radiance gradually radiate away over billions of years until the stars are completely black and dead.
Although Sirius B is largely composed of a carbon-oxygen mixture, its core is overlaid by an envelope of lighter elements. Hydrogen, being lightest, forms the outermost layer (which is why the little star currently appears uniformly white).