Portal:Stars

Canopus (/kəˈnoʊpəs/; α Car, α Carinae, Alpha Carinae) is the brightest star in the southern constellation of Carina, and the second brightest star in the night-time sky, after Sirius. Canopus's visual magnitude is −0.72, and it has an absolute magnitude of −5.65.

Canopus is a supergiant of spectral type F. Canopus is essentially white when seen with the naked eye (although F-type stars are sometimes listed as "yellowish-white"). It is located in the far southern sky, at a declination of −52° 42' (2000) and a right ascension of 06h24.0m. Its name comes from the mythological Canopus, who was a navigator for Menelaus, king of Sparta.

Canopus is the most intrinsically bright star within approximately 700 light years, and it has been the brightest star in Earth's sky during three different epochs over the past four million years. Other stars appear brighter only during relatively temporary periods, during which they are passing the Solar System at a much closer distance than Canopus. About 90,000 years ago, Sirius moved close enough that it became brighter than Canopus, and that will remain the case for another 210,000 years. But in 480,000 years, Canopus will once again be the brightest, and will remain so for a period of about 510,000 years.

The main sequence is a continuous and distinctive band of stars that appear on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung-Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell. Stars on this band are known as main-sequence stars or "dwarf" stars.

After a star has formed, it creates energy at the hot, dense core region through the nuclear fusion of hydrogen atoms into helium. During this stage of the star's lifetime, it is located along the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and other factors. All main sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers. The strong dependence of the rate of energy generation in the core on the temperature and pressure helps to sustain this balance. The main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. Stars below about 1.5 times the mass of the Sun (or 1.5 solar masses) primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the proton-proton chain. Above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of carbon, nitrogen and oxygen as intermediaries in the CNO cycle that produces helium from hydrogen atoms.

Energy generated at the core makes its way to the surface and is radiated away at the photosphere. The energy is carried by either radiation or convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both.

Main sequence stars with more than ten solar masses undergo convection in the core region, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. When core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen. For stars with lower masses, this convective core is progressively smaller until it disappears at about 2 solar masses. Below this mass, stars have cores that are radiative but are convective near the surface. With decreasing stellar mass the convective envelope increases, and main sequence stars below 0.4 solar masses undergo convection throughout their mass.