Object of the Week - Feb 4th, 2018 - NGC 1555 "Hind's Variable Nebula" and NGC 1554 "Struve's Lost Nebula"
NGC 1555 "Hind's Variable Nebula" (Sh 2-238, HH 155)
R.A.: 4h 21m 57s
Dec.: +19° 32' 04"
Size: ~0.5arcmin
Type: RN (YSO, HH)
NGC 1554 "Struve's Lost Nebula"
R.A.: 4h 21m 44s
Dec.: +19° 31' 16"
Size: ?
Type: RN?
High in the sky, both objects, or better the one visible (NGC 1555) presents perhaps one of the most fascinating, because of there variability, objects in the sky.
The history can fill books and reads like a good thriller. To make the long story short - NGC 1555 was discovered from the English astronomer John Russell Hind in 1852 with a 7-inch refractor. In the following years, other astronomers confirm the sighting (Chacornac, Breen, d'Arrest, Auwers).
Then, around 1858 the nebula began to die away. Chacornac could not find the nebula a second time in 1858 with a 10-inch refractor. Goldschmidt, Schönfeld, Auwers, d'Arrest and Baxendell also couldn't see the nebula visually.
With a more powerful telescope, Struve and Winnecke could detect the nebula again in 1862 and 1863 with the large 15-inch Merz refractor.
Strangely enough, simultaneous observations could not confirm the positive results. Leverrier and Chaconac (31-inch!), Secchi, Lassell (48-inch!) and the discoverer itself - Hind could not detect anything.
The visibility changes again from 1863 when Struve and Lassell could detect the nebula with the large 48-inch telescope from Malta. Later, between 1865 and 1866 Vogel could confirm the positive observation with a small 4.6-inch refractor.
But the story began to be even more confused. Struve discovered with the powerful 15-inch Merz refractor another nebula 4' away - which was catalogued as NGC 1554 in 1868, while NGC 1555 was again invisible at the same time. D'Arrest confirmed the new nebula, Bigourdan only suspected NGC 1554 in 1886, while other astronomers failed (Parsons, Copeland, Engelhardt).
With the even most powerful telescope at the time Burnham and Barnard confirmed NGC 1555 with the large 36-inch Lick refractor in 1890 and 1895. Struve's NGC 1554 was again invisible. Both astronomers discovered another very small nebula directly around T-Tauri. Keeler (36-inch Crossley, photographic) and Aitken/Perrine (36-inch Lick refractor) could confirm NGC 1555 visual again in 1899. In the following years Hind's nebula could be photographed several times. Interestingly the last professional visual! observation of NGC 1555 was through the 100-inch Hooker telescope by Hubble and Baade who assumed a light up of the nebula.
All in all, now we know that NGC 1555 is clearly a variable object which explains the coming and going sightings. NGC 1554 is probably an observing failure of Struve, d'Arrest and Bigourdan who confuse a "new nebula" with faint stars.
Astrophysical, the 400-light years distant NGC 1555 is a refection nebula. Some sources describes the nebula as a Herbig-Haro object. These objects were formed by fast moving narrow jets of gas out of a young aged star or better young stellar object (YSO). When these jets collides with nearby gas and dust, a resulting shock wave gets visible. T-Tauri, the very neighbour star and culprit of this jets changes his magnitudes between around 8.5mag and 13.5mag.
Today Hind's Variable Nebula seems to be quit stable in brightness. That means it should be visible in mid size telescopes. One help to see the faint glow can be to cover T-Tauri with a shutter or cross-hair in the eyepiece near the focal plane.
The challenge could be to detect structures within the nebula or even the small nebula structures directly around T-Tauri.
10'x10' DSS blue
NGC1555_DSS10b.jpg
Capella Team(Palaiologou, Binnewies, Pöpsel)
NGC1555_Capella.jpg
sketch: 16", 189x, NELM 6m0+
NGC1555.jpg
As always, give it a go and let us know
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