Arcturus

Arcturus Setting just above the Trees

As I write this in late September, the reddish star, Arcturus, sets soon after the Sun, so it isn't nearly a dazzling at mid-summer when it reaches its peak in the early evening.  It lies just above the trees in the photo with the rest Bootes extending upward. But that is not why I'm writing about it.  At eleven parsecs Arcturus is the nearest giant star to Earth, and because its mass is similar to the Sun, it gives a picture of what the Sun might look like in five billion years.   Because it is so close and a giant star, it is one of the few normal stars from which we can detect radio emission.   This emission is analogous to the radio waves from the quiet sun, but because Arcturus is so much larger than the Sun (about twenty solar radii) its radio emission is inherently much brighter.

This got me thinking about all of the stars like Arcturus and further evolved in the bulge of our Galaxy. These are the sources for surveys of gravitational microlensing such as OGLE.  How would they appear if we observed a microlensing event in the radio?  It turns out that about eight percent of the OGLE-2 sources could be detected by a planned radio telescope called the Square Kilometer Array, and if the lens had a small enough mass, the radio flux would oscillate as the lens passed in front of the source, telling us the mass of the lens.   Pretty neat!  If you want to learn more, read http://arxiv.org/abs/1002.3007.

Of course, being the brightest star in the northern half of the sky, there are lots of stories about Arcturus from how its light opened the world's fair in Chicago to various myths.   The one that I found most interesting is how the declination of Arcturus is the latitude of the island of Hawaii, so when you are at the latitude of Hawaii, Arcturus passes directly overhead.   The Polynesians would sail northward from Tahiti and the Marquesas islands until Arcturus was directly overhead at its highest point and then sail west until they landed at Hawaii.  They called it Hōkūleʻa, the "Star of Joy", so when you look at Arcturus, you can imagine it at the zenith above a beach on Hawaii, and you too will feel the joy.  By the way, the Polynesians used Sirius for the return trip.