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Arcturus: The Mysteries of the Red Giant Star

arcturus, the brightest star tonight
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The Brightest Star in the Northern Sky (No Star Chart Needed)

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Astronomy is filled with wonders, but it is also frustrating if you can’t see the sights. So, we will focus on the #1 brightest star in our northern sky. Its name is Arcturus. No star charts are needed. Discover this orange-colored giant star with is fascinating mythology.

If you love the night sky as I do, you know that there are mind-stretching realities and visual wonders capable of impressing anyone.

Unfortunately, the night sky also has a darker side, if you’ll excuse the paradox, because it’s packed with caveats and limitations. “You’ll see a meteor a minute if you get away from city lights,” we caution every August. Or, “You’ll likely enjoy vivid auroras if the Moon is absent.” Or maybe the news article will promise the Northern Lights “. . If you travel to near the Arctic Circle. Stuff like that.

Let’s get real. You won’t hop a Trailways bus and leave the city to see a few meteors. Nor does your appointment calendar say, “Thursday: Arrive at the Arctic Circle.” And you don’t want to need to fool with a star chart. 

My job is to highlight a few sky-objects or sky-happenings that you can instantly locate no matter where you live, and we’ll make sure they’re cooly enveloped in myth, mythology, science, or strangeness so that you can instantly impress friends or just yourself.

What Is Arcturus?

Arcturus is the brightest star in the northern sky, easily visible without a star chart. This brilliant orange-red star, found in the constellation Boötes (boh-OH-teez) the Herdsman, is around 36.7 light-years from Earth. Arcturus’ distinctive reddish hue reflects its temperature. It’s a cool star (in the thermometer rather than the hip sense), about 7,300 degrees Fahrenheit.

When winter ends, our planet starts facing away from the brilliance of the Milky Way—and the nursery filled with blue super-hot baby stars. The spring sky initially takes on a dim, forlorn appearance. We’re ready for new, bright objects to hold our attention.

Perfect timing. So happens, a genuinely brilliant star—the best we’ll see all spring and summer—has now risen, ready for our admiration. And it has an easily pronounceable name. Arcturus. That’s rare. Two-thirds of star names came to us from the ancient desert-dwelling Arabs who cursed them with titles like Zubenelgenubi, Libra’s brightest star. 

Follow the Arc to Arcturus

Around 9:30 P.M., gaze toward the sunrise position. High up in that direction shines the brilliant orange-white star of the first magnitude.

This is Arcturus.

If it’s around midnight, just look overhead, and Arcturus is simply the only brilliant star shining, with few celestial neighbors. 

Nothing more is needed for identification, but just to make things kindergarten-easy, you might recall the helpful phrase in every beginner’s astronomy book: “Follow the arc to Arcturus [and then speed on to Spica].” Meaning, follow the Big Dipper’s curving handle. If you continue the arc of that ladle, it curves precisely to the first bright star, Arcturus. (If you continue the line further, it needs to bright Spica.)

If you can commit this phrase to memory, you’ll easily be able to go out on a clear night and find the star Arcturus. This is how ancient stargazers “followed the arc to Arturus.”

About Arcturus

Credit: Credit: Babak Tafreshi, TWAN

Color

Arcturus has a beautiful orange-white color. The giveaway that you’re looking at Arcturus is its distinctive reddish hue.

Star Type

Arcturus is a “red giant,” a star that is slowly on its way to death (though that may take billions of years). Like other red giants in the northern sky, this color indicates the star type. To be a red giant, a star must have between half our Sun’s mass up to eight times our Sun’s mass.

Size

Today, Arcturus’ diameter is roughly 25 times greater than our Sun.

Temperature

It’s a cool star, about 7,300 degrees Fahrenheit. This is several thousand degrees cooler than the surface of our Sun. Young stars like nearby Vega are blue-white, which stems from their youthfulness.

Age

It’s far older than our Sun, believed to be an estimated 7.1 billion years old (compared to our Sun, roughly 4.5 billion years old).

Constellation

Arcturus is the brightest member of a kite-shaped constellation called Boötes the Herdsman. I’m not quite sure what a herdsman looks like, but it’s easy to see the kite. 

Distance From Earth

Part of the reason Arcturus looks so bright is that it’s pretty nearby. Arcturus is just 36.7 light-years from Earth, while its pal Vega is a mere 25 light-years. Think of what you were doing 36 years ago. That was when tonight’s Arcturus’s light left that star. 

Any star closer than around 50 light-years is considered nearby, the average naked-eye star is 100 to 300 light-years away.

3 Fun Facts About Arcturus

Arcturus has not one but three oddities. 

  1. The first is that starting in 1898, astronomers used radiometers to measure its total energy, including infrared or heat. They found that despite having the same brightness and nearly the same distance from us as Vega, Arcturus puts out so much infrared that its heat reaching our faces as we stare at it is more than twice what we get from Vega. Not that you could “starbathe” by facing Arcturus with a tanning sun reflector. But turns out, if you hold up your palms toward Arcturus and carefully feel its warmth, you’d find it equal to the heat from a single candle located five miles away. 
  2. Here’s another strange one. Our galaxy is flat like a pancake, with almost all its stars revolving around the galactic core every 225 million years. But Arcturus doesn’t follow the flat plane of the Milky Way. Instead, this star circles the galaxy’s center at a 90-degree angle. It has just dived down toward our solar system from above instead of orbiting next to us like adjacent horses on a carousel. Because of this, Arcturus was totally invisible a mere 500,000 years ago. Brightening ever since, it’s now at very nearly its closest point to us, explaining its great brilliance.
  3. Arcturus gained its greatest fame for a strange reason. In 1933, when the new Chicago World’s Fair opened, the authorities focused the light from Arcturus onto a photoelectric cell, a new-fangled device back then, to throw the switch to turn on all the lights of their dazzling, famous ‘Century of Exposition’ pavilion, thus officially opening the fair. Why did they use Arcturus? That’s the cool part. Arcturus was believed to lie 40 light-years away, meaning that the light that left that star just as the previous Chicago Worlds Fair was closing in 1893 would now open the new one!

Myth and Meaning

Arcturus is a Greek word that generally means “guardian of the bear” or “bear keeper.” This herdsman is placed near Ursa Major, whose Latin name means “greater bear.” So, Arcturus drives Ursa Major across the sky. 

In the Bible, stars are rarely named, but Arcturus is. (“Which maketh Arcturus, Orion and Pleiades, and the chambers of the south” – Job 9:9, KJV, and “Canst thou bring forth Mazzaroth in his season? or canst thou guide Arcturus with his sons?” – Job 38:32, KJV.)

The old red giant dates back to antiquity, and literature speaks of Polynesian sailors who used this high-up star to sail by starlight. The Ancient Romans foretold the weather with it.

Arcturus in Pop Culture

Perhaps because Arcturus is so bright, such a cool color, and so easy to pronounce, it’s been explored in literature, film, and pop culture many times, including: 

  • Isaac Asimov’s Foundation book series
  • David Lindsay’s novel, A Voyage to Arcturus
  • Douglas Adams’s series, The Hitchhiker’s Guide to the Galaxy
    “This compares to an ancient Arcturan Proverb, “How ever fast the body travels, the soul travels at the speed of an Arcturan Mega-Camel. –OOlan Colluphid” 
  • “Doctor Who” television series
  • “Star Trek” television series
  • The movie “Aliens”

So salute the orange fire of the star Arcturus the next clear evening, high in the east at 9:30 or nearly overhead at midnight! And if you want to learn about its pal, read my 10 facts about Vega, the summer star.

About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman

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