Procyon, << PROH see on, >> also known Alpha Canis Minoris, ranks as the ninth brightest star in Earth’s sky. Only the sun and seven other stars appear brighter. Procyon is the brightest star in the constellation Canis Minor, the Lesser Dog. In the Northern Hemisphere, Procyon appears to rise into view shortly ahead of Sirius, also known as the Dog Star. The name Procyon, in fact, comes from Greek words meaning before the dog. Procyon is actually a binary star, a pair of companion stars that orbit each other. Nearly all the light reaching Earth from Procyon comes from the larger companion, called Procyon A.
Procyon A is a yellowish-white subgiant star. A subgiant star forms when a star like our sun begins to run out of hydrogen fuel in its core. The star swells slightly, becoming a subgiant. This expansion begins a series of changes that will transform the star into a type of huge reddish star called a red giant.
Procyon A’s yellowish color results from relatively warm surface temperatures of around 6500 K. One kelvin (K) equals one Celsius degree above absolute zero (–273.15 °C). Procyon A has a diameter about twice that of the sun and contains about 11/2 times as much mass (amount of matter). Because it has a larger size and hotter surface, Procyon A gives off about 7 times as much light as does the sun.
Procyon A’s companion, Procyon B, orbits the star at roughly the same distance that Uranus orbits the sun. Procyon B is a white dwarf, a type of burned-out star that can no longer sustain nuclear fusion. Procyon B has about 2/3 as much mass as the sun has, but it probably measures not much larger than Earth in size. Procyon B gives off about 1/15,000 as much light as Procyon A does. Observing Procyon B requires a telescope.
Procyon does not give off an unusually large amount of light for a star, but it appears bright because it is relatively close to Earth. Procyon lies about 11.5 light-years away. One light-year equals the distance light travels in a vacuum in a year, about 9.46 trillion kilometers.
Procyon A has the spectral type F5IV. For more information on spectral classification, see Star (The Hertzsprung-Russell diagram) .
