The Big Dipper will shine bright in our evening skies

It almost goes without saying that the most prominent star pattern in our current evening sky is the Big Dipper.

The Big Dipper is presently high in our northern sky during the evening hours.

All seven stars of the Dipper are relatively bright; the four westernmost constitute the Dipper’s bowl – which is upside-down in its current configuration – and the other three stars form a handle that attaches to the bowl at one corner.

The three handle stars – four, if one includes the bowl’s corner star – are not in a straight line, but rather form a bit of a curved arc. In looking at the nighttime sky, one can extend this pattern beyond the handle and make an arc – to Arcturus, a very bright star currently shining high in our eastern sky during the evening hours. Indeed, Arcturus is more than just a bright star; it is the fourth-brightest star in the entire nighttime sky.

Arcturus is within the constellation of Bootes, the herdsman, and in fact marks the herdsman’s left foot. Its name comes from Greek words meaning Guardian of the Bear, a reference to the nearby constellations Ursa Major and Ursa Minor, the Great and Little Bears, respectively.

Being as bright and prominent as it is, Arcturus has had its place in all sorts of cultural references. It is one of the few stars mentioned by name in the Bible (in the book of Job). Its distance north of the celestial Equator is the equivalent to the latitude of Hawaii – which means it passes directly overhead as seen from there – and thus it was very useful to the ancient Polynesian sailors who were attempting to reach Hawaii. All they had to do was sail north from, say, Tahiti, until Arcturus was directly overhead, and then sail eastward with the trade winds, keeping Arcturus overhead, until they arrived.

Arcturus also has its place in relatively modern culture as well.

Its light was used to trigger the opening of the Chicago World’s Fair in 1933, this being done under the assumption that Arcturus is 40 light-years away from us and that the previous Chicago World’s Fair had been held in 1893. More modern measurements indicate that Arcturus’ distance is actually 36.7 light-years; thus, the light we see from it tonight left on its way to us around July 1980.

Arcturus is a giant star, roughly 25 to 30 times the diameter of our sun, which contributes to its appearing so bright in our nighttime sky. It is somewhat older than our sun, with estimates ranging from approximately 6 billion to 8 billion years, and apparently started out being slightly more massive than our sun. Some time ago it used up all the hydrogen in its core during the nuclear fusion process, and it is now fusing hydrogen in a shell surrounding the core; at some point in the future it will actually begin fusing the helium that presently resides in its core. Our own sun will undergo similar activity a few billion years from now, thus when we study Arcturus we are, in effect, studying our sun’s distant future.

One of Arcturus’ most important roles in our understanding of the surrounding universe came in the early 18th century, when around 1718 the British astronomer Edmond Halley noticed that Arcturus, along with a handful of other bright stars, were not in the same places, relative to other stars, that had been meticulously catalogued by the Greek astronomer Hipparchus over 1,800 years earlier. Up until that time the stars were believed to be fixed relative to each other, however Halley’s measurements showed that this is not the case; Arcturus, and indeed all-stars, including our sun, are on the move throughout our galaxy, and over time the various constellations will slowly change shape.

Arcturus turns out to have one of the largest so-called proper motions, i.e., motion along our line of sight, of the bright naked-eye stars. It is traveling through space at a speed of 76 miles per second relative to our solar system; it will pass closest to the solar system in about 4,000 years when it will be a small fraction of a light-year closer than it is now, and after that will begin pulling away from us.

In the early 1970s the American astronomer Olin Eggen pointed out that Arcturus is one of a little over 50 stars that are traveling through space together as a group; these stars have collectively been dubbed the Arcturus Stream. There has been recent speculation that the Arcturus Stream is actually the remnant of a small galaxy that was cannibalized by the Milky Way hundreds of millions to billions of years ago; if this is true, then the bright star we currently see in our nighttime sky actually originated in a separate galaxy from our own.

However, a study completed three years ago by Swedish astronomer Thomas Bensby and his colleagues suggests that, chemically, the stars in the Arcturus Stream aren’t dramatically different from other stars, which casts considerable doubt upon this idea of a separate galaxy. For now this must remain an open question.

At this time there is no convincing evidence that suggests that Arcturus is accompanied by any planets, although it is entirely possible that planets are indeed present but that we are unable to detect them at this time. Any close-in planets would be dramatically affected by the changes – i.e., expansion, etc. – that Arcturus has undergone over the last few millions to tens of millions of years, and of course any life that might have developed on any such worlds would be tremendously affected.

A prelude to what will eventually happen to life here on Earth – although, fortunately, we still have quite a bit of time left.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is http://www.earthriseinstitute.org.