June 18, 2019

Astro-Event: R Geminorum Rising.

Mira, a prototype variable similar to R Gem as seen by the Hubble Space Telescope.

(Credit: NASA/ESA).

Tired of observing M31 for the nth time and ready to do some backyard astrophysics? Eventually, we all go there; you may have even read a recent “how-to” post of our not-so-secret addiction: variable star observing. Unlike much of deep sky astronomy, variable stars actually change and show activity on time scales that you might live through… and a sure bet is ramping up this coming month; the variable star R Geminorum. Located at Right Ascension 7 Hours 7’ 21”  and Declination +22N 42’ 13”, this Mira-type variable star has a period just over a year, at 370 days. This year should see R Geminorum (R Gem) just breaking binocular visibility on December 1st at magnitude +8.5 on its way towards a peak visibility of +6 by year’s end. In fact, you may just be able to spy reddish R Gem very near 44 Geminorum with the naked eye from a deep sky site; both will vie for that magical +6 magnitude visibility barrier. Also given the designations HD 53791 and SAO 79070, this variable star was first discovered by John Russell Hind (he of Hind’s Crimson star fame) in 1848.

R Gem is very near 44 Geminorum… (Created by the Author in Starry Night).

R Geminorum spends most of the year down in the 14th magnitude range, visible in only the largest of backyard telescopes. At its peak brightness it shines a ruddy to orange color, with a B-V index of around 2.1.  R Geminorum is a Mira-type long period variable and is a subcategory known as an S-type star, a breed of red giant closely related type M-Giants. This asymptotic branch of giant stars are at the end of their lives and are fusing such exotic elements as technetium, yttrium and zirconium oxide via the nuclear S-process (“S” as in slow neutron capture). In fact, elements such as technetium are exclusively man-made on Earth, and the discovery of stars such as R Geminorum creating said element on its own via natural processes in 1952 came as somewhat of a shock. This was determined to be the case because Technetium-98 has a half live of about 4.2 million years, too short a time for said element to have been pre-existing when it was incorporated into the present star.

1 Degree field centered on 44 Gem…North is up. (Created by the Author in Starry Night).

The presence of such an exotic element in the spectral lines of stars like R Geminorum could only have been placed there by ongoing nuclear synthesis. Unlike typical red giant stars which continue to fuse hydrogen and helium in their inner shells via the proton-proton chain of nucleosythesis up through to oxygen and heavier elements, technetium stars are prone to periodic “drench ups” and helium shell burning periods lasting 100,000 years. We’re watching R Geminorum go through one of those periods now, and the relatively short time-frame involved is perhaps why we don’t see many of these types of variable stars in the sky. And yes, I can hear that Astrophysics 101 student in the back- you are correct; this brand of nucleosythesis runs well above iron at atomic number 26; more energy is going into creating these heavy elements than is coming out. Stars like R Gem can’t keep this “heavy metal” act up for long before finally shedding those outer layers and ending its days as a white dwarf. Chi Cygni, another variable star, is the brightest of the S-type stars known. Keep an eye on reports and send your own observations of R Gem to the American Association of Variable Star Observers (AAVSO).

This December, do give this curious variable a look while sweeping through the constellation Gemini. The constellation rises around 9PM local time by mid-month, placing it high overhead by midnite. And keep an eye out for that eclipsed Moon only a constellation away on December 10th, and the Geminid meteors on the 14th… more to come on those soon in this space!

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