How big is Procyon?

Procyon is a pair of stars in the constellation Canis Minor, about 11.4 light-years from Earth, where a light-year is the distance light travels in one year. The bright star, Procyon A, is about 2.8 million kilometers (1.7 million miles) across, about twice the width of the Sun, and that is the star the explorer shows. Its faint companion, Procyon B, is a white dwarf only a little wider than Earth.

Size

A star has no solid surface, so its size is the width of the glowing layer that its light comes from. Procyon A's apparent width was measured with the European Southern Observatory's Very Large Telescope Interferometer in Chile, which combines light from separate telescopes. It is 5.448 thousandths of an arcsecond, where an arcsecond is 1/3,600 of a degree. Combined with the star's distance, measured by the Hipparcos satellite, that gives a width 2.05 times the Sun's.

The same study found that the average of all published measurements gives the same result. A 2012 reanalysis, using 3D computer simulations of the star's outer layers, found a radius 2.02 times the Sun's, within about 1.5% of the first result. The rhythm of the star's oscillations confirmed it.

Size comparisons

Procyon A is about twice as wide as the Sun.
About 220 Earths side by side would stretch across it.
Altair, which has the same size in the explorer, is almost exactly as wide at its equator.
Vega, the next larger star in the explorer, is about 3.8 million kilometers wide at its equator, about one-third wider than Procyon A.
Procyon B is about 17,000 kilometers (11,000 miles) across, about 1.3 times the width of Earth. Procyon A is about 170 times as wide.
If the Sun were shrunk to the size of a basketball, Procyon A would be a ball about 49 centimeters (19 inches) across, and Procyon B a bead about 3 millimeters wide.

A companion found by its pull

In 1844, the astronomer F. W. Bessel noticed that Procyon and Sirius wobble slightly as they move across the sky. He suggested that unseen companions were pulling on them. The companion of Sirius, Sirius B, was seen in 1862. Procyon's is far fainter and stays within about 5 arcseconds of its bright partner, so it was not seen until 1896, when J. M. Schaeberle spotted it with the Lick Observatory's 36-inch (91-centimeter) telescope.

The two stars orbit each other every 40.84 years, on average about 15 times as far apart as Earth is from the Sun. Hubble Space Telescope images taken over two decades show that Procyon A has 1.48 times the Sun's mass and Procyon B 0.59 times. Procyon B began as a star with about twice the Sun's mass. What remains is a white dwarf, packing more than half the Sun's mass into a ball not much wider than Earth.

An aging star

Procyon A is about 2.7 billion years old. Models show it slowly growing larger as it ages, but they disagree on how close it is to using up the hydrogen at its center.