by Rex Parker, PhD director@princetonastronomy.org

Happy New Year!  Let us renew our hope for the new year and commit to connecting with other members of the amateur astronomy community and AAAP especially.  I look forward to seeing you in the upcoming Zoom sessions.  The first meeting of 2022 will be on January 11 (7:30pm) with an astronomer who has strong Hubble Space Telescope connections.  Please see program chair Victor Davis’s section below for more on the program. 

Astrovideo Live Winter Sessions.  We are renewing the hit sessions which debuted last winter.  The new dates are Jan 7, Feb 4, and Mar 4, Friday evenings close to a new moon. All members are welcome to join in these live observing sessions.  Those of you making progress with your own astrovideo telescope setups are urged to contribute your own video stream to the Zoom sessions – please contact me if interested. An email will be sent with the Zoom link a few days before each date.

The Greenness of Comets.  The current hit movie Don’t Look Up (Netflix) portrays the government and industry attempting to acquire the valuable rare minerals in a newly discovered comet on a fatal collision course with earth.  Trillions of dollars of wealth and massive job opportunities are seen, blinding the authorities from seeing the impending disaster from the collision that the astronomers (played by Leonardo DiCaprio and Jennifer Lawrence) calculate from the data.  This is a fitting allegory for climate change and human denial, yet in a literal sense the color of comets really is green.  Why is that?

The images below of comet C/2021 A1 are from my home observatory in NJ using a 12.5” telescope and ZWO ASI071 camera, taken on Dec 8 just before morning twilight.  The green glow is quite apparent in these images which are carefully color-balanced.  The stars are trailing because the mount is tracking the comet (13x2min subframes, left panel;  6x2min subframes, right panel).  Known as Comet Leonard after its discoverer at the University of Arizona, it makes a good stand-in actor for the movie Don’t Look Up.  Although it won’t collide with the earth, Comet Leonard was found using Catalina Sky Survey’s 1.5m infrared telescope on Mt Lemmon.  Just like the movie, the Catalina survey is supported by NASA and the Near Earth Object Observation Program under the Planetary Defense Coordination Office.

From an astrochemistry view the emission of green or blue-green color from a comet’s nucleus but not the tail is an intriguing puzzle, only recently solved.  It is not the same photochemistry mechanism as others in astronomy, for example the fluorescent blue-green emission from ionized oxygen seen in planetary nebulae in the telescope. The nucleus of a comet is an agglomeration of rock, dust, and frozen gases.  As it gets closer to the sun, increasing heat causes the gases to sublimate and form a nebulous envelope around the nucleus known as the coma.  The tail of a comet is an extension of the coma drawn out by the solar wind.  Yet the green around the nucleus disappears in the tail which instead displays a reddish brown color.  

It has been thought for years that a comet’s green comes from the breakdown of the reactive molecule dicarbon (C2).  Dicarbon is an abundant molecule in the universe although not on earth, and multiple valence electronic states in its chemistry give it a rich spectroscopy.  The famous British scientist Wollaston reported the emission spectra of blue-green flames as early as 1802, the first glimpse of dicarbon.  Now a new study has solved the question of green in comets.  In this laboratory work, dicarbon chloride (C2Cl4) was irradiated by UV-laser, a way to generate dicarbon for spectral analysis (Borsovszky et al., Photodissociation of dicarbon: How nature breaks an unusual multiple bond. Proc Natl Acad Sci USA 2021, Vol 118, No 52).  Further irradiation at longer wavelengths generates a metastable state of the C2 molecule (a radical) which decays and radiates a characteristic greenish photon.  The emission spectrum of dicarbon is known as the Swan band, after the Scot physicist William Swan in the 1850s.  Swan bands are a characteristic of the spectra of carbon stars and some nebulae as well as comets.  Dicarbon photoemission in the Swan band requires two “forbidden” electron transitions which are favored in the environment of space but not on earth.  The spectral pattern (color) is a sensitive probe of local environment.  In their 2021 paper, Borsovszky et al. determined that the half-life of the C2 radical is a little under 2 days under the conditions of a comet at ~1 AU distance from the Sun.  This is the first solid explanation of why the head of a comet but not the tail glows green, because the dicarbon radical with its short half-life is dissipated as material streams out to the tail.  

The Unjournal Club Wants You.  Doing astronomy in AAAP is a little different when we cannot meet in person for regular meetings.  For now, the best way to keep the comm channels active is to use our monthly Zoom meetings to highlight club activities and facilitate member conversations.  These take place during the 2nd hour after the main speaker, when the informal “Journal Club” presentation by a member is slotted each month.  Help us break the boundaries set by Zooming by volunteering to give an “unjournal” club session!  I say “unjournal” here because these short episodes don’t need scholarly, journal-like topics, they only need to engage members with what you care about in astronomy.  It works great with Zoom screen sharing of PowerPoint slides, JPEG’s, etc from your home computer or mobile device.  To get on the schedule for an upcoming meeting, please contact me or program chair Victor Davis.

Progress on the New Initiatives

• Merchandise store.  Thanks to member Rich Sherman who did all the setup, the merchandise store is launched now.  The club will benefit with 15% of each sale, and it is not limited to members although at present we are not advertising.  The store link is  http://aaap1962.logosoftwear.com/ (also posted on the upper right of the first page of each issue of ST on the website);  the password is SiderealTimes.

• Social Media and the Discord AAAP Server.  Thanks to Debbie Mayes who provided an initial social media action plan which is under review by the board.  Len Cacciatore, Dave Skitt, Debbie Mays, Rich Sherman and I did an initial test of two social text-like apps (Groupme and Discord).  We decided to go forward with Discordfor member trial — it has excellent features and potential to help communication in the club.  The AAAP Discord server has been set up with three channels– General, Observatory, and Astrophotography.  Members were invited to join with a link sent Dec 14.  If you missed that invite and are interested, keep tuned in for a new invitation to be sent by email around Jan 5.  The invitation-only sign-up helps make this private for members of AAAP.  Please give it a try-out and provide some feedback to me or others on the Board.   

• Telescope Loaner Program.  We are getting the equipment organized and a system is being set up to track loan outs.  Member Todd Reichart is spearheading this initiative with input from Dave S. and me, and it is nearly ready to launch.  

• NASA/JPL Night Sky Network.  NSN and its usefulness in the club needs more time and thought but has great promise.  One idea is that we could begin using it to handle some of the roster functions.  Ira Polans and the Board are currently working on this.