How big is Deneb?

Deneb is a blue-white supergiant star in the constellation Cygnus, the Swan. It is estimated to be about 280 million kilometers across, roughly 200 times the width of the Sun. That estimate depends on Deneb's distance, which is uncertain, and the star could be only about half that size.

Size

Stars have no solid surface, so a star's size is worked out from how wide or how bright it looks and how far away it is. Interferometers, which combine the light from several telescopes, show that Deneb looks about 2.4 thousandths of an arcsecond wide. An arcsecond is 1/3,600 of a degree, so Deneb looks about as wide as a human hair seen from 8.6 kilometers (5.3 miles) away.

A 2008 study put Deneb's radius at 203 times the Sun's, which makes it about 280 million kilometers across. That study assumed Deneb belongs to Cygnus OB7, a group of stars about 2,600 light-years away. A light-year is the distance light travels in one year. At that distance, Deneb's apparent width gives the same size, and a second method of estimating the distance gave a similar answer.

The Hipparcos space mission found Deneb much closer, at about 1,400 light-years. A 2010 study pointed out that this would shrink Deneb's size estimates by almost half, to around 150 million kilometers across. Even for relatively nearby supergiants such as Deneb and Rigel, the study noted, distances remain hard to pin down.

Size comparisons

At the 2008 estimate, Deneb is about 200 times as wide as the Sun, which is about 1.39 million kilometers across.
At that size, if Deneb took the Sun's place, its surface would reach past the orbit of Venus, almost to the orbit of Earth.
Its width is almost twice the distance from Earth to the Sun, which is about 150 million kilometers.
R Doradus, one of Deneb's larger neighbors in the explorer, is about 1.5 times as wide as Deneb.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away, and Deneb would be a ball about 49 meters (160 feet) across, almost half the length of a football field.

A brilliant supergiant

Taking the 2008 study's distance of 2,600 light-years, Deneb gives off about 196,000 times as much energy as the Sun. Its surface is about one and a half times as hot as the Sun's. The 2008 study found that Deneb now has about 19 times the Sun's mass. It concluded that the star began with about 23 times the Sun's mass and is now evolving toward the red supergiant stage.

A 2024 study points out that this is not settled for stars like Deneb. Some of them may instead have been red supergiants already and now be on their way back.

How Deneb pulsates

Deneb's other name is Alpha Cygni, and a group of variable stars, the Alpha Cygni variables, is named after it. Deneb's brightness and the motion of its surface rise and fall irregularly, roughly every 12 days.

These pulsations grow stronger suddenly, fade away over several cycles, and then start again. Satellite measurements taken between 2003 and 2011 showed that they most often restart after 100–120 days. In 2024, a catalog of variable stars listed more than a hundred Alpha Cygni variables, including Rigel in Orion.