How big is Sirius A?

Sirius A is the brightest star in the night sky and the larger of the two stars in the Sirius system, about 8.6 light-years from the Sun. It is about 2.4 million kilometers (1.5 million miles) across, about 1.7 times the width of the Sun. That figure is the width of its photosphere, the glowing layer that looks like a star's surface.

Size

A star has no solid surface, so its width is measured to the photosphere. Sirius A has been measured with interferometers, which combine the light from several telescopes so that together they see much finer detail than any one of them. In 2003 a team led by Pierre Kervella used the European Southern Observatory's Very Large Telescope Interferometer. In 2011 John Davis and colleagues used the Sydney University Stellar Interferometer.

Both found that the star appears about 6.04 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. Combined with the star's distance, that gives a radius about 1.71 times the Sun's. A 2017 study led by Howard Bond adopted 1.7144 times the Sun's radius, a width of about 2.39 million kilometers, uncertain by about half a percent.

Sirius A belongs to a different class of star from the Sun: A1 V, where the Sun is G2 V. Its surface is about 9,600 degrees Celsius, compared with about 5,500 for the Sun's. It has about twice the Sun's mass.

Size comparisons

Sirius A is about 1.7 times as wide as the Sun and about five times as large by volume.
It is about 187 times as wide as Earth.
It is about 1.4 times as wide as Alpha Centauri A, the next smaller star in the explorer.
Its companion, the white dwarf Sirius B, is about 11,300 kilometers across, a little smaller than Earth. Sirius A is about 210 times as wide.
If Sirius A were shrunk to the size of a basketball, Sirius B would be a speck about 1 millimeter across. The two would circle each other between about 120 and 470 meters apart.

Why Sirius looks so bright

Sirius is bright in the sky for two reasons: it is close, and it gives off a lot of light. At about 8.6 light-years, it was the seventh-nearest known star system to the Sun as of 2017. A light-year is the distance light travels in one year, about 9.5 trillion kilometers. Sirius A also gives off about 25 times as much light as the Sun, because it is both wider and much hotter.

Sirius is also young for a star. Its age is estimated at between about 230 and 250 million years, compared with about 4.6 billion years for the Sun.

The companion, Sirius B

In 1844 the astronomer Friedrich Bessel found that Sirius drifts back and forth slightly as it moves across the sky. He concluded that an unseen companion must be pulling on it. In 1862 Alvan G. Clark and his father first saw that companion, now called Sirius B.

The two stars orbit each other every 50.1 years, between about 8 and 32 astronomical units apart, where one astronomical unit is the average distance from Earth to the Sun. Sirius B is a white dwarf, the kind of dim, dense star that the Sun will become at the end of its life. It packs about the Sun's mass into a ball a little smaller than Earth. Astronomers estimate that it began as a star of about five times the Sun's mass, more massive than Sirius A is now, and shed most of that material as it aged.