This is a bit better photo of Comet 17P / Holmes, it was taken on Nov. 2, 2007 from the roof at the University of Tennessee Knoxville Physics Building. This was a photo I made from a TV monitor that was attached to a large telescope. The nucleus of the comet is the bright white dot within the illuminated gas and dust cloud (the coma). Unfortunately, my camera did not pick up the even larger, paler ring that was visible on the monitor. Scientists estimate, at this point, that the coma is wider than the diameter of the orbit of the Moon around the Earth (nearly 500,000 miles)! The tail cannot be seen because the comet is coming straight toward Earth and is behind it. It was exciting to see this comet because it is rare for them to increase in brightness as rapidly as this one did over 3 days! The white spot on the right is the star Algol (a binary star). This photo was taken in manual mode, I had to hand-hold the camera because I didn't think it would go over too well to set up a tripod in a crowd of ~100 people packed onto the roof!
Comets can be thought of as "little (relatively speaking!) cosmic crumbs" of water ice, dry ice, dust and rock left over from the formation of the solar system. Since they are made of ice, they are obviously, quite cold. They are not hot and do not make their own light, what we see from earth is light reflected from the Sun.
The "P" designation in 17P / Holmes signifies that this is a "periodic" comet, it has an orbital period of less than 200 years. It was first discovered on November 6, 1892 by Edwin Holmes, for whom it was named. It went through a similar outburst of brightness at that time too.