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)
Hen1-5_DSS5b.gif

Keck Telescope
(6-10-1996)
sge-fg-keck.jpg

sketch: 16", 180x, [OIII], NELM 6m5+, Seeing II
(10-9-2007)
Hen1-5.jpg

sketch: 27", 586x, no filter, NELM 6m5+, Seeing III
(11-16-2012)
Hen1-5_27.jpg