February 19, 2019

AstroEvent: A Close Binary Occultation.

Astronomical occultations are always fun to catch. Unlike other astronomical events that often happen over glacial time scales, occultations happen with abrupt swiftness. And besides just being plain cool, occultations can produce real scientific value, data that you can contribute to from your own backyard… and there’s no bigger occulting body in the night sky than our own Moon. This week, I’d like to bring to your attention a fairly bright and interesting star that is currently undergoing a series of lunar occultations this year; Sigma Scorpii. This star shines at magnitude +2.9 in the heart of the constellation Scorpius and is itself a close binary difficult to separate with a telescope. This star is also known as Al Niyat, or Arabic for the “Shield of the Heart,” possibly referring to its visual proximity to brilliant Antares. Sigma Sco is itself a complex system, with a 9th magnitude companion about 20” distant.

But wait, there’s more. Sigma Scorpii A is itself an eclipsing, spectroscopic binary, with a type O companion on a 33 day orbit. Farther out, another star orbits at about 120 Astronomical Units at a very tough spread of less than half an arc second… this companion was first suspected during the lunar occultation of March 12, 1860 of Sigma Sco as seen from the Cape of Good Hope. With some luck, you might be able to catch the split as this star “winks-out” in a stepwise fashion…this would also be a very video-able event. The action begins early on the morning of Saturday, April 3rd, (see link) and favors viewers from Quebec and the Maritime provinces down to Georgia. This is third in a set of Sigma Sco occultations this year; the others occur on April 30th, May 28th, June 24th, and July 21st.  During this weeks’ event, the Moon will be at 78% illumination. The system itself is about 735 light years distant…good luck, and let us know what you see!

This week’s astro-term is Pulsating Variable. A pulsating variable is a type of star that physically shrinks is size and has predicable amplitude of brightness in comparison to its radius. The most famous of all variable types are the Cepheids, which serve as a standard candle for measuring stellar distances. Sigma Scorpii A is a variable of the Beta Cepheid type, sometimes also known as the Beta Canis Majoris class of pulsating variable stars. This class is marked by short pulsation period (in the case of Sigma Sco, about 6 hours) and slight brightening during contraction by about 0.1 to 0.3 magnitudes. Another subclass of pulsating variables are the longer term Mira class stars, which undergo larger fluctuations sometimes spanning the year.

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