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The guardian of the bears


darrell heath

By Darrell Heath

Astronomy Columnist 

It was 9:15 pm on the evening of May 27, 1933 and a crowd of 30,000 people were waiting to see something magical happen. The occasion was the Chicago World’s Fair Century of Progress Exposition. Forty years previously, in 1893, Chicago had hosted its first World’s Fair, the World’s Columbian Exposition. That first fair celebrated Christopher Columbus’ arrival to the New World and this one was dedicated to celebrating a century of technological and scientific innovation. One of the avenues in which science had made progress was in honing the distances to the stars. Arcturus, the brightest star in the constellation of Bootes, had recently had its distance calculated at 40 light years away. Retired astronomer Edwin Frost had made the connection that the previous fair had taken place forty years ago and that fact could allow Arcturus to play some kind of role in the opening of the 1933 fair. He hit upon the idea of using the star’s light to trigger a photoelectric cell, which would then throw a switch that would turn on the bright lights announcing the fair’s grand opening. This is what the throng had gathered to witness.
After a few speeches the appointed time had come and word was given to four observatories across the country whose job it was to harvest some of Arcturus’ light and send it to Chicago. At 9:15 p.m. the light from a star that had been traveling for forty years across interstellar space hit the photocell and a huge display board lit up for the crowd to see as did a spotlight atop the Hall of Science. The crowd was ecstatic! This ingenious idea for marking such an auspicious occasion paid off big time.
There is a catch however. We have since refined our knowledge about Arcturus’ distance a bit more and we now have it pegged at 37 light years away. Oh well, I doubt that if you were to have told a member of the crowd this news that they would have cared all that much.
You may recall that I introduced Arcturus in last week’s article. It’s the star that you can find quite easily by locating the Big Dipper’s handle and then extending that handle out by it’s length one more time until you hit the fourth brightest star in all the night sky, Arcturus. The name means “bear keeper” or “bear guardian” because it always seems to follow Ursa Major and Ursa Minor around the sky.
To the eye Arcturus appears to be a pale orange color. Take a pair of binoculars to it and you can really see the color much better. The reason Arcturus is orange is because its surface temperature is rather cool, around 7200 degrees Fahrenheit (73% as hot as our Sun). At one point in its life Arcturus was a lot like the Sun but it has stopped fusing hydrogen and has now switched to fusing helium within its core. During this transition the core became much hotter than it was before and that increase in temperature has caused Arcturus’ outer layers to swell. The result is a star 26 times as big as the Sun. As the star’s outer layers swell the surface cooled down but because the surface area has now increased the star also becomes much brighter. When you factor in all of the radiation the Arcturus is emitting then it is some 200 times as bright as the Sun. Red giants like Arcturus are elderly stars that are approaching the end of their lives. When you look at Arcturus you are getting a sneak peek at the destiny of our own Sun.
A peculiar thing about Arcturus is its speed and direction through the galaxy. Arcturus is a real speedster, traveling at 122 km per second relative to our solar system. Its direction of travel is also a bit weird. Rather than moving along with the stream of stars that make up the disc of our Milky Way Galaxy (which all travel in a circular motion about the supermassive black hole at the galaxy’s center) Arcturus’ motion is perpendicular to the disc. But it isn’t just Arcturus, there are about 52 other stars that form what is known as the “Arcturus Group” that are doing the same thing. There is an intriguing idea that Arcturus and its group may not even be an original component of our galaxy, they might be stars that belonged to a small galaxy that our own Milky Way cannibalized billions of years ago. If true then Arcturus could be an alien sun from another galaxy! And that’s just cool to think about. So, step outside this evening and take a look at this amazing star in our night sky.