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 Hubble's
Variable Nebula is a beautiful mystery. Discovered by William
Herschel in 1783, this was the first object photographed by the
200-inch Hale telescope at Palomar. What makes this nebula so
interesting is its variability. We are used to the unchanging faces
of nebulae, yet this one apparently changes in brightness and
structure over weeks or months. It was Edwin Hubble in 1916 who
first noticed the changes in photographs.
The star at the end of
the fan-shaped nebulosity is R Mon, an Orion type variable that
varies in brightness by as much as 2 magnitudes. Orion variables are
irregular, eruptive variables connected with bright or dark diffuse
nebulae or observed in the regions of these nebulae. They have only
recently formed and are still in the process of becoming a main
sequence star. Strong stellar winds blow material away from the
star.
R Mon appears to be
surrounded by a disk of material, which is typical for such stars.
It is thought that the planets in our solar system formed from a
similar disk around our sun billions of years ago. As material from
this disk falls into the star much of it gets ejected back outward.
The thick disk that lies at the star's equator blocks the outflow of
ejected matter through it, but the way out at the poles of the star
is relatively clear. The result is a directed flow of matter in two
opposing jets.
In the case of R Mon,
one of these jets flows into the nebula. Dust forms as the material
flows outward producing dark streamers. These streamers cast shadows
onto the walls of the nebula. The nebula is visible to us via
the reflected light of the star.
The nebula itself is a
dusty, hollow shell. When we look at it we primarily see the closer
side of this shell, but parts of the other side also show through.
Some of the structure we see is due to the shadows of relatively
immobile clouds of dust which make more or less permanent features
on the nebula. The shadows of the fast moving streamers are what
cause the apparent changes in brightness as they flow into the
hollow interior. For more information regarding this fascinating
object see the pages from the Glenlea
Astronomical Observatory.
This
finder chart shows the location of the nebula. Stars are drawn to
9th magnitude. The orientation is that of the naked eye. NGC 2261
lies to the east of Betelgeuse near the Cone Nebula and Christmas
Tree Cluster nebula (NGC 2264).
The field in an 6-inch at
50x. North is down and east is to the right.
The nebula is quite
bright and obvious in my 18-inch truss Dob. It looks very much like
the photographs. The surface brightness is high enough to allow for
high magnification, so don't be shy about putting in a high power
eyepiece. Things to look for are light and dark knots in the nebula,
and the faint nebulosity that lies beyond the wider end. R Mon
itself can be difficult to distinguish from the bright nebulosity
that surrounds it.
Have a look at Mark
Birkmann's Hubble's Variable Nebula page for an excellent
drawing made with a 40".
NGC 2261 makes an
interesting target for CCD imaging. Several amateurs have made
animations showing the changes in the nebula.
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