Object of the Week Junly 26th, 2015 – Henize 1-5 and the variable CS Fg Sge

Henize 1-5 (Hen 1-5)
PNG 60.3-7.3

Saggita

RA 20 11 56
DEC +20 20 05

Type: Planetary Nebula
Size: 36"

Magnitude: 16.2
Central Star Mag: V Fg Sge (actual ~12mag-18mag)


To compare the limiting magnitudes of the Michigan – Mount Wilson southern Hα survey (Lamont-Hussey Observatory / South Africa) with that of the northern survey, Karl G. Henize took some objective-prism plates in the Cygnus-Vulpecula region. The result was a catch of seven new objects which were further photographed with the 40" and 60" telescope at Mount Wilson and which were all classed as planetary nebula.
In the 1961 published paper (1961PASP...73..159H) object number five gets an extra long comment. Henize describes a bright central star with strong emission lines and a variation in brightness – he observes the PN already twice in 1955. He started a discussion of the morphology of the object – the story behind Hen 1-5 began.

Long before the discovery of the PN in 1961 started, Cuno Hoffmeister of the Sonneberg Observatory / Germany discovered a variable star “FG Sge” and cataloged the star as “irregular” because of the unknown light curve.

In the Seventies bigger and more modern observation confirm a uniform increase from 12mag from the Forties up to 9.5mag in the Seventies (1968IAUS...34..383H). This uniform increase stops in the Eighties with around 9.0mag, before the brightness of the central star drops more or less rapidly and vary up to date from around 12mag down to 18mag.

Back to the PN. The irony of the magnitude dropping of the CS is that the visibly of the PN rises. Behind a 9mag star the faint PN will be visible very hard, same effect than Abell 13. But behind a say 14mag star, the PN is visible much easier. Different to the abnormal CS, the PN seems to be a typical planetary nebula. As far as we know the planetary shell is around 6000 years old (1973ApJ...183..491F) with a distance of around 1,5 kpc (1970PASP...82.1333F)

To come to the practical observations - positive observations of the nebula should be possible from 8" up. When the CS is at its minimum, a 12.3mag star which lies only 8" E of the center of the PN should be the first detail what is visible. The UHC or [OIII] filter shows an eccentrically round glow around the star with hard defined edges. Bigger telescopes shows more stars (around 14mag) to the E and with luck the CS directly W of the 12.3 star.

Because of the variable CS it would be very interesting to hear your observation.

POSS II 5'x5' blue
(with bright CS)
Attachment 1739

Keck Telescope
(6-10-1996)
Attachment 1740

sketch: 16", 180x, [OIII], NELM 6m5+, Seeing II
(10-9-2007)
Attachment 1741

sketch: 27", 586x, no filter, NELM 6m5+, Seeing III
(11-16-2012)
Attachment 1742

Great OOTW, Uwe.

I only have a single observation back in 2001 with my former 17.5-inch -- I don't why I never went back to take another look. I guess it's about time! Here's what I logged ---

Although FG Sge is currently below 15th magnitude, a mag 12.5 star ~10" E of the central star was easily identified at 220x as the third of four in a 6' chain NW to SE consisting of mag 12-13 stars. An extremely faint halo, ~20" diameter, was barely visible with the star superimposed off-center. With a UHC filter, the nebulosity was more evident although there was only a mild contrast gain.

Hi Uwe
Super OOZW of Heinze 1-5.
In particular, the difference of 16 inches to 27 inches you have excellent drawn on paper.
This object I try with 16 inches!!
Thank you for this presentation.

Hajü
www.astromerk.de

Sweet object. Here is my observation from back in 2008.

22” (287 and 460x) – Very faint, round glow with a very bright central star. Responds well to O-III filter. It appears as a faint round glow over a vB star. About 0.4’ across.

After reading this observation, looks like I have to look at it again...but won't be until 2016 or 2017, as my largest scope is a 4" refractor at this point.

Steve, Hajü and Alvin,

thanks for your response.

@Steve
Summer 2001 - I guess you catch the minimum off FG Sge at around 16mag. To tough for the 17.5".

@Hajü
Stay in touch with that object. Very interesting PN besides the many brighter NGC/IC and more famous fainter Abells.

@Alvin
Very nice to hear a comment of you, good news that you are still in on it, even with smaller aperture. Your observations sounds that you catch the brighter "companion". But it is of course difficult to place that faint glow exactly between the stars.