Rigel

How to find it

Find Orion's three Belt stars on a winter evening. Below the Belt, seen from the Northern Hemisphere, two bright stars mark the Hunter's knees or feet. The one on the lower right, shining a crisp blue-white, is Rigel. Its partner on the opposite corner, above the Belt, is orange Betelgeuse.

From 40°N, Rigel climbs to about 42 degrees above the southern horizon in mid-January evenings. A line from the Belt straight down toward the horizon passes close to it. Do not confuse it with Sirius, which is brighter and lies farther east and lower, to the lower left of the Belt.

Rigel is also the southwestern corner of the Winter Hexagon, a large ring of bright stars that runs from Rigel through Aldebaran, Capella, Pollux, Procyon and Sirius. Once you know Rigel, the rest of the winter sky falls into place around it.

What kind of star it is

Rigel is a blue supergiant of type B8. It was born only a few million years ago, a short time by cosmic standards, as a star of more than 20 solar masses. Such heavy stars burn their fuel quickly, and Rigel has already used up the hydrogen in its core and begun to expand.

It is now about 70 to 80 times as wide as the Sun, with a surface temperature of about 21,000°F, roughly twice the Sun's. Its total energy output is tens of thousands of times the Sun's; some estimates reach about 120,000. If Rigel were as close as Sirius, it would cast shadows at night.

Mass is the key to this short, bright life. The Sun will spend about 10 billion years quietly burning hydrogen; a star as heavy as Rigel gets through the same stage in only a tiny fraction of that time, which is why every blue supergiant we see is young.

Notable features

Rigel's distance is usually given as about 860 light-years, based on the Hipparcos satellite, though other estimates come out nearer 800. Either way, it is one of the most distant stars that look this bright, a clue to its huge power.

Its brightness wavers slightly and irregularly as the star's outer layers pulse. It belongs to a group called Alpha Cygni variables, named after Deneb, another blue-white supergiant.

Rigel is not alone. About 2,200 times the Earth–Sun distance away lies Rigel B, roughly 400 times fainter. Rigel B is itself a close pair of stars, and studies suggest a further companion, so the whole system may hold four stars. All are expected to outlive Rigel, which will most likely end its short life as a supernova.

Name and history

The name comes from the Arabic rijl jauzah al-yusra, 'the left foot of al-Jawza', the giant figure that Arab sky lore saw in Orion. Ottoman astronomy texts gave a related form, Riclü'l-cebbâr, 'the foot of the giant'. Like many star names, it reached Europe through medieval translations of Arabic astronomy.

When Johann Bayer labeled the stars of each constellation with Greek letters in 1603, he gave Rigel the second letter, Beta Orionis, and Betelgeuse the first. Today Rigel is usually the brighter of the two, partly because Betelgeuse varies.

Observing tips

Rigel's blue-white color is easiest to appreciate next to Betelgeuse; look from one to the other a few times and the difference becomes obvious. Like all bright stars, it twinkles strongly when low after rising, then settles as Orion climbs higher.

Rigel B is a classic test of a telescope and of steady air. It sits very close to Rigel and is lost in its glare in binoculars. Use a telescope of about 4 inches or more at high magnification on a calm night, and look for a faint point just beside the brilliant star.

Since Orion is visible from almost everywhere on Earth, Rigel is a star you can share with observers in both hemispheres. From the south it appears near the top of the upside-down Hunter.