Some of the most difficult, but spectacular objects to be seen at night are dark nebulae. The image to the left is called the 'Horsehead Nebula' and is located in the constellation of Orion. But believe me, you will probably never see this good of a view through a telescope. It is often an extremely difficult object to view, because the background is just barely lighter than the foreground object. Dark nebulae are clouds of gas and dust that absorbs some light from behind them. The light that is absorbed heats up the dust particles, and thus these particles re-radiate, or emit, some of the absorbed energy as infrared light (Infrared light is on the opposite side of the light spectrum from ultraviolet). Some of the background light is also scattered into the surrounding interstellar medium and thus can reduce the contrast between background and foreground, making viewing them even more difficult.