How big is Alnitak?

Alnitak is the easternmost of the three bright stars in Orion's Belt, and it is made of three stars. The largest, a hot supergiant called Alnitak Aa, is about 28 million kilometers (17 million miles) across, about 20 times the width of the Sun. That width is uncertain by about one-sixth, mostly because Alnitak's distance is not known precisely.

Size

Alnitak, also called Zeta Orionis, is so far away that Alnitak Aa looks only about 0.48 thousandths of an arcsecond wide, where an arcsecond is 1/3,600 of a degree. That is the width of a human hair seen from about 43 kilometers (27 miles) away. This value was measured in 1974 with an interferometer, an instrument that combines light collected at separate points to see very fine detail.

Turning that angle into a true width needs the distance, and the estimates disagree. A 2013 study followed Alnitak Aa and its close companion through a full orbit, which pointed to a distance of about 960 light-years. The brightness of the third star suggested about 1,260 light-years, closer to the roughly 1,300 light-years usually assumed for the star-forming regions of Orion. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

The study adopted the larger distance, which gives Alnitak Aa a radius of 20 times the Sun's, give or take 3.2. A 2008 study that assumed about 1,350 light-years found 25 times the Sun's radius, which would make the star about 35 million kilometers across. At the closer, orbit-based distance, the same apparent width would give only about 15 times the Sun's radius.

Size comparisons

Alnitak Aa is about 20 times as wide as the Sun and about 2,200 times as wide as Earth.
About 8,000 Suns would fit inside it, by volume.
If it took the Sun's place, its surface would reach about a quarter of the way to the orbit of Mercury.
It is about 1.7 times as wide as the larger of the two giant stars in Capella, the next smaller star in the explorer. Arcturus, the next larger, is about 1.3 times as wide as Alnitak Aa.
If the Sun were shrunk to the size of a basketball, Alnitak Aa would be a ball about 4.8 meters (16 feet) across.

Three stars in one

Alnitak Aa's close companion, Alnitak Ab, was discovered in 2000 with the Navy Precision Optical Interferometer. The two stars orbit each other every 7.4 years, and they are too far apart to have ever passed gas from one to the other. Alnitak Ab is about seven times as wide as the Sun. A third star, Alnitak B, lies 2.4 arcseconds away in the sky.

Alnitak Aa's surface is about 29,500 kelvins, about five times as hot as the Sun's surface. The kelvin scale measures temperature up from absolute zero, the coldest possible temperature. Alnitak Aa is the brightest star of its type, called O-type, in the sky.

The star also blows off a strong wind of gas. A 2008 study estimated that it loses about two-millionths of the Sun's mass every year, which is about two-thirds of Earth's mass. It is young, too: estimates of its age range from about 3.6 million to 7 million years, compared with 4.6 billion years for the Sun.

Alnitak's neighborhood

Alnitak belongs to a loose group of hot, massive stars, part of which is centered on Orion's Belt. A short way south of Alnitak in the sky is the Horsehead Nebula, a dark cloud shaped like a horse's head, about 1,375 light-years away. The Horsehead is the closest giant star-forming region to Earth and is part of the vast Orion molecular cloud.