May 25, 2020

AstroEvent of the Week: 27.04.09: Epsilon Aurigae.

The American Association of Variable Star Observers wants you to help gather data on a very enigmatic astronomical object; the variable star Epsilon Aurigae. This seemingly ordinary star varies in a very peculiar way. The primary is a type F0 super-giant star, and what is known is that every 27.06 years an unseen mass dims its light from its usual +3.0 magnitude to about +3.8 for about a year.

Successive dimming and brightening on either side of the minimum often lasts over half a year. This hints at a large but unseen companion. Known to be a variable since 1821, this system has performed this feat only seven times since discovery and is expected to do so again in late summer of 2009. Here’s were you, the avid observer, come in. Epsilon Aurigae is bright enough to see from even light polluted urban areas with the unaided eye;  catch it now before it fades behind our local star, the Sun. It probably won’t be visible again until late June in the early dawn, and by that time the eclipse may well have begun. Also known as Haldus, this star has stubbornly refused to conform to the astrophysical paradigm. Whatever the visible primary (itself no light-weight at about 15 solar masses!) orbits, it is much less luminous in the visible spectrum, and semi-transparent, to boot. Some of the more outlandish theories have been a black hole with a large accretion disk or a large cool “superstar,” which would be one of the most massive and least dense objects known. Current leading theories are a B5-type star nestled in a tenuous envelope of gas.   The system lies about an estimated 2,000 light years from Earth. Photometry is encouraged, but this is also a good naked eye variable to cut your teeth on. Estimates can be simply done by comparing the observed brightness with the brightness of known stars. After a bit of practice, an accuracy of 0.1 of a magnitude can be easily achieved. Check out this bizarre variable, and we welcome any thoughts as to what might be going on in this curious system!

The Astro-phrase of the week is eclipsing binary. This is simply a pair of stars that have their orbit inclined to our line of sight, and thus we witness a dip in brightness as they pass in front of and behind each other from our point of view. Algol, the “Demon Star” in the constellation Perseus is the most well known of these types of variables, and was known to the Arabs before telescopic times. Transiting exoplanets are discovered in a similar fashion; by teasing and separating out the individual spectra of each component star, data such as composition and radial velocity can be deduced.

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