7 Facts About Arcturus, the Red Giant Star

By: Valerie Stimac  | 
An artist's conception of Arcturus, a red giant star easily visible from Earth. Pablo Carlos Budassi/Wikimedia/CC BY-SA 4.0

When the 1933 World's Fair opened in Chicago, it was marked by a mechanism that captured the light of a distant object in the night sky: the star Arcturus, shining in the constellation Boötes. World's Fair organizers had chosen Arcturus, because at the time, it was believed that the star was roughly 40 light-years from Earth – and the previous Chicago World's Fair had taken place 40 years earlier in 1893.


How Far Away is Arcturus Star?

In fact, bright Arcturus is 36.7 light-years from earth, as astronomers discovered over the intervening decades. It's the brightest star north of the celestial equator. This is just one of the many incredible discoveries that we've made about Arcturus in the 20th and 21st centuries, though the star dates back to antiquity in historical records and stories to the Polynesian navigators who took advantage of this zenith star and used it to sail by starlight in the night sky and the Ancient Romans who foretold weather with it.

If you're curious to learn about Arcturus, one of the brightest stars in the night sky, read on for interesting facts on how it can be spotted with the naked eye, fascinating discoveries, and more interesting facts about Arcturus star.


1. You Need to "Follow the Arc to Arcturus"

There's an oft-spoken phrase when you ask an astronomer how to find Arcturus, the fourth brightest star in the Milky Way: "Follow the arc to Arcturus [and then speed on to Spica]." What this means is that if you follow the arc of the handle of the Big Dipper, the first bright star you'll see by drawing an imaginary line will be Arcturus; if you continue the line further, you'll next see bright Spica in the neighboring constellation of Virgo.

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. The other giveaway that you're looking at the right star is Arcturus' distinctive reddish hue. Like other red giants in the northern sky, this color is an indication of the star type.


As twilight fades to darkness, Arcturus in the constellation Bootes shines brightest. It's the most visible one shining over the trees.
Jeff Dai/Stocktrek Images/Getty Images

2. Arcturus Is a Powerhouse Red Giant Star

Speaking of Arcturus' star type, it can officially be classified as the brightest K-type, a red giant of spectral type K0III. While this all sounds like jargon, it means that Arcturus is a bright, large star that has moved beyond the main sequence (the phase our own sun is in).

Arcturus is so bright that it ranks fourth among the brightest stars in the sky (behind Sirius, Canopus, and the binary system Alpha Centauri), making it one of the most luminous stars in the Northern Hemisphere. It shines roughly 113 times brighter, despite Arcturus only being about 1.5 times bigger than our sun. Arcturus also releases roughly 215 times more heat than the sun, proof that it is generating a ton of energy despite its size.


3. Arcturus will Become a White Dwarf

Though it happens on the billions of years' scale, stars move through life cycles just like almost every other known object in the universe. For bright stars like Arcturus, it has already moved through the main sequence, fusing hydrogen in its core as our sun is currently doing it. When Arcturus' supply of hydrogen was fully depleted, it transitioned into its red giant status and astronomers believe it is now fusing helium into carbon in its core instead (which helps explain why it shines brightly and produces so much heat).

When the helium in Arcturus is all gone, what's next? Arcturus will begin to lose mass, shedding layers of gas and slowly shrinking to enter the final phase of its life as a white dwarf star. White dwarfs are the small, dense cores of former stars; astronomers have identified just eight white dwarf stars in the systems around our sun because of their faintness.


4. Arcturus is about 7.1 Billion Years Old ... Maybe

Since Arcturus is in a later stage of life than our sun, astronomers believe it is likely older than it, too (our sun is believed to be roughly 4.5 billion years old). Since we can't ask Arcturus its age, some astronomers have used measurements of the star and different elements like iron and helium to estimate its age at about 7.1 billion years old, but in a range between 6.0 and 8.5 billion years old due to the way it was measured.

However, other astronomers measuring carbon in Arcturus have debated this estimate (though there hasn't been a better estimate put forward), so for now we'll assume we need to buy a lot of birthday candles no matter what.


5. Arcturus is Moving Toward the Sun

As the universe is expanding, Arcturus is on the move. Its current trajectory is actually bringing Arcturus closer to the sun; the two stars will be at their closest in roughly 4,000 years – but don't worry: Arcturus will only be a few hundredths of a light-year closer to us by that point.


6. Part of The Arcturus Stream

Speaking of Arcturus moving in the sky, Arcturus is also part of a series of 53 stars known as the Arcturus Stream, which all appear to be moving in a similar way through the galaxy – possibly as a result of "dynamical perturbations" (normal abnormalities) in the Milky Way.


7. We Once Thought Arcturus had a Planet Way Bigger than Jupiter

At its mature age and stability, astronomers have wondered whether Arcturus might be a binary star, or have its own planet or planetary system. In the late 20th century, astronomers studied certain movements displayed by Arcturus as well as two other giant stars, Aldebaran and Pollux. They determined that it was possible that movement was caused by a giant planet, roughly 12 times larger than Jupiter, orbiting each star at roughly the distance from the earth to the sun.

In the end though, the astronomers concluded that since all three stars – all varying sizes, ages, and distances from the earth – showed the same results, it was more likely these stars didn't have similar super-Jupiters and the movement was intrinsic to stars like them.


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