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Northern Rockies Sky for June: Ursa Major and the Big Dipper

May 29, 2013
Constellation chart

A monthly look at the night skies of the northern Rocky Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

If you look directory overhead right after sunset this month, you will see the easily recognizable constellation Ursa Major and the Big Dipper. The Big Dipper is a part of the Big Bear. H.A. Rey, the creator and author of the children’s book series “Curious George,” had a unique way of viewing the stars and the big bear, as seen in this month’s chart.

Ursa Major, the big bear, is one of the oldest constellations and is referenced in the Bible and in Homer’s writings. In Greek mythology, it is associated with Callisto, the beautiful maiden sworn to chastity. She eventually was seduced by Zeus, had a son, and lived in the forest as a bear.

The Big Dipper, also known as the Plough, has seven very bright stars. In Hindu legend, these stars represent the seven Sages. Since the Big Dipper is used to find the north direction, mariners, campers and even the slaves in the Civil War’s Underground Railroad have used it.

Today, we know the properties of these seven stars, the two brightest being Alioth and Duhbe (means the back of the bear). The most interesting of the seven stars is Mizar, a double binary star with a fainter nearby companion, Alcor, another binary star.

June Planet Alert: Right after sunset, toward the west, you will see Venus, and nearby, Mercury, which is at its greatest elongation from the sun. Saturn is seen near Spica between the constellations Virgo and Libra.

Oh, the summer solstice is June 21, so welcome summer -- we badly need it.

Astronomy from Orbit: Gamma-ray Bursts --The Interplanetary Network

 (Best URL: )

Last month we discussed the Vela satellites -- designed to monitor the Earth for nuclear explosions, but which also discovered gamma-ray bursts (GRBs) of cosmic origin in 1967.

Since GRBs are brief (durations spanning less than a second to a few minutes), one-time events, the direction to a GRB is a priori unknown. Some gamma-ray instrumentation of that era was collimated with slats (like a venetian blind) and used to point at known sources, but these instruments could be of no use for determining directions to GRBs.

Instead, by analyzing the difference in time of arrival of an intensity spike in a GRB time profile -- as observed by two Vela satellites -- the GRB's direction could be determined to within a few degrees accuracy. Here's how that works: Geometrically, the difference in arrival time of the intensity spike for the satellite pair translates into a source position lying on a hyperboloid of revolution. Extrapolated to a large distance (e.g., way outside the solar system), the intersection of a hyperboloid with the distant star sphere is a circle.

Three participating satellites yield three circles that happen to mutually intersect in two points; four satellites reduce the position solution to one point. With wider satellite separations (therefore longer light travel times between pairs) and narrower intensity spikes, the GRB localizations are more accurate. The Vela satellites were in high Earth orbit separated by an order 100,000 miles, so the difference in light travel time was roughly a large fraction of a second. The resulting few-degree accuracy eliminated all terrestrial and solar system sources, but left the rest of the cosmos suspect.

By 1978, the Velas' capability was augmented with the addition of spacecraft flying to Venus, including NASA's Pioneer Venus Orbiter, and the Russian Venera 11 and 12, as well as the German Helios 2 orbiting the sun. Then, with the light travel time differences between these spacecraft of up to a few minutes, GRB localizations were reduced to a few arc minutes.

Still, no apparent counterparts at visible, ultraviolet, infrared or radio wavelengths were found within the smaller localization error regions that the "Interplanetary Network" (IPN) produced. As the years have gone by, satellites have expired and others have joined to IPN, and GRBs continue to be localized by the IPN to this day.

However, clues to the GRB distance scale and its eventual elucidation awaited development and launch of GRB instrumentation in the 1990s -- to be discussed next time.

Big Dipper

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