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A closer look at Hubble's Variable Nebula.

A favorite object for winter deep-sky observers is NGC 2261, a little fan-shaped reflection nebula in Monoceros. Since 1916 it has also been known as Hubble's Variable Nebula, following Edwin Hubble's discovery that it can change shape on a time scale of months (CCD Astronomy: Winter 1996, page 42). Embedded in the fan's bright tip is the hazy variable star R Monocerotis. Dark dust clouds moving near the star cast their shadows on the nebula, changing its appearance.

Now a team of astronomers at the University of Hawaii and elsewhere has brought new telescopic power to bear on this enigmatic object. Laird M. Close and colleagues examined the nebula's brightest part in archival Hubble Space Telescope images. They also studied it at infrared wavelengths using the 3.6-meter Canada-France-Hawaii telescope, which they have equipped with their own adaptive-optics instrument to largely cancel out the blurring effects of atmospheric turbulence. The ground-based images they obtained are about 10 times sharper than any taken previously. Among their findings:

* R Monocerotis itself is hidden from view. The 10th-magnitude starlike object we see is a small, dense dust cloud that's brilliantly lit from within. At infrared wavelengths, however, the dust can be penetrated and the star seen directly.

* The star is a binary. Just 0.7 arcsecond west of the primary the astronomers found a previously unknown companion star that's 200 times fainter. The primary is hot and massive, with a B-type spectrum and signs of matter accreting onto its surface. The secondary star seems to be an ordinary, less-massive T Tauri star no more than 300,000 years old. The presence of such a young star implies that the primary also is likely to be extremely young.

* Both stars are buried in dense disks or doughnuts of dust smaller than even the Hubble telescope can resolve. The familiar fan-shaped reflection nebula is a hollow cone resulting from hot gas blowing out perpendicular to R Monocerotis's disk. The nebula shows twisted filaments that may highlight magnetic lines of force spiraling around the cone's walls. The filaments appear to form a double helix.

As Close and his colleagues explain in the November 1, 1997, Astrophysical Journal, their study also enabled them to crudely estimate R Monocerotis's distance: 2,500 light-years. This strengthens the notion that the unusual young star is somehow affiliated with NGC 2264, a young, gaseous cluster about 90 arcminutes to the northeast.

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Title Annotation:fan-shaped reflection nebula discovered by Edwin Hubble
Publication:Sky & Telescope
Date:Mar 1, 1998
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