Brightest Stars

Ruby red Antares is the Scorpion’s Heart

Star chart superimposed over a photo of the Milky Way, with an arrow pointing to Antares.
Outline of constellation Scorpius, with red star Antares at the Scorpion’s Heart. Adapted from a photo by Akira Fuji. Via rondeauprovincialpark.ca

Antares is an eye-catching star, shining with a distinctive bright red sparkle on northern summer evenings. In the Southern Hemisphere, it’s a red beacon in winter evening skies. This star, also known as Alpha Scorpii, lies about 550 light-years away. It’s the brightest star in the zodiacal constellation Scorpius the Scorpion, which has figured prominently in the sky lore of ancient cultures. Antares is often called the Scorpion’s Heart.

Today, we know that Antares is a very large massive star – a red supergiant – in the final stages of its life.

Star field with faint orange gas regions and a central bright orange star, Antares.
Red Antares, via Fred Espenak at AstroPixels. Used with permission.

Antares as a red supergiant

It’s classified as an M1 red supergiant star.

The M1 designation means that Antares is reddish in color and much cooler than many other stars. Its surface temperature is about 6,100 degrees F (3,400 degrees C). That’s in contrast to our sun’s surface temperature of about 10,000 degrees F (5,800 degrees C).

So Antares is relatively cool, and its surface temperature is relatively low. Yet the star appears very bright to us. That’s because Antares is a truly enormous star. Its surface area – the surface from which light can escape this star – is gigantic. If you could hold our sun and Antares side by side, you’d find Antares more than 11,000 times brighter than our sun, and with 680 times the diameter!

And that’s just in visible light. When you consider all the various wavelengths of electromagnetic radiation, Antares pumps out about 75,900 times the energy of our sun!

Like all M-type giants and supergiants, Antares is close to the end of its lifetime. Someday soon (astronomically speaking), it will effectively run out of fuel and collapse. The resulting infall of its enormous mass – some 11-14 times the mass of our sun – will cause an immense supernova explosion, ultimately leaving a tiny neutron star or possibly a black hole. This explosion, which could be tomorrow or millions of years from now, will be spectacular as seen from Earth, but we are far enough away that there likely is no danger to our planet.

And the meantime, astronomers love to explore huge Antares. In 2017, the European Southern Observatory released a detailed image, taken in infrared wavelengths, of features on Antares’ surface. The also found that there was a lot of turbulence in the star’s atmosphere and that gases were being expelled further away from the star than expected.

An orange disk with two prominent elongated yellowish features on it.
The most detailed image to date of Antares’ surface (and any other star than our sun). This image, from 2017, was observed in infrared wavelengths using an imaging technique called interferometry. Image via K. Ohnaka / ESO.

Just how large is Antares?

At 680 times the sun’s radius, Antares is a truly enormous star. That’s more than 3 astronomical units (AU). One AU is the Earth’s average distance from the sun. If by some bit of magic Antares was suddenly substituted for our sun, its surface would extend well past the orbit of Mars!

Recently, astronomers discovered more details about Antares’ outer surface. In 2020, a study of data taken by radio telescopes showed that Antares’s chromosphere (that’s the layer above the star’s surface) extended out by 2.5 times the star’s radius, far more than previously thought. In comparison, our sun’s chromosphere is only 1/200th of its radius.

They also saw that some of the gaseous material ejected by Antares was being lit up by its companion star, Antares B.

Annotated size comparison chart with an orange orb in different layers and a ruler stretching out from its center to the right.
View larger. | In this diagram, the inner disk represents the surface of Antares. If this star replaced our sun, it would engulf everything past the orbit of Mars. New data, published in 2020, from radio telescopes (marked with the acronyms ALMA and VLA in the figure) show that Antares’ chromosphere would extend past Jupiter. Gases expelled from the star are detectable even farther out. Image via S. Dagnello / NRAO/ AUI/ NSF.

Antares and Antares B

Antares isn’t alone. It has a companion, called Antares B. As the image below shows, Antares B is hard to see next to its much brighter companion.

The companion is a blue-white main sequence star with a visual magnitude of just 5.5. This near the edge of what you can see with the unaided eye. Antares itself varies in brightness, and its visual magnitude ranges from 0.6 to 1.6. Antares B is a big star, too, bigger than our sun. It’s about 7 times the sun’s mass and 5 times the sun’s size. But Antares B is no match for the size of mighty Antares.

An overexposed red star, with a tiny dot, a companion star, at its side.
Antares B, the companion to the red supergiant Antares. The photo was taken using an 11″ SCT scope with diagonal and Hex Mask at 220X on August 21, 2011. Ernest R. Evans took the photo, and Scott MacNeill processed it using Registax 6 and Photoshop. Image via Astronomy.com.

Antares in history and skylore

Both the Arabic and Latin names for the star Antares mean heart of the Scorpion. If you see this constellation in the sky, you’ll find that Antares does indeed seem to reside at the Scorpion’s heart.

Antares is Greek for rival of Ares, meaning rival of Mars. Antares is sometimes said to be the “anti-Mars.” All this rivalry, as told in ancient celestial lore, stems from the colors of Mars and Antares. Both are red in color, and, for a few months every couple of years Mars is much brighter than Antares. Most of the time, though, Mars is near the same brightness or much fainter than Antares. Every couple of years, Mars passes near Antares, which was perhaps seen as taunting the star, as Mars moves rapidly through the heavens and Antares, like all stars, seems fixed to the starry firmament.

As is typical, more mythology attends the full constellation of Scorpius than the star Antares. Perhaps the most well known story of Scorpius is that the Earth goddess, Gaia, sent him to sting arrogant Orion, who had claimed his intent to kill all animals on the planet. Scorpius killed Orion, and both were placed in the sky, although in opposite sides of the heavens, positioned as if to show the Scorpion chasing the Mighty Hunter.

In Polynesia, Scorpius is often seen as a fishhook, with some stories describing it as the magic fishhook used by the demigod Maui to pull up land from the ocean floor that became the Hawaiian islands. According to the University of Hawaii’s Institute for Astronomy, the Hawaiian name for Antares, Lehua-kona, seems to have little to do with the constellation. It means “southern lehua blossom.”

A painting of a bluish-green scorpion with stars marked on it.
Scorpius, as depicted in Urania’s Mirror, a set of star chart cards that were published in 1824. Image via Wikimedia Commons.

How to see Antares

If you look southward in early evening from late spring to early fall, you’re likely to notice the fishhook pattern of Scorpius the Scorpion, with ruby Antares at its heart. If you think you’ve found Antares, aim binoculars in its direction. With the eye alone, and with binoculars, you should notice its reddish color. If you have binoculars and a dark sky, also scan just to the right of Antares. You should see a little star cluster, M4.

Antares is the 16th brightest star in the sky. It’s located in the southern half of Earth’s sky and is a beautiful sight from both the Northern and Southern Hemispheres. From our northerly latitudes, we see it arc across the south. Because we’re sometimes looking at it through a greater thickness of Earth’s atmosphere in the direction toward the horizon, we see Antares twinkle fiercely.

From the Southern Hemisphere, Antares appears higher in the sky. Your chance of seeing this star on any given night increases as you go farther southward on Earth’s globe. If you traveled to about 67° south latitude, you’d find that Antares is circumpolar. That means that, from Earth’s southernmost regions, Antares never sets and is visible every night of the year.

From most of the Northern Hemisphere, short of the Arctic, Antares is within view. Well, not quite the Arctic, but anywhere south of 63° north latitude can – at one time or another – see Antares. (Helsinki, yes, Fairbanks, no)

The midnight culmination of Antares is on or near June 1. That’s when Antares is highest in the sky at midnight (midway between sunset and sunrise). It is highest in the sky at about dawn in early March and at about sunset in early September.

A star map showing the stars in Scorpius.
Map of the constellation Scorpius, showing Alpha Scorpii — or Antares — the brightest star in the constellation. Image via IAU and Sky & Telescope/ Wikimedia Commons.

Antares’ position is RA:16h 29m 24s, dec: -26° 25′ 55″.

Bottom line: Antares is a brilliant ruby red star in summer for the Northern Hemisphere (winter for the Southern Hemisphere). It’s an enormous red supergiant star, whose constellation – Scorpius the Scorpion – has a rich history in skylore.

Posted 
June 22, 2021
 in 
Brightest Stars

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