Hubble Heritage Image

Hubble's Variable Nebula


Glenlea Astronomical Observatory, University of Manitoba

NGC 2261 is a fan-shaped reflection nebula immediately north of R Monocerotis. It became known as "Hubble's Variable Nebula" after Edwin Hubble discovered that it varies in both shape and brightness month to month (Hubble 1916).
L. M. Close et al. (1997) made high-resolution (FWHM=0.2") near-infrared images of R Mon and developed a model for the star and its optically thick envelope. They concluded that R Mon is surrounded by an accretion disk which is tilted 20 degrees toward us and has an axis of rotation at  P.A. 350 degrees. A strong polar wind along this axis has cleared a "throat" through the envelope that surrounds the star and accretion disk. Infalling envelope material is entrained into this wind, ejected through the throat and now forms a roughly parabolic dust shell (NCG 2261) north of R Mon.

The walls of this nebula reflect and polarize the light from R Mon. Both Close and Lightfoot (below) concluded that dust streamers forming 1-2 AU from the star cast shadows up through the throat and onto the walls of the nebula. The pattern of light is complicated by the irregular shape of the nebula walls and additional shadows cast by dusty filaments that occupy the interior of the shell.

To understand the combination of fixed and changing patterns that occur, J. F. Lightfoot (1989) studied hundreds of photographs of NGC 2261 that where taken by C.O. Lampland at the Lowell observatory between 1916 and 1951. These included two dramatic shadow transits that occurred in 1934-35 and 1939-40. (See the Analysis section for animated images of the shadow transits.) The major features of the model Lightfoot developed are show below beside D.F. Malin's B-band photograph of the nebula taken with the 3.9 m Anglo-Australian Telescope.

D.F Malin - AAT

Lightfoot's model includes

Lightfoot created a computer model to calculate the shape of the interior filaments that cast shadows onto the walls of the nebula. His model, shown in his Figure 8 at right, used loops created by a small helix that was itself curled into a larger helix.

He noted that the loops did not have to be continuous, that short arcs would accomplish the same effect.

Lightfoot also noted that the filaments north of the nebula and the shadows cast by filaments inside the nebula had not changed noticeable since 1920.

To summarize then, the visible surface of the NGC 2261 is made up of

The present study was undertaken to determine the current condition of the nebula and advance the model developed by Lightfoot (1989). Images were taken on several nights during the 1998-99 observing season using a 0.4 m reflecting telescope and CCD camera. During that time one shadow transit event was observed as well as small-scale and large-scale variability of the near-side component of the nebula. Changes were detected in as little as three days.

Two methods are provided for observing the changes:

For an excellent introduction to Hubble's Variable Nebula see Steve Toew's CCD project.

For a detailed discussion of the nebula's variability read, download or print J.F.Lightfoot's 1989 paper Shadowplay in Hubble's variable nebula. Choose Full Refereed Scanned Article to see the paper on-screen.

Close et al provide a model for the outflows from R Mon in their 1997 paper Adaptive Optics Infrared Imaging Polarimetry and Optical HST Imaging of Hubble's Variable Nebula. Note that this paper can also be read on-line by choosing the appropriate link on that page.

Richard Hall of Northern Arizona University created a "movie" of the hundreds of images of NGC 2261 taken by C.O. Lampland between 1916 and 1951. The Lampland images and animations at this site were created by capturing frames from a video tape of the movie.

Tom Polakis observed the nebula between January 22 and April 18, 1999 (a period that overlaps this study) and created an animation of the changes he observed.

Introduction Analysis and Animations References
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Last modified July 7, 2000