Antares

How to find it

Face south an hour or two after dark in late June or July. Low in the sky you will see a bright star with a clear reddish tint. To its right, a short, nearly vertical arc of three stars marks the Scorpion's head and claws, and a long curved tail of stars hooks down and to the left of Antares toward the horizon.

From 40°N, Antares rises no higher than about 24 degrees above the southern horizon, so you need a clear, low view to the south. The farther south you live, the higher it climbs; from southern Türkiye or the southern United States it is much better placed. In late summer it sets earlier each evening, sinking into the southwest by September.

What kind of star it is

Antares is a red supergiant, a massive star in a late stage of life. It probably began with about 15 times the Sun's mass and has since lost around 3 through strong winds, leaving about 12. Its outer layers have swollen to about 700 times the Sun's width; in the Sun's place it would reach well beyond the orbit of Mars.

Its surface is cool for a star, around 6,000°F, which gives it its red-orange color, yet its vast size makes it many thousands of times as luminous as the Sun. Its distance is usually given as about 550 light-years, with some estimates near 600.

Notable features

In 2017 astronomers used ESO's Very Large Telescope Interferometer, which links several telescopes in Chile, to map the motion of gas across the surface of Antares. It was the most detailed map ever made of the surface of a star other than the Sun. It showed huge, turbulent clumps of gas rising and falling in its extended atmosphere, motions that astronomers are still working to explain.

Antares has a companion, Antares B, a hot blue-white star of about magnitude 5.5. It lies only about 3 arcseconds away, hundreds of times the Earth–Sun distance, and probably takes many centuries to complete one orbit. It was discovered in 1819 by the astronomer Johann Tobias Bürg, who saw it shine alone for a few seconds after the Moon had covered the bright star.

That trick still works. The Moon is now in a long series of occultations of Antares that began on 25 August 2023 and runs to 27 August 2028, 68 events in all. Each is visible only from part of the Earth, so check an astronomy app or almanac for your location.

Name and history

The name comes from the Greek anti-Ares, 'rival of Ares', the god whom the Romans called Mars. When the planet Mars passes near the star, which happens every couple of years, the two really do look like rivals. Arab astronomers called it qalb al-aqrab, 'the heart of the scorpion', which describes its place in the figure.

In Turkish the constellation is Akrep, from the same Arabic word for scorpion, and Antares is often described as the Scorpion's heart. In China it belongs to the Heart mansion, one of the 28 lunar mansions, and many cultures in the Pacific saw these stars as a great fishhook rather than a scorpion.

Observing tips

Choose a site with a low, open southern horizon, such as a beach or hilltop, and a dry night after a weather front. Haze near the horizon dims Antares and makes it twinkle strongly, sometimes flashing other colors.

Antares B is a real challenge. Because the companion is so close and the main star so bright, you usually need a telescope of 6 inches or more, high power and very steady air. Observers often describe its color as greenish; this is mainly an effect of contrast with the red glare beside it. Watching a lunar occultation through a telescope is the easiest way to glimpse it.

With binoculars, look just over 1 degree west of Antares for the globular cluster M4, a soft round glow of very old stars about 7,200 light-years away.