NGC 2261 - Hind's Variable Nebula
"Hinds variable nebula" is a reflection nebula in the constellation Monoceros with the catalog designation NGC 2261, which changes not only its brightness
but also its shape over a long period of time. The object was discovered two days after Christmas in 1783 by F.W. Herschel. Its apparent magnitude is given
as 9.0mag and the size as 3x2' (arcminutes). The distance to NGC 2261 is 2.600 light-years, so its true size can be calculated to 1.6x2.3 light-years.
The nebula reflects the light of the giant star "R Mon", which is hidden from our curious eyes by some dust in front of it. It is a pre-main sequence star, i.e. a
star still in its evolutionary phase. The star is extremely young with an assumed age of just 100,000 years. The mass is estimated to be 3- 10 solar masses. "R Mon"
belongs to the spectral class B0...B8 and changes its brightness in the range of 10.6...12.5mag. It is also surrounded by a dust disk with a mass of 0.007 solar
masses and a radius of 150AE.
The variability of the nebula is caused when dust clouds block the light of the star, casting shadows on the surrounding nebular masses. Changes can be seen
even in smaller telescopes after weeks or months.
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NGC 2261 offers a beautiful view in my 20" f/3.2 Dobsonian. The surrounding area is very rich in stars and two wonderfully bright stars provide a great contrast. :)
The nebula itself is relatively small, so it is best observed at high magnifications of 270...380x. Then it shows many exciting details. The tip of the nebula is of
course the brightest part and extremely compact. This part fans out towards the north. In particular, the eastern flank is quite bright, well-defined and extends far
to the north, while the western flank is also bright, breaks into two pieces and does not extend quite as far north. In the middle of the nebula another bright nebular
knot can be seen.
Definitely a highly exciting object and absolutely worthwhile!
