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Northern Rockies Skies for August: Scorpius and the Perseid Meteor Showers


In August, always a good time for viewing the Milky Way, one can easily explore a myriad of faint stars, dark dust clouds and many star clusters with the naked eye and binoculars. Visible on the lower southern horizon around 10 p.m. is Scorpius, the scorpion, and its brilliant red star, Antares.

Located near the center of the Milky Way, Scorpius is one of the oldest and most interesting constellations. With its inverted “question mark and curlicue” formation, Scorpius houses many intriguing stars, star clusters, dust-forming regions and gaseous nebulae. In Greek mythology, it was the scorpion that killed Orion the hunter.

Antares, the red star “rival of Mars,” is so large its visible surface would reach outside the orbit of Mars!  There are many star clusters and nebulae within the constellation. One of the most unique is the “Cat’s Paw Nebulae,” which is a vast region of concurrent star formation. Get out those binoculars and small telescopes, and view the Scorpion’s wonders.

The Perseid meteor showers will last up to four days and is centered around its peak dates, Aug. 13-14. You may see up to 60 meteors per hour. Look in the direction of the constellation Perseus, in the northern sky after midnight.  

You can see Venus on the western horizon and Saturn in the southwest near Spica, in Virgo, right after sunset. Jupiter and Mars will rise in Gemini after 4 a.m.

Astronomy from Orbit: Gamma-ray Bursts - BeppoSAX and Swift

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For 30 years (1967-1997), gamma-ray bursts (GRBs) were observed by space instruments that did not have the capability to image the burst positions to better than about a degree accuracy. Hence, no association had yet been made between the gamma radiation and possible counterparts in the X-ray, ultraviolet, visible, Infrared or radio portions of the spectrum.

At these longer wavelengths, many object classes with arcsecond-sized positional error boxes are known, and their astrophysical properties are relatively well understood. (An arcsecond is 1/60th of an arcminute, which is 1/60th of a degree or 1/360th of a circle). On Feb. 28, 1997, the Italian/Dutch satellite "BeppoSAX" detected a GRB with one of its wide field cameras -- coded aperture X-ray /low-energy gamma-ray instruments with fields of view 40 degrees square and angular resolution as good as one arcminute.

Rapid communication to the ground and follow-up of the GRB position by large optical telescopes revealed a faint, long-lived optical afterglow coincident with a galaxy at a redshift of 0.695, corresponding to a distance of 8 billion light-years. BeppoSAX continued to detect many GRBs with X-ray, optical and radio afterglows until the satellite fell into the Pacific Ocean in 2003.

BeppoSAX's discovery that GRB sources are in galaxies at cosmological distances led to a competition for a new dedicated GRB satellite that would map the distribution of GRBs as a function of redshift, or distance. The winner was "Swift," selected by NASA for construction in 1999. It was launched in November 2004 and is still operating.

Swift has three instruments. The Burst Alert Telescope (BAT), a hard X-ray coded aperture telescope with field of view covering 1/6 of the sky, produces arcminute positions. Upon a BAT alert, Swift is re-oriented so that the two narrow field-of-view instruments, the X-ray Telescope (XRT) and Ultraviolet Optical Telescope (UVOT), can examine the BAT position for afterglows.

The BAT, XRT and UVOT localizations are relayed to the ground on a timescale of minutes. Ground-based observatories, with small to very large telescopes, monitor the positions for afterglows. In the visual, most detected afterglows range from about the 16th to 21st magnitude (10,000 to a million times fainter than the naked eye). Swift has detected about 750 GRBs (one every three days or so) and has determined distances for about 200 of them.

The GRB "co-moving" distance distribution peaks around 20 billion light-years. The "look-back time" distribution peaks near 12 billion years ago, about 1.8 billion years after the Big Bang, when the GRB host galaxies were very young.

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

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