Arcturus

Arcturus is the brightest star in the constellation Bootes. To the naked eye, orange-yellow Arcturus has a visual magnitude of -0.04, making it the brightest star north of the celestial equator, and the fourth brightest star in the night sky, after -1.46 magnitude Sirius, -0.86 magnitude Canopus and Ğ0.27 magnitude Alpha Centauri. However, Alpha Centauri is a bright binary star, whose unresolved components to the naked eye are both fainter than Arcturus. This makes Arcturus the third brightest individual star, just ahead of Alpha Centauri A, whose visual magnitude is -0.01.

Arcturus is visible from both hemispheres in the sky, as it is located less than 20 degrees north of the celestial equator. The star culminates at midnight on about the 30th April, being visible during the northern spring or the southern autumn. From the northern hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find Spica, “Arc to Arcturus, then speed to Spica.” The star is also a member of the Local Interstellar Cloud.

Arcturus takes its name from its nearness to the sky Bears, Big and Little Bears, Ursa Major, and Ursa Minor. From Arktouros or Arctophilax, “the Bear Guard” and also called “the Bear Watcher”. The “Herdsman”, or “driver of oxen” are other titles. Arcturus is the brightest star in the constellation of Bootes the Herdsman so it is also called Alpha Bootis. The pattern of stars in this constellation forms the shape of a kite or an ice cream cone; seeing a Herdsman driving the bears around the sky.

Arcturus is believed to be one of the first stars named by ancient observers. It is translated as “Guardian of the Bear” and is a name that was once used for the entire constellation of Bootes. It is easily found by noting that the curve of the handle of the Big Dipper is part of a circle – an arc – and we can just follow the arc to Arcturus.

Arcturus is a giant with a diameter about 18 times our Sun’s and four times as much mass. Its surface temperature is about 1500 degrees lower than Sun’s but its much greater surface area results in an outpouring of energy at a rate making it 105 times as luminous as Sun.

Arcturus is notable for its high proper motion, larger than any first magnitude star other than nearby ? Centauri. It is now at its closest point to the Sun, and is moving rapidly relative to the solar system. Arcturus is thought to be an old disk star, and appears to be moving with a group of 52 other such stars. Its mass is hard to exactly determine, but may be about the same as the Sun, and is no more than 1.5 solar masses. Arcturus is likely to be considerably older than the Sun, and much like what the Sun will be in its red giant phase.

According to the Hipparcos satellite, Arcturus is 36.7 light years (11.3 parsecs) from Earth, relatively close in astronomical terms. From this satellite’s observations, Arcturus is now known to be slightly variable, by about 0.04 magnitudes over 8.3 days. It is believed that the surface of the star oscillates slightly, a common feature of red giant stars. In the case of Arcturus, this was an interesting discovery as it is known that the redder (more towards or within the M spectral class) a giant gets, the more variable it will be. Extreme cases like Mira undergo large swings over hundreds of days; Arcturus is not very red and is a borderline case between variability and stability with its short period and tiny range.

Hipparcos also suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the very limits of our current ability to make it out. The most recent studies of the issue are generally coming down on the side of it being a single star, however.

Arcturus has the largest “proper motion” – motion across the sky – of any of the bright stars except Alpha Centauri, the nearest star to our solar system. In 100 years Arcturus moves across the sky a distance equal to about half the width of your little finger held at arm’s length.

At its distance of nearly 37 light years, this motion, when combined with its motion along our line of sight measured spectroscopically using the Doppler shift, yields a space velocity of about 76 miles per second with respect to our Sun. Most stars in our vicinity are moving relatively slowly with respect to Sol because of our common motion carrying us around the center of the Milky Way galaxy every 250 million years. Arcturus is in an elongated orbit around the Galaxy’s center that carries it out into the Galaxy’s halo.

It was formed in the halo of the Milky Way and is an interloper in our neighborhood. It has been visible to the naked eye for only about half a million years. It will be a little closer in a few thousand years, but then will recede from our view in another half million years as it continues its journey on a different orbital path.

Arcturus’s great brilliance makes it possible to obtain very detailed spectra and to determine its chemical composition. Arcturus is deficient in elements such as silicon, aluminum, and iron which are formed in stars. It contains only about a fifth as much of these elements as Sun. These elements are formed inside stars, mixed into the interstellar medium as stars explode, and incorporated into subsequent generations of stars.