The Caldwell Objects

46
4 6
Hubble’s Variable Nebula;
Hersxhel’s Forgotten Fan
NGC 2261
Type: Reflection Nebula
Con: Monoceros
RA: 06 h 39.2 m
Dec: +08° 44'
Mag: -10.0
(nebula; variable)
Dim: 3.5 x 1.5'
Dist: 2,500 light-years
Disc: William Herschel, 1783
W. H ERSCHEL: [26
December 1783] Considerably bright and fan shaped
About 2 minutes long from the center. (H IV-2)
T HERE IS A CURIOUS REFLECTION NEBULA IN
Monoceros that, when photographed, looks like
the head of a match just beginning to ignite. It's
called NGC 2261, and it is one of the most
unusual objects in the Caldwell Catalog. William
Herschel discovered the fan-shaped glow in 1783.
The nebulous object includes R Monocerotis, a
l0th-magnitude "star" at the southern tip of the
fan. (I've placed "star" in quotes, because, as we
shall see, the star that ultimately illuminates the
nebula cannot be seen at visual wavelengths.)
While using the 6-inch refractor at Athens
(Greece) Observatory in 1861, director and
energetic variable-star observer J. F Julius .
Schmidt discovered that "R Mon" fluctuates in
brightness, though at irregular intervals.
Historically the star has varied between
magnitude 9.5 and 13.0. While comparing
photographs of NGC 2261 taken between 1900
and 1916, American astronomer Edwin Hubble
found that the nebula's shape
182
GC / NGC: Bright, very much extended toward position angle
330°, nucleus cometic and equal to a star of 11th magnitude.
and brightness also vary over time scales ranging
from a few weeks to months. Ever since that
discovery, NGC 2261 has been called Hubble's
Variable Nebula, leading many to believe that
Hubble actually discovered the nebula. Out of
respect for Sir William, I like to call NGC 2261
"Herschel's Forgotten Fan."
Inspired by Hubble's discovery, Carl Otto
Lampland began, in 1916, to monitor the nebula
with the 42-inch reflector at Lowell Observatory.
By 1951 he had produced a valuable series of 900
photographs. Like snapshots of a candle flame
flickering in the wind, these images showed the
nebula's shape and intensity changing over the
years. (The photographs on page 183, taken more
recently, only hint at the phenomenon.)
Curiously, the variations in the nebula did not
coincide with the irregular fluctuations of the
"star" that supposedly illuminated it.
Furthermore, individual details in the nebula
could be "displaced" by as much as
Deep-Sky Companions: The Caldwell Objects