How big is V354 Cephei?

V354 Cephei is a red supergiant, a huge, cool and very luminous star, in the constellation Cepheus. A 2019 estimate puts it at about 950 million kilometers across, about 690 times the width of the Sun. That is the width of its visible surface, and it is far from certain: an earlier study found the star more than twice as large.

Size

The width of V354 Cephei has not been measured directly. Astronomers calculate it instead. A star's total light output depends on its surface temperature and its size, so knowing its light output and temperature gives its size. The light output, in turn, is worked out from how bright the star looks and how far away it is.

In 2019, Maria Messineo and Anthony Brown used a distance from the Gaia space telescope of about 2,100 parsecs, or 6,800 light-years, where a parsec is about 3.26 light-years. With a surface temperature of about 3,600 kelvins (degrees measured from absolute zero), they found a radius of 685 times the Sun's. That gives a width of about 950 million kilometers.

In 2005, Emily Levesque and her colleagues placed the star at about 9,000 light-years, the distance of a group of stars it seems to belong to. They found a radius of 1,520 times the Sun's, making it about 2.1 billion kilometers across. Dust between Earth and the star dims its visible light about 260 times, which adds to the uncertainty. Like most red supergiants, V354 Cephei also varies slowly and irregularly in brightness.

Size comparisons

By the 2019 estimate, V354 Cephei is about 690 times as wide as the Sun.
If it took the Sun's place, its surface would reach past the orbit of Mars, about 60% of the way to the orbit of Jupiter. By the 2005 estimate, it would reach well past Jupiter.
By the 2019 estimate, it is almost exactly as wide as Antares, its neighbor in the explorer, and Betelgeuse is about one-tenth wider.
If the Sun were shrunk to the size of a basketball, V354 Cephei would be a ball about 160 meters (530 feet) across, about one and a half times the length of an American football field.

A member of a young group

V354 Cephei appears to belong to Cepheus OB1, an OB association: a scattered group of young stars that includes hot, massive ones. Astronomers use such groups to find red supergiants and to estimate their distances, but a star's membership is never certain, and V354 Cephei's has been questioned. Red supergiants are young by the standards of stars. They are 4–30 million years old and have 9–40 times the mass of the Sun.

How big red supergiants can get

The 2005 study measured 74 red supergiants in the Milky Way. The four largest, V354 Cephei among them, came out at roughly 1,500 times the Sun's radius. That is about the largest size that theories of how stars change as they age allow a red supergiant in the Milky Way to reach. The other three were KW Sagittarii, KY Cygni and Mu Cephei, sometimes called Herschel's Garnet Star.