This photo combines a series of November 2013 observations by NASA's NEOWISE (Near-Earth Object Wide-field Survey Explorer) mission, spanning half a degree. The red color, which makes Lovejoy stand out so clearly, is due to gas and dust in the comet's coma.
Image credit: NASA/JPL-Caltech
At the time the images were taken, Lovejoy was 1.7 astronomical units — AKA 1.7 times the distance of between the Earth and Sun — from the Sun. The red color is derived from a strong signal in the NEOWISE 4.6-micron wavelength detector. If you want to see the comet for yourself, learn how here.