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Northern Rockies Skies for May: Leo the Lion

April 25, 2013
One of the 12 Vela satellites that were launched to monitor the Earth’s surface.

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 take the pointer stars in the Big Dipper and go south, away from the North Star, you run into the large and easily recognized constellation, Leo the Lion. Its two brightest stars are Regulus and Denebola, and the backward question mark emanating from Regulus, called “the sickle,” represents the Lion’s head.

The ancient Egyptians associated Leo with the annual flooding of the Nile since the sun was placed in Leo during that time. Since the Nile was the lifeline of the Egyptians, many believe that ancient lion-headed fountains were a tribute to this cosmological symbol.

If you happen to be in a dark location and have a moderate-size telescope, look carefully between Regulus and Denebola, where many interesting galaxies can be seen.

This month, the Eta Aquarids meteor shower peaks around May 4-5 and is best viewed after midnight toward the constellation Aquarius (in the southeast). Jupiter is seen right after sunset and Saturn can be seen all night long near the star, Spica, in Virgo.

Astronomy from Orbit: First Detection of GRBs - The Vela Satellites

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Cosmic gamma-ray bursts (GRBs) were first detected 46 years ago by the Vela satellites. GRBs are brief (lasting a fraction of a second to hundreds of seconds) blasts of beamed, high-energy electromagnetic radiation arising from the collapse of massive stars or possibly from coalescence of binary compact objects -- neutron stars and/or white dwarfs.

When a bright GRB happens to be beamed in our direction, present-day GRB instrumentation can detect it from all the way across the observable part of the universe.

The original purpose of the Velas (short for velador, "watchman" in Spanish) was to monitor the Earth's surface for nuclear explosions. The Velas were launched, in pairs, into orbits of about 65,000 miles altitude (one-third of the moon's distance), allowing each of the pairs to monitor almost half of the Earth's surface, as well as a large portion of the sky. In all, 12 Velas were launched between 1963-70, with the last one operating until its shutdown in 1984.

The Velas had different detector types for recording various emissions: Scintillation detectors (sodium iodide crystals viewed by photomultiplier tubes) detected X-rays and gamma-rays, and silicon photodiodes ("bhangmeters") detected the visible flash from a nuclear explosion.

The characteristic visible time profile (intensity vs. time) of a nuclear explosion shows two peaks that rise and fall in less than a second, whereas GRBs have varied, unpredictable time profiles.

In July 1967, two Vela satellites detected a flash of gamma radiation markedly different from the nuclear explosion profile. During the next few years, the Velas recorded several more GRB time profiles.

By using light travel time analysis ("triangulation") between two or more Velas, directions to the GRB sources were determined to within a few degrees of accuracy. The resulting approximate random distribution of positions precluded, as the GRB origins, all possible terrestrial and solar system sources.

Thus, the bursts were judged to be coming from distances at least as far as the hypothetical Oort cloud of comets (about one light-year). However, localization error regions were far too large to isolate any visible wavelength counterpart to a GRB.

For 30 years, generations of GRB instruments on satellites could not localize bursts well enough to exclude most kinds of cosmic sources -- flaring stars, active galaxies, supernovae, etc.

Next time, we will discuss the Interplanetary Network of satellites, which assisted in eliminating some kinds of sources as the origin of GRBs.

To view this month's sky chart, click here.

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