How big is VV Cephei A?

VV Cephei A is a red supergiant, a huge, cool star, and the larger of the two stars in the eclipsing pair VV Cephei in the constellation Cepheus. Every 20.3 years it passes in front of its small, hot companion and hides it for more than a year. Its size is uncertain: a 2018 study used a radius of about 1,000 times the Sun's, which makes it about 1.4 billion kilometers across. Older studies of its eclipses found it larger still.

Size

VV Cephei A eclipses its companion, so its size can be worked out from the orbit of the two stars. The key is how long the companion stays hidden. Studies published in 1971 and 1977 gave radii of 1,200 and 1,600 times the Sun's, and a 1980 study set an upper limit of 1,900 times. Those figures would make the star 1.7–2.2 billion kilometers across, and at most 2.6 billion.

The results depend on the tilt of the orbit, which is not known exactly, and on how the edge of the star is defined. In 2018 a team studying its latest eclipse used a smaller radius, about 1,000 times the Sun's. A 2019 estimate based on the star's distance gave only about 390 times. But distances measured for stars like this are unreliable, because their orbital motion and large size disturb the measurement.

VV Cephei A is also a variable star. Like other red supergiants, it changes in brightness on its own, with a main period of about 150 days.

Size comparisons

By the 2018 figure, VV Cephei A is about 1,000 times as wide as the Sun.
If it took the Sun's place, its surface would reach almost to the orbit of Jupiter. By the older estimate of 1,600 times the Sun's radius, it would reach past Jupiter, about three-quarters of the way to the orbit of Saturn.
It is about 1.3 times as wide as Betelgeuse and about 1.5 times as wide as Antares, two of its neighbors in the explorer.
Light, which travels about 300,000 kilometers per second, takes about 77 minutes to cross it.
If the Sun were shrunk to the size of a basketball, VV Cephei A would be a ball about 240 meters (790 feet) across, more than twice the length of an American football field.

An eclipsing pair

In 2018, VV Cephei was the brightest known eclipsing pair in the sky with a red supergiant like it. Its companion is a hot star of the kind astronomers call early B-type, much smaller in radius than the supergiant. A study published in 1977 found that each star has about 20 times the mass of the Sun. The two follow a stretched, oval orbit, and their average distance apart is about five times the supergiant's radius.

The most recent total eclipse began in October 2017 and lasted about a year and a half. The next is due around 2038. Because the companion shines through the supergiant's outer layers as it passes behind them, eclipses give astronomers a chance to study the atmosphere and wind of a red supergiant.

Gas flowing between the stars

The supergiant sheds gas in a slow wind of about 20 kilometers per second, losing about one Sun's worth of mass every 670,000 years. Some of that gas falls onto the companion, where it glows brightly in the red light of hydrogen. In November 1997, the Hubble Space Telescope made the first ultraviolet observations of one of the system's total eclipses, and it followed the system until 2003.