What causes Hubble's Variable Nebula to vary? The unusual nebula pictured above changes its appearance [ http://www.psiaz.com/polakis/n2261/n2261_1999.html ] noticeably in just a few weeks. Discovered over 200 years ago and subsequently cataloged [ http://antwrp.gsfc.nasa.gov/apod/ap990429.html ] as NGC 2661, the remarkable nebula is named for Edwin Hubble [ http://antwrp.gsfc.nasa.gov/apod/ap980621.html ], who studied it [ http://adsbit.harvard.edu/cgi- bin/nph-iarticle_query?1916ApJ....44..190H ] earlier this century. Hubble's Variable Nebula [ http://heritage.stsci.edu/public/99oct7/ngc2261/ngc2261table.html ] is a reflection nebula [ http://antwrp.gsfc.nasa.gov/apod/ap990301.html ] made of gas and fine dust [ http://antwrp.gsfc.nasa.gov/apod/ap990509.html ] fanning out from the star R Monocerotis. The faint nebula is about one light-year [ http://chandra.harvard.edu/photo/cosmic_distance.html ] across and lies about 2500 light-years away towards the constellation [ http://www.astro.wisc.edu/~dolan/constellations/constellations.html ] of Monocerotis [ http://www.astro.wisc.edu/~dolan/constellations/constellations/Monoceros.html ]. A leading variability explanation [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1978ApJ...226..455A ] for Hubble's Variable Nebula [ http://www2.localaccess.com/adamc/ngc2261.html ] holds that dense knots of opaque dust pass close to R Mon [ http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997ApJ...489..210C ] and cast moving shadows onto the reflecting dust seen in the rest of the nebula.