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Meet Arcturus: Guardian of the Bear – Sky & Telescope

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Arcturus Vitals

Official identify Arcturus
Different designations Alpha Boötis, HIP 69673, HD 124897, HR 5340
Obvious Magnitude -Zero.05
Distance from Earth 36.7 light-years
Sort Purple large (K1.5III)
Colour Reddish orange
Mass 1.Zero–1.5 M
Radius ~25 R
Constellation Boötes, the Herdsman
Proper Ascension 14h 15m 39s
Declination +19° 10′ 56
A number of system? Probably
Variable star? No
Exoplanet standing None identified
Possible destiny Planetary nebula / white dwarf

Bodily Traits

Arcturus’s reddish-orange shade is clear on this picture. (The star seems bigger than a degree of sunshine right here as a result of it is so brilliant.)
Daniel Johnson

Arcturus is a superb instance of a crimson large star — impressively giant, accordingly rarefied, brilliantly brilliant, and surprisingly cool. Arcturus possesses solely barely extra mass than the Solar, however expands that materials right into a radius about 25 instances bigger.

So far as stars go, Arcturus is fairly near us at solely about 37 light-years away — appreciably greater than the four or eight light-years of Alpha Centauri or Sirius, respectively. Nevertheless it’s a lot nearer than the a whole lot of light-years that separate us from different crimson giants similar to Betelgeuse or Antares. This proximity to Earth is actually a contributing issue to Arcturus’s obvious brightness. Actually, Arcturus is the fourth-brightest star within the night time sky, outshone solely by Sirius, Canopus, and Alpha Centauri, and the second brightest for a lot of the Northern Hemisphere.

Arcturus is a star on the transfer — it speeds alongside at about 122 km/s (273,000 mph). As a result of it’s so near us, its correct movement, or motion on the sky, is pretty simple to detect: It quantities to about 2.three” a yr. With cautious remark by way of a telescope, and plenty of endurance, you may detect the star’s movement over the course of a few decades. Arcturus can be transferring towards us, however that movement isn’t observable with no spectroscope.

Historic astronomers held the view that the celebrities have been fastened and motionless within the heavens — an inexpensive assumption for observers with out optical help. You possibly can thank the 18th-century astronomer Edmond Halley (of Halley’s Comet fame) for locating the right movement of Arcturus (and by implication, different stars). He did this by evaluating the star’s place throughout his lifetime towards information within the Almagest.

Origin / Mythology

Bootes and the Big Dipper
This diagram reveals Arcturus, its constellation Boötes, the Herdsman, and the Huge Dipper asterism that’s a part of Ursa Main.

Arcturus is a Greek identify, and also you’ll discover a couple of totally different delicate translations of the which means: “guardian of the bear,” “bear follower,” “keeper of the bear,” and others. All of those indicate the identical fundamental truth: Arcturus “chases” Ursa Main across the sky.

After all, an alternate set of views depicts the Huge Dipper part of Ursa Main as a plow — a standard idea within the UK, for instance. Mythology has a pleasant connection between Arcturus for that view as effectively. The star’s constellation, Boötes is the Herdsman — a farmer, in different phrases. On this case, Boötes the Herdsman turns into the person driving the plow. Presumably, oxen pull the plow however alas, we’re left to think about that ourselves.

The idea of Arcturus “following” or “tending” the Huge Dipper is widespread all through the mythology of a number of cultures as a result of Arcturus is near the Huge Dipper. Actually, for those who “rewind the clock,” Arcturus strikes even nearer to the Dipper with every passing millennium.

Historic cultures additionally put Arcturus to make use of as a navigational help. Homer’s Odysseus sailed the Mediterranean along with his eyes “fastened on . . . late-setting Boötes.” Polynesian sailors took benefit of the truth that Arcturus is a zenith star from Hawai‘i’s latitude, which means that the star passes instantly above the islands when at its highest level. Thus, they might use it to estimate their very own latitude in relation to the archipelago.

The best way to See Arcturus

"Arc to Arcturus"
The dashed line right here demonstrates how one can “arc to Arcturus” from the Huge Dipper.
Daniel Johnson

Arcturus is a superb spring and summer season star, driving excessive within the southwest round Summer time Equinox. Nevertheless it’s additionally a pleasant autumn star, hanging decrease within the west simply after sundown within the fall. The star’s brightness and apparent crimson shade make it simple sufficient to search out, however in case you are in want of a useful guidepost, you may’t do higher than by discovering the Huge Dipper and “Arcing to Arcturus.”

To do that, discover the Huge Dipper and try the arc that’s shaped by the Dipper’s three deal with stars (these can be Alioth, Mizar, and Alkaid). Then observe the course of that curve till you run into Arcturus (it’s shut however not good!). If Spica is above the horizon as effectively, you may then “Pace on to Spica,” for those who’d like.

As a result of the Huge Dipper is so outstanding, the phrase is a useful technique of finding Arcturus — and a terrific studying device to show others. The plain orange shade of Arcturus can be a very good instructing second; somebody unfamiliar with astronomy might have by no means seen that the celebrities are available in a rainbow of colours.

Daniel Johnson is a Wisconsin-based freelance author and professional photographer and the coauthor of over a dozen books. He’s a longtime newbie astronomer and lucky sufficient to stay in a rural area with glorious seeing circumstances. You possibly can view a few of Dan’s images (he does plenty of animals!) at www.foxhillphoto.com.

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