May 25, 2020

AstroChallenge: The Sub-Giants of Delta Serpentis.

You might’ve noticed that we here at Astroguyz have a “thing” for double stars. This can be a controversial affliction among deep-sky observers, as many an astro-imager sees mere “stars” as boring compared to their quest for faint nebulae and galaxies. But for many an observer trapped in the ‘burbs, double stars offer a deep sky target that still holds up well under light polluted skies when those +14th magnitude galaxies just don’t stand a chance of ever shining through. This week, we look at the fascinating system of Delta Serpentis.  This yellow white pair is 210 light years distant in the constellation Serpens Caput (The Head of the Serpent). A complex system, the bright pair has 4.1” arc seconds of separation & magnitudes +4.2 (Component A) and +5.2 (Component B), respectively. Position angle of the A-B pairing is 178°. Also, sharp-eyed viewers may be able to catch another associated pair 66” distant at position angle 14°, with 14th and 15th magnitude components with a 4.4” arc second spread.  The A and B components are both F-type sub-giants off of the main sequence shining at 76 and 31 times the luminosity of our own Sun. The primary pair is embraced in a 3200 year orbit, while the distant has an unknown period.

Delta Serpentis: A finder chart. (Created by the Author in Starry Night).

The position for the system is as follows;

Right Ascension: 15 Hours 34.8’

Declination:  +10° 32’

The pair is well positioned to the west after sunset for the next month…good luck, and let us know of your triumphs and tales of hunting down double-star prey!

The Astro-Word of the Week is Delta Scuti Variable. Also sometimes referred to as a Dwarf Cepheid variable stars, this is a rare form of variable of which the A component of Delta Serpentis is a “card carrying” member. Hey, you knew something was special about the pair, right?  Lesser known than traditional Cepheids, these smaller F-type stars also have a period-luminosity relationship and also serve a crucial standard candles. Credit for discovery of the variability of the prototype Delta Scuti goes to a paper written by W. W. Campbell and W.H. Wright in 1900. Drawings depict Delta Scuti as far back as Flamsteed’s 1781 southern sky star catalog.  These stars occupy a remarkable branch of the Hertzsprung-Russell diagram; a metal poor branch of instability near the center of the traditional “lazy-S” curve. Most of these stars are between 1 and 2.5 solar masses and have a very short range of variability of 0.03 to 0.3 days.   Why study Delta Scuti variables? Well, astrophysical modeling on these stars has proven crucial to the emerging field of astro-seismology. Delta Scuti-style variables can be further divided into high amplitude and low amplitude variables. This fast variability characteristic coupled with an asymmetric non-radial pulsation enables astronomers to map and model “star-quakes” within the interior of Delta Scuti type stars much the same as helioseismologists study acoustic waves resonating through our own Sun.  Backyard observers have also gotten into the act, and some suspected Delta Scuti type variables awaiting validation include V703 Scorpii, YZ Bootis, and CY Aquarii. These swiftly evolving F-type stars do not reside in the instability region of the H-R diagram for long relative to their lifetimes, a prime reason that this particular class of variable stars is comparatively rare in the sky. It’s amazing that we can glean so much information from a few photons of light!