Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek Seirios ("scorcher"), possibly because the star's appearance was associated with summer. The star has the Bayer designation α Canis Majoris (α CMa, or Alpha Canis Majoris). What the naked eye perceives as a single star is actually a binary star system, consisting of a white main sequence star of spectral type A1V, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B.

Sirius appears bright due to both its intrinsic luminosity and its closeness to the Earth. At a distance of 2.6 parsecs(8.6 ly), the Sirius system is one of our near neighbors. Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old. It was originally composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around 120 million years ago.

In astronomy and physical cosmology, the metallicity (also called Z) of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively. Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be "metal-rich" in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of "metal"; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Normal chemistry therefore has little or no relevance in stellar interiors.

The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Stellar populations are categorized as I, II, and III, with each group having decreasing metal content and increasing age. The populations were named in the order they were discovered, which is the reverse of the order they were created. Thus, the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are population I. While older stars do have fewer heavy elements, the fact that all stars observed have some heavier elements poses something of a puzzle, and the current explanation for this proposes the existence of hypothetical metal-free Population III stars in the early universe.