June 3, 2020

AstroChallenge: Beta Monocerotis: A True Triple.

This week, we’d like to turn your attention towards an interesting object in an often overlooked constellation; Monceros. Sandwiched between the flashier constellations of Orion and Canis Major, this rambling constellation sports an interesting multiple star that should be part of your spring repertoire; Beta Monocerotis.

This is a true ternary system discovered by Sir William Herschel in 1781. Low power will easily split the AB pair at about 7.4” arc seconds, and a bit of a boost under clear steady skies will split the B-C pair at 2.8” The visual magnitude for the system is +4.6, and the entire system resides about 700 light years distant. The entire system weighs in at about 18 solar masses, and the C component is suspected but not confirmed to be a double itself!

The Astroterm for this week is the Annual Aberration of Starlight. This term refers to the apparent displacement of stars as a result of our movement about the sun. This curious effect is separate from Parallactic, or true apparent shift that astronomers measure to gain a distance estimate. First noticed in 1725 by astronomer James Bradley, this motion gave astronomers a first hint that the speed of light was indeed finite. The annual aberration of starlight is distance independent, meaning that the same value of displacement is seen for objects across the celestial sphere. The analogy often given is the apparent tilted motion of raindrops as you move forward in the rain; in this case, you are the Earth bound observer, and the rain is the incoming starlight. A smaller diurnal aberration also occurs because of the Earth’s rotation. All of this had to be untangled before astronomers could measure the first true parallax measurements of stars in the mid-19th century. The yearly value for the annual aberration of starlight is 40” in a circular motion for stars inclined 90 degrees to the ecliptic, and linear for those along the ecliptic plane. A system like Beta Monocerotis would exhibit a flattened oval due to this effect if it were carefully plotted over the course of a year.

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