How big stars are

Sirius B is a whole star yet slightly smaller than Earth; Betelgeuse is about 700 times wider than the Sun.

From Earth-sized to the size of an orbit

Start the model at the far left. The faint blue ball is Sirius B, the companion of the brightest star in our sky. ESA gives its diameter as 12,000 km, a little less than Earth's 12,742 km, yet it holds 98% of the Sun's mass. It is a white dwarf, the leftover core of a star that used up its fuel. Next to it, Earth is slightly bigger. Then come Jupiter and the red dwarf Proxima Centauri, about one seventh of the Sun's width and about 1.5 times Jupiter's.

Keep going and the Sun appears, then Alpha Centauri A, a Sun-like star 4.37 light-years away, and Sirius A, which ESA lists as 2.4 million km across, about 1.7 times the Sun. Vega is a bit larger again. These are all main-sequence stars, fusing hydrogen in their cores, and on the main sequence size grows only slowly with mass. The big jump comes with stars that have left the main sequence: Arcturus is about 25 Suns wide, Aldebaran about 45, the blue supergiant Rigel about 79.

Betelgeuse in place of the Sun

The red supergiants Antares and Betelgeuse are at the far right. NASA's Marshall team estimates Betelgeuse's radius at about 724 times the Sun's. Multiply: 724 × 695,700 km is about 504 million km, or 3.37 astronomical units (1 AU is Earth's distance from the Sun, 149.6 million km, which is 215 solar radii). Turn on "Planet orbits" in the model. Betelgeuse swallows the orbits of Mercury, Venus, Earth and Mars at 1.52 AU, and its surface reaches past the main asteroid belt, but Jupiter's orbit at 5.2 AU stays outside.

Some NASA pages say Betelgeuse "would stretch past Jupiter's orbit". That would need a radius of about 1,120 Suns. Older measurements gave numbers of that kind, and a few still do, because two things are hard to pin down. One is distance: NASA pages give about 548 and about 700 light-years. The size comes from the star's apparent width in the sky multiplied by its distance, so a 30% change in distance is a 30% change in radius. The other is the edge: a supergiant's outer layers thin out gradually and look different at different wavelengths. Treat any single number for a supergiant's size as an estimate with a wide margin.

Width, volume and density

Volume grows with the cube of the radius. A star 724 times wider than the Sun has 724³, about 380 million, times the Sun's volume. Yet NASA puts Betelgeuse's mass at only about 15 Suns, so its average density is extremely low: spread 15 Suns of mass through that volume and you get about 0.00006 kg per cubic metre, roughly 20,000 times thinner than the air around you. The opposite holds for white dwarfs. Sirius B packs almost a Sun's mass into a ball the size of Earth, about 2 billion kg per cubic metre on average. NASA puts it this way: a teaspoon of white dwarf material would weigh more than a pickup truck.