How big is Albireo?

Albireo is a well-known double star in the constellation Cygnus, the Swan. Its largest member, a giant star called Albireo Aa, is about 86 million kilometers (53 million miles) across, roughly 62 times the width of the Sun. That figure is the width of the giant's visible surface, and it is an estimate.

Size

Stars have no solid surface, so their sizes are worked out rather than measured directly. A 2021 study estimated Albireo Aa's radius from how much light it gives off and how hot its surface is. The result was 62 times the Sun's radius, which makes the star about 86 million kilometers across. The same study found that Albireo Aa has about 5.2 times the Sun's mass.

A second method gives a similar answer. Interferometers, which combine the light from several telescopes, show that the star looks about 4.8 thousandths of an arcsecond wide in the sky. An arcsecond is 1/3,600 of a degree, so that angle is about the width of a human hair seen from 4 kilometers (2.5 miles) away.

At Albireo's distance, that angle also works out to about 86 million kilometers. A 2022 study that used another method found a slightly smaller star, about 77 million kilometers across.

Both answers depend on the distance, which is uncertain. Albireo A is too bright for the Gaia space mission to measure reliably, so the 2021 study used the distance of its neighbor, Albireo B: about 390 light-years. A light-year is the distance light travels in one year.

Size comparisons

Albireo Aa is about 62 times as wide as the Sun, which is about 1.39 million kilometers across.
If Albireo Aa took the Sun's place, its surface would reach about three-quarters of the way to the orbit of Mercury.
Its width is a little more than half the distance from Earth to the Sun, which is about 150 million kilometers.
Rigel, one of Albireo's larger neighbors in the explorer, is about 1.3 times as wide as Albireo Aa.
If the Sun were shrunk to the size of a basketball, Earth would be a 2-millimeter bead 26 meters away, and Albireo Aa would be a ball about 15 meters (49 feet) across.

A double star with hidden members

Through a small telescope, Albireo splits into two stars of strongly contrasting colors, about 35 arcseconds apart. The brighter one, Albireo A, is itself a close pair. It holds the cool giant Albireo Aa, which officially carries the name Albireo, and a smaller, hotter blue star, Albireo Ac. The two are about 0.4 arcseconds apart and take about 122 years to orbit each other.

The 2021 study found that Albireo A has more mass than its two known stars can explain, which suggests another member that has not been seen. In 2022, astronomers found another star in the system, Albireo Ad, with about 8.5 percent of the Sun's mass, circling Albireo Aa about once every 371 days. The study that found it does not say whether it is the missing member.

Whether A and B belong together

Astronomers have long debated whether Albireo A and Albireo B orbit each other. Measurements from the Hipparcos and Gaia space missions suggest that the two stars may be about 30 to 65 light-years apart along the line of sight. At that distance they are very likely not held together by gravity.

They are still probably related. The 2021 study found that the stars appear to be the same age and to move through space at almost the same speed and in almost the same direction. A chance lineup of two unrelated stars like these is very unlikely. The astronomers concluded that A and B probably formed together in the same cluster of stars, even if they are not bound today.