Sky & Telescope - March 2025 - 23

refractor. John Dreyer remarked in his original NGC catalog
(published in 1888) that this may be a rediscovery of
NGC 2238, which German astronomer Albert Marth found
a year earlier using William Lassell's 48-inch reflector on the
island of Malta. In 1886, Swift reobserved the Rosette and
noted another section of the nebula, which is now designated
NGC 2246.
Curiously, neither William nor John Herschel noted
any of the Rosette's nebulosity. Perhaps the small field of
view of their instruments combined with the brightness
of NGC 2244 washed out the subtle glow of the 1.3°-wide
Rosette Nebula.
The multiple NGC numbers for the nebula's brightest segments
have sometimes caused confusion about the designation
of the entire object. (In fact, Sky & Telescope's Pocket Sky
Atlas lists it as NGC 2237-8/46.) As it doesn't have an overall
NGC number, some astronomers simplify by referring to it
using its Sharpless designation, Sh 2-275.
There are four open clusters near the nebula: Collinder 97,
Cr 104, Cr 107, and NGC 2252. To my eye only NGC 2252
- and possibly Cr 97 - stand out well enough to resemble
clusters, with the others blending into the starry field of view.
Different methods for measuring the distance to the
Rosette place it anywhere from 4,200 to 5,100 light-years away,
which means its apparent diameter translates to 95-115 lightyears.
It's superimposed on the southern edge of the Monoceros
Loop supernova remnant, G205.5+0.5, and recent measurements
disagree as to whether G205.5+0.5 and the Rosette
are at the same distance. Depending on which study you find
more plausible, the two objects may or may not be interacting.
Nevertheless, the European Space Agency's Gaia mission's
parallax measurements of the NGC 2244 cluster stars place
them at 5,053 light-years, which greatly supports a distance
of 5,100 light-years for the nebula. The powerful nature of the
O-type stars in NGC 2244 (as the illuminating source) argues
even more strongly for the cluster and nebula to be at the
same distance. Even so, an article published in the Monthly
Notices of the Royal Astronomical Society in 2022 discussed
the possible distance discrepancy as real, putting the cluster
hundreds of light-years behind the nebula! (If nothing else,
this corroborates how notoriously difficult it is to pin down
astronomical distances - see, e.g., S&T: Oct. 2022, p. 12.)
Night One: O III Observations
I used filters for all my observations of the Rosette - namely,
an O III and a hydrogen-alpha (Hα) filter (more on the latter
FIRST DIP Top: I started the night's observing by pointing my 8-inch
scope fitted with an O III filter at the Rosette Nebula and NGC 2244. I
saw the open cluster NGC 2252 as more subtle than I've drawn here.
Also note the dipper shape of NGC 2244 - it even has a hook at the end
of its short handle. North is up.
GOING DEEPER Bottom: This sketch shows what the Rosette looked
like through my 30-inch with an O III filter. Notice the increased detail
within the nebula and additional stars, especially NGC 2252 at upper left
- overall, a memorable view!
sk yand tele scope .o r g * MARCH 2025 23
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Sky & Telescope - March 2025

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