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June 27, 2013 — 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.
Immediately after sunset, almost directly overhead, the third brightest individual star in the night skies appears, the orangish giant Arcturus. Arcturus is the first bright star on the imaginary arc from the handle of the Big Dipper to the star, Spica, in Virgo. Arcturus also is the brightest star in the Herdsman, the constellation Bootes.
As the mythological story goes, the herdsman drives the oxen cart of Ursa Major around the celestial pole, accompanied by his two companion hunting dogs, Canes Venatici, located slightly to the west. Mythology tells us that it is the herdsman that keeps the night skies in a constant rotation around Polaris.
Arcturus, the “guardian of the bear,” is an individual orange giant with a surface temperature that is slightly cooler than our sun’s. Although Arcturus is nearly 40 light years from the sun, it is about 10 times larger than the sun and gives off more than 100 times more energy, making it one of the brightest stars in the sky.
July planet watch: Venus, the brightest object in the night, will be on the western horizon after sunset. Saturn will be located near Spica (Virgo) just south of Bootes.
Astronomy from Orbit: Gamma-ray Bursts – BATSE
Up through the 1980s, gamma-ray burst (GRB) space instruments that formed the Interplanetary Network continued to record many GRBs. However, there were still insufficient detections to determine if the burst sources were among the nearest stars, or if they were detected from as far out as the edges of our Milky Way galaxy or, perhaps, even from far beyond our galaxy. The larger the distances of GRB sources, the more energetic they would have to be. Therefore, most GRB researchers in the 1980s believed the GRB sources to be in the galaxy and, that if enough could be detected, the GRB distribution on the sky would track the planar distribution of the Milky Way itself.
During this time period, the Gamma Ray Observatory (GRO) -- the second of NASA’s “Great Observatories”--was being planned, and researchers had convinced NASA to include a GRB instrument onboard. It was named the Burst and Transient Source Experiment (BATSE) and led by the Marshall Space Flight Center. GRO itself was the size of a step van, and weighed about 37,000 pounds, with a good portion of the weight being in the large sodium or cesium iodide crystals that are used to detect gamma rays.
The geometry of the BATSE experiment was distributed as eight large (20-inch diameter) sodium iodide detectors situated on the eight corners of the roughly rectangular-solid GRO satellite. With this arrangement, the detectors covered the whole sky (and the Earth). After launch in April 1991, GRO was named for Arthur Compton, who received the 1927 Nobel Prize in physics for elucidating the scattering of high-energy photons with electrons. When two or more BATSE detectors “triggered” an increase in the gamma-ray rate, onboard circuits began recording the event, subsequently sending the recorded GRB information to the ground for analysis.
During the nine-year span of the GRO mission, BATSE detected more than 2,700 bursts, about one per day. The surprise was that the distribution of burst positions on the sky was uniform -- so uniform that the Milky Way and our sister galaxy, Andromeda, were effectively eliminated as the sources of GRBs. Instead, the weight of the evidence from BATSE suggested that bursts came from very large distances, from the far reaches of the universe.
Toward the end of the Compton GRO mission, this new hypothesis was confirmed via observations from additional satellites and ground-based observatories -- to be discussed next time.
To view this month’s sky chart, click here.
The Compton GRO satellite being deployed by the Space Shuttle. Four BATSE modules are visible on corners of the satellite’s top side.