The Brightest Star

WHAT IS SIRIUS?

Sirius is the brightest star in Earth's night sky. It is located in the constellation Canis Major, which gives it the nickname "the Dog Star." Sirius appears exceptionally bright partly because it is relatively close to the Solar System and partly because its main star, Sirius A, is intrinsically more luminous than the Sun.

SIRIUS A

Sirius A is a hot main-sequence star classified as an A-type star. It is considerably hotter and more luminous than the Sun. Its surface temperature is roughly 9,900 kelvin, compared with about 5,800 kelvin for the Sun. This high temperature gives Sirius A its distinctive white appearance.

SIRIUS B

Sirius is not a single star. It is a binary system containing Sirius A and Sirius B. Sirius B is a white dwarf — the dense remnant of a star that has already passed through its giant phases. Although Sirius B is physically small and extremely dense, Sirius A is so bright that Sirius B is difficult to observe.

WHY IS SIRIUS SO BRIGHT?

The brightness of a star in Earth's sky depends on both its intrinsic luminosity and its distance from us. Sirius is relatively close at about 8.6 light-years. Its main component, Sirius A, also emits considerably more energy than the Sun. The combination makes Sirius the brightest star visible in the night sky from Earth.

KEY IDEAS

THE DOG STAR

Sirius is located in Canis Major and has been associated with the "Dog Star" for thousands of years.

WHITE STAR

Sirius A appears white because its surface temperature is much higher than the Sun's.

BINARY SYSTEM

Sirius consists of a bright main-sequence star and a much fainter white dwarf companion.

WHITE DWARF

Sirius B is roughly Earth-sized but contains a large fraction of the Sun's mass.

8.6 LIGHT-YEARS

The light you see from Sirius today began its journey toward Earth roughly 8.6 years ago.

NOT THE MOST LUMINOUS

Sirius is the brightest star in our sky, but many stars are intrinsically far more luminous.

DID YOU KNOW?

Sirius is so bright that it can sometimes appear to flash different colors when it is low above the horizon because Earth's atmosphere bends and scatters its light.

WHAT DOES “BRIGHTEST” MEAN?

A star can be called bright in different ways. Apparent brightness describes how bright it looks from Earth and depends strongly on distance, while intrinsic luminosity describes how much energy the star actually radiates. Sirius is the brightest star in Earth's night sky by apparent visual magnitude, excluding the Sun.

Claims about the most intrinsically luminous known stars are more difficult because distances, extinction, multiplicity and stellar models introduce uncertainty, and catalogues can change as measurements improve.