Antares: Heart of the Scorpion

Antares is the brilliant red star at the heart of Scorpius, one of the most visually striking stars in the summer sky. Its deep reddish color, reminiscent of Mars, earned it the Greek name that means "rival of Ares" (the Greek name for Mars). This enormous red supergiant is one of the largest stars visible to the naked eye and a fascinating object for study. Find it in the southern summer sky with StarGlobe.

How to Find Antares

During summer evenings, look low in the southern sky (from northern latitudes) for a distinctly red star. Antares is the brightest object in a region that also features the curving tail of Scorpius and the dense star clouds of the Milky Way. Three stars in a vertical line to the upper right of Antares form the scorpion's head, helping to confirm its identity.

Antares is sometimes confused with Mars when the planet passes nearby, which happens roughly every two years as Mars moves along the ecliptic. Both appear reddish, but Mars does not twinkle as much as Antares does, and their positions change relative to each other over days and weeks.

Physical Characteristics

Antares is a red supergiant of spectral type M1.5Iab, with a diameter estimated at about 680 times that of the Sun. If placed at the center of our solar system, its surface would extend well past the orbit of Mars. It is one of the few stars whose apparent disk has been directly resolved by interferometric techniques, revealing a mottled surface with large convective cells.

The star is approximately 550 light-years away and has a luminosity roughly 75,000 times that of the Sun when its infrared output is included. Its surface temperature of about 3,400 Kelvin makes it one of the coolest bright stars. Antares is a semi-regular variable star, fluctuating between about magnitude 0.6 and 1.6 over a period of roughly 4.5 years.

Antares has a mass of about 12 to 15 solar masses and is losing mass through a strong stellar wind, shedding material into a surrounding nebula visible in deep images. Like Betelgeuse, Antares is expected to end its life in a supernova explosion within the next million years or so.

The Companion Star

Antares has a companion star, Antares B, a hot blue-green star of magnitude 5.5 orbiting at a separation of about 2.5 arcseconds. Despite being bright enough to see easily on its own, Antares B is extremely difficult to observe because it is overwhelmed by the glare of the red primary. When conditions allow it to be glimpsed, the color contrast between the fiery red primary and the blue-green companion is stunning.

The best opportunities to observe Antares B occur during lunar occultations, when the Moon covers Antares A first, briefly revealing the companion before it too disappears behind the lunar limb. These events are rare and highly prized by observers.

One of the Royal Stars

Antares was one of the four Royal Stars of ancient Persia, marking the autumnal equinox around 3000 BCE (the equinox has since precessed away). The other Royal Stars were Aldebaran, Fomalhaut, and Regulus. These four stars divided the sky into quadrants and served as important seasonal markers.

In Polynesian navigation, Antares was known as Rehua and played a role in determining direction during ocean voyages. In Chinese astronomy, it was the heart of the Azure Dragon of the East, one of the four great celestial animals. Many cultures recognized its red color and associated it with fire, war, or blood.

Best Time to Observe

Antares is best observed during June, July, and August, when Scorpius crosses the meridian during evening hours. The star culminates at its highest around early July. Because Antares is far south on the ecliptic (declination -26 degrees), it never rises very high from northern latitudes above about 40 degrees. A clear southern horizon is essential for good observation.

From the Southern Hemisphere, Antares rides much higher in the sky and is one of the most conspicuous objects during southern winter evenings.

Neighboring Stars and Constellations

Antares sits in the midst of Scorpius, with the scorpion's head to the upper right and the curving tail to the lower left. Sagittarius lies to the east, toward the galactic center. Libra, the former claws of the scorpion, sits to the west. The globular cluster M4 lies just over a degree west of Antares and is easy to find using the star as a guide. Explore the heart of the scorpion with StarGlobe.

Quick Facts

Antares has a variable apparent magnitude averaging about 1.06, a distance of approximately 550 light-years, and a luminosity roughly 75,000 times the Sun (bolometric). Its diameter is about 680 solar radii, and its surface temperature is approximately 3,400 Kelvin. Its right ascension is 16h 29m, and its declination is -26 degrees 26 minutes.