Rival of Mars: Getting to know the Summer Star Antares

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Happy summer everyone! Or, at least, happy summer to everyone in the northern hemisphere (happy winter to any southern hemisphere residents who happen to be reading this). This time of year marks the return of long, hot days marked by extremely late sunsets. And then, once the Sun has finally set, the reliable sight of a bright red point of light in the southern sky.

This is the star Antares, whose return to the evening sky has always been the marker of summer for me. It’s a massive, dying star, but it doesn’t get nearly as much attention as that other massive, dying star, Betelgeuse. So I’m going to rectify that a little bit. 

Join me for a deep dive into this fiery sign of summer, the star Antares!

The Basics

Okay, first and foremost Antares isn’t technically a star. It’s two. Antares is a binary system consisting of a massive, dying red giant, and a smaller (but still not small) star still in the prime of life. These are referred to as Antares A (the giant) and Antares B (the other one). 

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Although Antares B is a big star, over seven times the mass of our Sun and over five times the Sun’s radius, burning with a surface temperature more than three times hotter than our local star, it’s not what you’re seeing when you look at Antares from Earth. It’s just not bright enough compared to the giant.

It’s a perfectly normal star that, left alone, would live out a perfectly normal life and eventually turn into a perfectly normal white dwarf. Of course it’s only 50 billion miles from the giant and the giant is definitely going to explode someday, so that quiet fading into a white dwarf might not be in the cards for Antares B.

When people refer to “Antares”, they mean the giant. This is a star that started out big (it’s difficult to guess how big, but likely at least 15 times the mass of the Sun), which means it was destined to live fast and die young. Again, it’s hard to tell precisely how old this star is, but it’s less than 20 million years.

To give that a little perspective, the dinosaurs had been gone for several tens of millions of years by the time Antares first began to shine. The star is roughly as old as crows, which is a fun fact I now know.

Red Giant Rumble

Antares has a lot in common with the dying giant Betelgeuse, though they are coincidentally on almost opposite sides of the sky from each other. They are roughly the same size, roughly the same distance from Earth, and very close to each other in brightness. And they’re both gonna explode in a big way.

Yet a lot more people seem to be aware of Betelgeuse than Antares. While I can’t know for sure, my guess is that this is due to a couple of factors. First is their constellations. Betelgeuse is part of the bright, easily-spotted winter constellation Orion, which means finding Betelgeuse is fairly simple (finding Antares is fairly simple as well, because it’s much brighter than anything around it, but its constellation lacks anything as easily identifiable to astronomical newbies as Orion’s Belt).

Betelgeuse also seems to be a more temperamental star than Antares. Both are variables, fluctuating in brightness, but Betelgeuse fluctuates much more wildly, sometimes in ways that garner mainstream headlines (anybody remember the Great Dimming that Betelgeuse underwent in 2020?).

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Betelgeuse is also likely to blow before Antares does. Nobody can precisely predict very much in advance when a supernova will go, and technically either could go any day (and, in fact, may have already gone sometime in the last couple of hundred years and we wouldn’t know about it yet). But most predictions put Betelgeuse’s end somewhere in the next hundreds of thousands of years, while Antares is roughly estimated to have at least a million years left.

Of course, there’s also the fact that Betelgeuse has had a major movie character named after it and Antares hasn’t. Maybe that has something to do with it.

The Constellation

Antares is the heart of the scorpion. It’s the brightest star in the constellation Scorpius, which from the latitude of Boston takes up a big chunk of the southern summer sky. Boston is about as far north as you can get to see the entirety of this constellation. From here the scorpion’s tail curls down almost exactly to the horizon before arcing back up to end in the stinger.

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The identification of this group of stars as a scorpion was a widespread one in the ancient civilizations that spread out from the Mediterranean region, with everyone from Babylon to Egypt associating this group of stars (and Antares specifically) with scorpion-related stories and figures, though the exact mythology varied from group to group.

The “official” myth attached to it is, naturally, the Greco-Roman one, in which it was the scorpion that the goddess Artemis sent to attack the hunter Orion, which is why it’s never in the sky at the same time as Orion—Orion sets just as Scorpius rises and vice versa, because Orion is doing his best to stay far away from the angry scorpion. Which, really, is just showing good sense.

Of course, there are plenty of folks who saw this star group as something other than a scorpion. My personal favorite story (brought on, yes, by the movie Moana, which is perhaps a shallow reason for constellation story favoritism, but it’s also a really good movie) is the Hawaiian Islands mythology which identifies this group as the fishhook of the demigod Maui.

Martian Rivalry

The name “Antares” specifically translates to “Rival of Ares”, aka Mars. This is due to the visibly red coloring of the star and its appearance in a part of the sky that Mars also moves through. I like to think that ancient astronomers decided too many people were mixing the star and the planet up and so decided, more or less, to name the star “not Mars”.

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The red color, alongside its placement in the constellation, also leads to its identification as Scorpius’s heart. That color is due, of course, to the star’s low temperature. When stars expand into giants as they die, their outer layers get farther and farther from the burning heat of their cores, causing them to cool off and change color from white hot to merely red hot. 

And the surface of Antares is well away from its spicy core. If you were to put Antares in our own solar system in place of the Sun, its outermost layer would reach at least to the asteroid belt (some models put the outer layers of its atmosphere as far out as Uranus’s orbit) and the entire inner solar system would be history. Antares isn’t the kind of star that forms planets (according to our current models, anyway), but if it had it likely would have already eaten them.

Summer Skywatching

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Spotting Scorpius can be tricky, depending on your location and sky conditions, but spotting Antares in the sky is fairly easy, as long as you have a clear sky to the south (and as long as you live below the Arctic Circle, which something like 99.95% of Earth’s population does).

There’s nothing nearly so bright as Antares in its vicinity, so a good start is to face south after sunset during the summer and look for the brightest star you can see in that part of the sky. If that star has a reddish tint to it, that’s a good sign that you’ve spotted the rival of Mars, the heart of the scorpion, the dying giant Antares.

And while you’re out there, lay back and enjoy the rest of the sky. It’s summer after all—enjoy it while it lasts!