Arcturus: Scintillating facts about the 4th brightest star in the night sky

Actually, it was well known to the ancients.
Christopher McFadden
Deep space background stock photo.
Deep space background stock photo.titoOnz/iStock

Our venerable and magnificent Sun is the literal center of the Solar System and is vital for life on our planet. In a way, it can be thought of as our creator, keeper, and protector. It was even worshipped as a god in millennia gone by.

But, on the scale of the universe, even Sol, as some call it, is nothing to really write home about. There are other, far larger and more powerful, suns out there.

One such star is Arcturus.

One of the brightest objects in the night sky, this star was well known to the ancients and still fascinates us today. Let's find out more about this faraway miracle of nature.

What is Arcturus?

The brightest star in the constellation Boötes (meaning, roughly, "The Herdsman") is the red giant star Arcturus, One of the brightest stars in this constellation, and one of the brightest that can be seen from Earth, is Arcturus, and astronomers believe this star will end its days as a white dwarf.

Its name is derived from the Greek meaning "Keeper or Guardian of the Bear," which refers to its position adjacent to the tail of the constellation Ursa Major (the Great Bear).

Arcturus, the fourth brightest star in the night sky, is located around 37 light-years from Earth. Arcturus is most easily located by following the Big Dipper's "handle" curvature.

Arcturus' position in the night's sky.

If you want to spot it for yourself, an easy mnemonic is to "follow the arc to Arcturus and then speed on to Spica." The latter part of that phrase refers to the bright star Spica, which is actually a binary star.

More specifically, the location of Arcturus is:

  • 14 hours, 15 minutes, and 39.7 seconds of right ascension
  • +19 degrees, 10 minutes, 57 seconds of declination

Even though Arcturus only has a mass that is roughly 1.5 times that of the Sun, astronomers now understand that it has a powerful punch. According to Jim Kaler, an emeritus professor at the University of Illinois at Urbana-Champaign, Arcturus appears to shine 113 times brighter than the Sun to the unaided eye.

However, Arcturus is cooler than the sun, so a large portion of the red giant star's energy is released as heat. This means that Arcturus actually emits something like 215 times more heat than our Sun does.

The apparent and absolute magnitudes of Arcturus are -0.04 and 0.2, respectively. The brighter the star, the lower its apparent and absolute magnitudes.

This seems to indicate that the star is nearing the end of its natural life. Since the star is a red giant, Arcturus has stopped fusing hydrogen in its core, as our Sun does, and astronomers believe it is now starting to fuse heavier elements such as carbon.

According to Kaler, "such stars are not expected to have magnetic activity like the Sun, but very weak X-ray emission suggests that Arcturus, indeed, is magnetically active and has a hard-to-observe 'buried corona.'"

Arcturus' outer layers will probably flow away after its helium supply runs out, leaving behind a white dwarf remnant.

Arcturus is not only fascinating from a scientific point of view but also features heavily in various works of fiction.

For example. although fictional planets around Arcturus have been the subject of science fiction, no planets have actually been discovered to orbit this star. "A Voyage to Arcturus" by David Lindsay is one of the earliest examples (Methuen & Co. Ltd., 1920). The protagonist of the book visits Tormance, a made-up planet that revolves around Arcturus.

Detail of an antique brass planetarium showing leo and taurus

Arcturus, either the star or a fictional planet with the same name, has also appeared in many other science fiction references, including Isaac Asimov's "Foundation" book series, the "Doctor Who," and "Star Trek" television series, and the movie "Aliens."

In "The Hitchhiker's Guide to the Galaxy" universe, a fictional ancient Arcturan proverb goes, "However fast the body travels, the soul travels at the speed of an Arcturan mega-camel."

Arcturus quick-fire facts

  • Other name(s): Alpha Boötes
  • Distance From Earth: 37 light-years
  • Constellation: Boötes
  • Star Type: Orange Giant - K Class
  • Mass: 110% of the Sun
  • Luminosity: 100 to 200 times the Sun
  • Diameter: Approx 22 million miles (36 million km) - 26 times that of the Sun
  • Surface temperature: Approx 7200 degrees Fahrenheit (circa 4,000 degrees Celsius)
  • Estimated age: Approx 7 billion years old
  • Rotation Period: 2 years

Who is Arcturus in Greek mythology?

There are a number of legends told about the origin of Arcturus. According to one legend, the star Arcturus, which means "Guardian of the Bear," was named after Arkas (Arcas), an ancestor of the Arkadians (Arcadians). The bear that he is defending is his mother, Kallisto (Callisto), or Ursa Major (the Great Bear).

In Greek mythology, the legendary monarch Arcas was Zeus's son, and Arcadia is named in his honor. Zeus's son Arcas was born to Callisto, the daughter of King Lycaon of Pelasgia. Callisto was a member of Artemis' hunting retinue, so Zeus enticed her into the woods rather than in her father's palace, and she became pregnant.

Arcturus was named after Arcus, a son of Zeus.

As a virgin goddess, Artemis dismissed Callisto from her retinue after learning that one of her servants was pregnant. Even worse was to follow for Callisto; after giving birth to a son of Zeus, she was turned into a bear by a furious Hera, leaving Callisto to roam the same forests she had once hunted. If Zeus hadn't stepped in and taken the newborn infant away, Hera would have treated the kid of Callisto and Zeus in the same way or killed him.

Zeus would give his new son the name Arcas, and after handing the infant to the messenger god Hermes, he was raised by the Pleiad Maia (the mother of Hermes) to adulthood. Arcus would then, in turn, become the god-king of the Arcadians.

In one version of the legend, Arcas was reputed to be a good king who taught his subjects how to grow crops, make bread, and weave baskets after receiving agricultural training from Triptolemus, a Demeter follower.

In another legend, Arcas was out hunting and came across his mother Callisto, who had been transformed into a bear near the temple of Zeus. To prevent Arcas from killing his own mother, Zeus transported them both to the heavens, where Callisto became the constellation Ursa Major and Arcas the star Arcturus.

What is the biggest star known?

Our Sun, compared to Earth, appears to be an enormous star. After all, somewhere in the order of a million+ Earths could fit comfortably inside of it.

But, on a stellar scale, it is, in and of itself, a minnow of a star. Our Sun, for example, could easily be consumed inside any of over half of all observable stars. Especially gigantic stars like Arcturus. But, even this impressive star is not even close to being in the "big leagues" when it comes to star size.

Take UY Scuti, for example.

UY Scuti, the biggest star yet discovered, is an example of something called a "variable hypergiant" whose radius is almost 1,700 times greater than that of our Sun. To put it into perspective, consider that a sphere the size of UY Scuti could hold the volume of about 5 billion of our Sun.

That is truly massive.

This star was first observed and cataloged by German astronomers at the Bonn Observatory in 1860. It is known as a "variable star" because subsequent observations of it revealed it changes brightness and magnitude over a 740-day cycle.

Near the Milky Way's center, the star is located approximately 9,500 light-years from Earth. UY Scuti is a hypergiant star that sits within the constellation Scutum. Rare, brilliant stars called hypergiants are bigger than supergiants and giants. Rapid stellar winds cause them to shed a significant amount of their mass over time.

All is well and good, but there are some difficulties when it comes to estimating the size of distant stars.

According to astronomer Jillian Scudder of the University of Sussex, who told, "the complication with stars is that they have diffuse edges."

"Most stars don't have a rigid surface where the gas ends and vacuum begins, which would have served as a harsh dividing line and easy marker of the end of the star," he added.

Instead, astronomers tend to use the photosphere of a star to estimate its size. The photosphere is the region of the star where photons, or light particles, can leave the star and become transparent to light.

As far as astronomers are concerned, this is the star's surface because it is at this location that photons can escape the object.

With regards to UY Scuti, its photosphere would reach just past Jupiter's orbit if it were to take the place of the Sun at the Solar System's center. The gas nebula that is ejected by the star would stretch beyond the orbit of Pluto, about 400 times farther than the distance between Earth and the Sun.

Needless to say, if that were to ever happen, Earth and all inner rocky planets would cease to exist in very short order.

What are some scintillating facts about the star, Arcturus?

We've already covered a lot of information about this literally and figuratively brilliant star. But, we have still only scratched its incredibly hot surface.

If you want to know more, here are some more scintillating facts about Arcturus.

1. Light from the star was used as an exhibition at the 1933 Chicago World's Fair

The 1933 Chicago World's Fair's planners were looking for a distinctive way to launch their show. The city had already hosted a world's fair in 1893, 40 years prior. The fair's planners came up with the concept to use Arcturus' light as a component of the new exhibition and as a means to honor the prior Chicago World's Fair because, at the time, it was thought that the star was around 40 light-years from Earth.

And so, at 9:15 p.m. (Central Time), on the 27th of May 1933, telescopes focused the light of the star on a number of photoelectric cells. The floodlights at the show grounds were turned on by flipping a switch that was activated by the electric current from the photocells that were fueled by light of the star.

It was only years later that astronomers calculated that Arcturus is closer to 37 light-years from Earth rather than 40. This means that the light used at the Chicago World's Fair actually started its journey to Earth in 1896, not 1893, as previously thought.

2. Arcturus could be 7.1 billion years old or more

Optical image of Arcturus.
NASA/Wikimedia Commons 

When it comes to stars, Arcturus is something of an old-timer. Astronomers believe Arcturus is a lot older than our Sun since it is in a later stage of life.

Our Sun is roughly 4.6 billion years old. However, by using measurements of Arcturus and the presence of several elements, including iron and helium, astronomers are pretty confident Arcturus is between 6.0 and 8.5 billion years old -- about 7.1 billion, give or take.

However, other astronomers measuring carbon in Arcturus have debated this estimate (though there hasn't been a better estimate put forward). In either case, Arcturus is a very old star.

3. Arcturus has no planets, but we used to think it did

Arcturus compared to the Sun.
alexbroPA/Wikimedia Commons 

Astronomers have pondered if Arcturus may have its own planet or planetary system, given its advanced age and stability. Astronomers observed specific movements in Arcturus and two other massive stars, Aldebaran and Pollux, in the latter part of the 20th century. They came to the conclusion that the movement might have been brought on by the presence of big planets, around 12 times as massive as Jupiter, orbiting each star at a distance corresponding to that between the Earth and the Sun.

However, since all three stars, which varied in size, age, and distance from the Earth, showed the same results, the astronomers came to the conclusion that it was more likely that these stars weren't orbited by super-Jupiters, but rather that the movement was inherent to stars like them.

If Arcturus did have any planets in its youth, these have probably long since been consumed by the star as it expanded into a red giant. We may find planets in the future, but for now, we cannot be entirely sure.

4. Apparently, Arcturus is moving towards our very own Sun

Image of the Milky Way.
ESO/Y. Beletsky/Wikimedia Commons 

As we are sure you are aware, it is widely believed that the universe is expanding. This means that Arcturus, being part of that universe, is also constantly on the move.

On its current direction of travel, it appears that Arcturus is moving gradually towards us. This means that our Sun and Arcturus should get as close as they ever will in around 4,000 years' time, give or take.

But don't worry, it won't be much closer to us by then—just a few hundredths of a light-year.

Speaking of how Arcturus appears to be traveling in the sky, it is one of 53 stars that make up something called the "Arcturus Stream." This collection of stars all appears to be moving in a similar way through the galaxy – possibly as a result of "dynamical perturbations" (normal abnormalities) in the Milky Way.

5. Even the brightest stars must one day die

Arcturus will one day become a white dwarf like Pegasi A and B.
RJHall/Wikimedia Commons 

Stars go through life cycles just like nearly every other known object in the cosmos. But with the exception that it is on a scale of billions of years.

Arcturus, a star similar to our Sun, has already passed through the main sequence and is fusing hydrogen in its core. Astronomers think Arcturus is now fusing helium into carbon in its core instead of transitioning into its red giant stage when its hydrogen supply was completely exhausted (which helps explain why it shines brightly and produces so much heat).

But, what happens when there is no more helium left in Arcturus' core?

As it progressively shrinks and starts to lose mass, Arcturus will reach the last stage of its life as a white dwarf star. White dwarfs are the compact, dense cores of extinct stars; despite their faintness, scientists have discovered candidates for high-confidence around 260,000 white dwarf stars, but it has been estimated that there are somewhere around 10 billion white dwarfs in the Milky Way galaxy.

6. Arcturus may have been captured by the Milky Way

Arcturus is believed to have originated outside of the Milky Way's plane, at the furthest reaches of the galaxy's dense halo.

Some researchers have also suggested that the other 50 or so stars that form the "Arcturus Stream' and share Arcturus' motion and trajectory through the Milky Way are actually the remains of a satellite galaxy that Arcturus may have formed in before being swallowed up by the Milky Way.

While this is a popular theory, other astronomers have discovered no differences in the chemical make-up of stars from the stream when they examined the chemical composition of F and G dwarf stars in the solar neighborhood, This would indicate an intragalactic rather than an extragalactic origin.

Another explanation for this co-moving group of stars is that the stream appeared in a manner similar to the so-called "Hercules group", which is hypothesized to have formed due to orbital resonance, such as Outer Lindblad Resonance, with the Galactic bar. However, it is unclear how this could produce an overdensity of stars in the thick disk.

7. Arcturus was known to the ancients

Sky at night

While in ancient Rome it was regarded as the sign of stormy, if not tempestuous weather, the ancient Greeks recognized Arcturus in Bootes as the protector of both Ursa Major and Ursa Minor. One of the few stars actually named in the Bible is Arcturus.

By the Middle Ages, it had been given magical characteristics by some and grouped with 14 other stars known as "Behenian fixed stars," which were thought to be particularly helpful in medieval astrology. The name comes from the Arabic word "bahman," meaning root, and each one was considered a source of astrological power for one or more planets.

The Wotjobaluk Koori (indigenous people of Australia) referred to Arcturus as "Marpean-kurrk," or the "mother of Djuit (Antares)" and Weet-kurrk ("Muphrid").

Arcturus was visible in the northern sky around the same time of year that wood-ant larvae, which the Aborigines in southeast Australia consumed, arrived. When Arcturus set in the west with the Sun, it also signaled the arrival of summer and the disappearance of wood-ant larvae.

8. Arcturus might actually be a binary star

According to data from the Hipparcos spacecraft, Arcturus might be a binary star with a companion in a close orbit. It is extremely challenging to see the hypothesized companion because it appears to be around 20 times fainter than Arcturus and is in a close orbit.

The Hipparcos results were supported by later research that was released in 2005, even though it did not confirm the subgiant companion's existence. The study's analysis of data from near-infrared interferometric measurements suggested the presence of a subgiant companion.

Interestingly, one of Arcturus' neighboring stars also appears to be a binary star too.

The binary star Muphrid (Eta Boötis) seems to be close to Arcturus in the sky and is situated almost at the same distance from our Sun (37.2 light-years).

Only 3.3 light-years separate the two stars. This is so close that Arcturus would appear as bright as Venus (mag. -4.92 to -2.98) to an observer on a planet orbiting Muphrid. On the other hand, Muphrid would appear as a magnitude -2.5 star (about the brightness of Mercury) to an observer on a planet orbiting Arcturus.

9. Arcturus only recently popped up in the skies above Earth

Arcturus only appeared in the night's sky fairly recently.

Arcturus, as bright as it is, only really appeared in the skies above Earth around five million years ago. That is amazing in and of itself, but like most stars in the night sky, we won't be able to see it forever either.

It has been traveling in our direction and is currently closing in on the Solar System at a speed of around 76 million miles per second (122 kilometers per second).

It will get a few hundredths of a light-year closer than it is currently and will make the closest approach in about 4,000 years. After that, it will begin to drift southward, and in another half million years, it won't be at all visible to the unaided eye.

And that Arcturus-enthusiasts, is your lot for today.

As we have seen, Arcturus dwarfs our own Sun but is far from the biggest star out there. That being said, its conspicuous nature, when observed from Earth, has captured our imaginations for thousands of years.

It is more than certain that it will continue to be the subject of study for many millennia to come.

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