Sirius is the brightest star in the night sky and one of the closest stars to Earth, lying just 8.6 light-years away in the constellation Canis Major. Also known as the Dog Star, Sirius appears to be a single brilliant point of light, but there is much more to this familiar star than meets the eye.
Easy to Find Using Orion
Sirius is one of the easiest stars to find because the three stars of Orion’s Belt point almost directly toward it. Follow the line of the Belt downward toward the southeast and the first exceptionally bright star you reach is Sirius, in the constellation Canis Major. Once identified, it is difficult to mistake: no other star in the night sky appears brighter.
Brightest Star in the Night Sky
Sirius is the brightest star in the night sky, shining at an apparent magnitude of about −1.46. Yet Sirius A is not an exceptionally powerful star by cosmic standards. Much of its brilliance in our sky comes from being so close to Earth. In fact, Sirius appears almost twice as bright as Canopus, the second-brightest star in the night sky. Yet Canopus is actually far more luminous — Sirius outshines it largely because it lies just 8.6 light-years away, compared with about 310 light-years for Canopus.
About Twice the Mass of the Sun — But Much Brighter
Sirius A has about twice the mass of the Sun and is around 1.7 times wider, but it produces roughly 25 times as much light. Its surface temperature is close to 10,000 K (17,500°F), almost twice as hot as the Sun, giving Sirius A its distinctive blue-white color. It is an A-type main-sequence star, meaning that, like the Sun, it is still generating energy by fusing hydrogen into helium in its core.
Sirius A is also surprisingly young, at only around 250 million years old, compared with about 4.6 billion years for the Sun. More massive stars consume their nuclear fuel much faster, so despite being far younger than the Sun, Sirius A will have a much shorter overall lifetime.
Sirius Is Actually a Two-Star System
Although Sirius looks like a single star to the naked eye, it is actually a binary system made up of Sirius A and the much fainter Sirius B. The two stars orbit their common center of mass once every 50 years, separated by a distance that changes considerably during their elliptical orbit. Sirius A completely dominates what we see from Earth, while its tiny companion is extremely difficult to see beside its brilliant neighbor.
Sirius B Packs Almost the Sun’s Mass Into an Earth-Sized Star
Sirius B is a white dwarf containing almost as much mass as the Sun, yet squeezed into a sphere slightly smaller than Earth. This makes it incredibly dense — a teaspoon of its material would weigh several tons on Earth. Surprisingly, Sirius B is also more than twice as hot as Sirius A, with a surface temperature of around 25,000 K, but appears about 10,000 times fainter than Sirius A because its tiny surface radiates far less light overall.
Sirius B was once the more massive star of the pair, but burned through its fuel much faster before expanding into a giant, shedding its outer layers and leaving behind the white dwarf we see today. It was also discovered in a remarkable way: in 1844, astronomer Friedrich Bessel concluded from tiny irregularities in Sirius’s motion that an unseen companion must be pulling on it gravitationally. The mysterious star was finally observed in 1862 by Alvan Graham Clark. In other words, astronomers detected Sirius B through its gravity nearly 20 years before anyone actually saw it. You can learn more about how stars become white dwarfs in our guide to the life cycle of a star.
Sirius Can Appear to Flash Different Colors
Sirius is a blue-white star, but when it is low in the sky it can appear to flash rapidly between red, blue, green and other colors — an effect that has sometimes earned it the informal nickname “Rainbow Star.” Sirius itself is not changing color. The effect is caused by turbulence in Earth’s atmosphere, which bends different wavelengths of its light by slightly different amounts. Because Sirius is so bright, these colorful flashes can be especially striking and are sometimes visible to the naked eye.
The Dog Star Gave Us the “Dog Days” of Summer
Sirius is known as the Dog Star because it lies in Canis Major, the Greater Dog, traditionally depicted as one of the hunting dogs of Orion. Orion’s other dog is Canis Minor, home to the bright star Procyon. The ancient Greeks associated the appearance of Sirius with the hottest part of summer, believing its heat combined with that of the Sun. This ancient association is the origin of the expression “Dog Days of Summer,” which is still used today. Sirius was also important in ancient Egypt, where its first appearance in the dawn sky each year was closely associated with the approaching annual flooding of the Nile.