December 16, 2018

Object of the Week; Gamma Arietis.

Double stars are often overlooked as astronomical targets, but tend to hold up well under light polluted, urban skies. I often show folks bright doubles at star parties to great effect, and a mental vocabulary of about a dozen or so can add to the usual crowd pleasers such as the Moon and bright planets. One of my favorite fall targets is Gamma Arietis, in the constellation Aries, the Ram.

Also known by the obscure name of Mesarthim, perhaps a connection to the Hebrew word for “to Minister”, Gamma Arietis is a fine double star of near equal brightness with a combined magnitude of +3.9. Even a small telescope will split the pairs’ 7.7 arc seconds of separation. Robert Hooke discovered the star’s double nature while tracking a comet through the constellation Aries in 1664. The pair hasn’t changed much in orientation, and is thought to orbit once every 5,000 years at a separation of 385 Astronomical Units (A. U.s). The system is about 160 light years distant. A fainter but unrelated pair of stars, sometimes called “Gamma Arietis C” are in the same field, about 220′ Arc seconds distant and at a collective magnitude of +9. these stars are thought to be unrelated due to separate measured proper motions.

The astronomical term for the week is Alpha Canum variable. This is a particular type of main sequence variable star characterized by a strong magnetic field and bright silicon, strontium, and chromium spectral lines in their emission. The southern member of the Gamma Arietis binary pair is a variable of this type. Alpha Canum variables typically vary over a range of 0.01 to 0.1 magnitudes (tough but not impossible to note with the naked eye!) over a period of 12 hours to 160 days. Gamma Arietis B itself has a slight amplitude of 0.02 magnitudes over a period of 2.6 days. Alpha Canum variables derive their name from the first example discovered and documented, Alpha Canum Venaticorum, also known as the binary star Cor Caroli.

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