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 the constellation Scorpius, with red star Antares at the Scorpion’s Heart. Adapted from a photo by Akira Fuji. Image 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’s nickname is the Scorpion’s Heart. Its magnitude varies between 0.6 and 1.6.

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

Star field with orange nebula, a bright orange star, Antares, and a small fuzzy round white object.
Red Antares, with globular cluster NGC 6144 to the upper right. Image via Fred Espenak at AstroPixels. Used with permission.

The Heart of the Scorpion is a red supergiant

Antares holds the stellar classification of a 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 place our sun and Antares side by side, you’d find Antares more than 10,000 times brighter than our sun. Plus it is about 700 times the sun’s 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.

For Antares, the end is nigh

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 rapid collapse 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.

In 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. They also found that there was a lot of turbulence in the star’s atmosphere and that the star was expelling gases further away than they expected.

Circular blob in concentric brilliant rainbow colors.
Turbulent discharge: The first relatively detailed image of the red supergiant Antares, which shows the stellar disk (yellow) with 2 brighter regions (white) and the extended atmosphere (blue and green) of the star. Its irregular shape, with several bulges and the variable distribution of gas, indicates that the star only loses matter in some regions, in turbulent currents. Image via K. Ohnaka et al. 2017/ Nature. Learn more about this image.

Just how large is Antares?

Antares is a truly enormous star with between 680 to 800 times the sun’s radius. That’s more than three astronomical units (AU). One AU is the Earth’s average distance from the sun. If by some bit of magic, we could substitute Antares for our sun, Antares’ surface would extend well past the orbit of Mars and into the asteroid belt.

Recently, astronomers discovered more details about Antares’ outer surface. In 2020, a study of data from 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 its companion star, Antares B, was lighting up some of the gaseous material that Antares was ejecting.

Size comparison chart with an orange orb reaching to orbits of outer solar system planets.
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. But scientists can detect gases expelled from the star even farther out. Image via S. Dagnello / NRAO/ AUI/ NSF.

Antares and Antares B

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

The companion is a blue-white main sequence star with a magnitude of just 5.5. This is 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 also a big star, bigger than our sun. It’s about seven times the sun’s mass and five times the sun’s size. But Antares B is no match for the size of mighty Antares.

Amateur astronomers can spot Antares B on a steady night with a telescope of at least 8-inch aperture and 200 power. The secondary star is about 2.5 arcseconds due west of Antares.

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

How to see the Heart of the Scorpion

Look southward in early evening from late spring to early fall to find the fishhook pattern of Scorpius the Scorpion, with ruby Antares at its heart. With the eye alone, and with binoculars, you should notice its reddish color. If you have binoculars and a dark sky, scan just to the right of Antares. You should see a little star cluster, M4.

Antares is the 16th brightest star in the sky. 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 near the horizon, we see Antares twinkle fiercely. In the Northern Hemisphere, anywhere south of 63° north latitude can – at one time or another – see Antares. (Helsinki, yes, Fairbanks, no.)

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 63° 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.

The midnight culmination of Antares is in mid-June. 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 late March and at about evening twilight in late July.

A star map with stars in black on white showing the stars in Scorpius.
Map of the constellation Scorpius, showing Alpha Scorpii – or Antares – the brightest star in the constellation. Image via International Astronomical Union/ Sky & Telescope/ Wikimedia Commons.

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 due to its competing red color. For a few months every couple of years Mars is much brighter than Antares. Also, every couple of years Mars passes near Antares, as if taunting the star. Mars moves rapidly through the heavens and Antares is fixed to the starry firmament.

The most well-known story of Scorpius, home to Antares, 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 now both reside in the sky on opposite sides of the heavens.

In Polynesia, Scorpius represents 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 published in 1824. Image via 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 26, 2022
 in 
Brightest Stars

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